.dt The Project Gutenberg eBook of The Heavens Above, by J. A. Gillet
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Transcriber's Note:
This version of the text cannot represent certain typographical
effects. Italics are delimited with the underscore character as italic.
Superscripts, such as P to the second power, are shown by the caret "^"
character before the superscript, such as P^2.
Subscripts are similarly shown by an underscore before the subscript which is
wrapped in curly braces, such as M_{2}.
The illustrations are also not present in this text edition.
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.ca Spectra Of Various Sources Of Light.
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[Illustration: SPECTRA OF VARIOUS SOURCES OF LIGHT.]
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.h1
The Heavens Above: A Popular Handbook of Astronomy
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THE HEAVENS ABOVE:
A POPULAR HANDBOOK OF ASTRONOMY.
BY
J. A. GILLET,
PROFESSOR OF PHYSICS IN THE NORMAL COLLEGE OF THE CITY OF NEW YORK,
AND
W. J. ROLFE,
FORMERLY HEAD MASTER OF THE HIGH SCHOOL,
CAMBRIDGE, MASS.
WITH SIX LITHOGRAPHIC PLATES AND FOUR HUNDRED
AND SIXTY WOOD ENGRAVINGS.
POTTER, AINSWORTH, & CO.,
NEW YORK AND CHICAGO.
1882.
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Copyright by
J. A. GILLET and W. J. ROLFE,
1882.
Franklin Press:
RAND, AVERY, AND COMPANY,
BOSTON.
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.sp 4
.h2 title='Preface.'
PREFACE.
.sp 2
It has been the aim of the authors to give in
this little book a brief, simple, and accurate account
of the heavens as they are known to astronomers
of the present day. It is believed that there is
nothing in the book beyond the comprehension of
readers of ordinary intelligence, and that it contains
all the information on the subject of astronomy that
is needful to a person of ordinary culture. The
authors have carefully avoided dry and abstruse
mathematical calculations, yet they have sought to
make clear the methods by which astronomers have
gained their knowledge of the heavens. The various
kinds of telescopes and spectroscopes have been
described, and their use in the study of the heavens
has been fully explained.
The cuts with which the book is illustrated have
been drawn from all available sources; and it is believed
that they excel in number, freshness, beauty,
and accuracy those to be found in any similar work.
The lithographic plates are, with a single exception,
reductions of the plates prepared at the Observatory
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at Cambridge, Mass. The remaining lithographic
plate is a reduced copy of Professor Langley's
celebrated sun-spot engraving. Many of the
views of the moon are from drawings made from
the photographs in Carpenter and Nasmyth's work
on the moon. The majority of the cuts illustrating
the solar system are copied from the French edition
of Guillemin's "Heavens." Most of the remainder
are from Lockyer's "Solar Physics," Young's "Sun,"
and other recent authorities. The cuts illustrating
comets, meteors, and nebulæ, are nearly all taken
from the French editions of Guillemin's "Comets"
and Guillemin's "Heavens."
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CONTENTS.
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I. THE CELESTIAL SPHERE #3:celestial-sphere#
II. THE SOLAR SYSTEM #41:solar-system#
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I. THEORY OF THE SOLAR SYSTEM #41:theory#
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The Ptolemaic System #41:ptolemy#
The Copernican System #44:copernicus#
Tycho Brahe's System #44:brahe#
Kepler's System #44:kepler#
The Newtonian System #48:newton#
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II. THE SUN AND PLANETS #53:sun-planets#
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I. The Earth #53:earth#
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Form and Size #53:earth-form#
Day and Night #57:day-night#
The Seasons #64:seasons#
Tides #68:tides#
The Day and Time #74:day-time#
The Year #78:year#
Weight of the Earth and Precession #83:earth-weight#
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II. The Moon #86:moon#
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Distance, Size, and Motions #86:moon-distance#
The Atmosphere of the Moon #109:moon-atmosphere#
The Surface of the Moon #114:moon-surface#
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III. Inferior and Superior Planets #130:planets#
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Inferior Planets #130:inferior-planets#
Superior Planets #134:superior-planets#
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IV. The Sun #140:sun#
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I. Magnitude and Distance of the Sun #140:sun-magnitude#
II. Physical and Chemical Condition of the Sun #149:sun-condition#
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Physical Condition of the Sun #149:sun-physical#
The Spectroscope #152:spectroscope#
Spectra #158:sun-spectra#
Chemical Constitution of the Sun #164:sun-chemical#
Motion at the Surface of the Sun #168:sun-surface#
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III. The Photosphere and Sun-Spots #175:photosphere-sunspots#
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The Photosphere #175:photosphere#
Sun-Spots #179:sunspots#
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IV. The Chromosphere and Prominences #196:prominences#
V. The Corona #204:corona#
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V. Eclipses #210:eclipses#
VI. The Three Groups of Planets #221:three-groups#
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I. General Characteristics of the Groups #221:group-chars#
II. The Inner Group of Planets #225:inner-group#
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Mercury #225:mercury#
Venus #230:venus#
Mars #235:mars#
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III. The Asteroids #241:asteroids#
IV. Outer Group of Planets #244:outer-group#
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Jupiter #244:jupiter#
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The Satellites of Jupiter #250:jupiter-satellites#
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Saturn #255:saturn#
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The Planet and his Moons #255:saturn-planet#
The Rings of Saturn #261:saturn-rings#
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Uranus #269:uranus#
Neptune #271:neptune#
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VII. Comets and Meteors #274:comets-meteors#
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I. Comets #274:comets#
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General Phenomena of Comets #274:comets-general#
Motion and Origin of Comets #281:comets-motion#
Remarkable Comets #290:comets-remarkable#
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Connection between Meteors and Comets, #300:comets-connection#
Physical and Chemical Constitution of Comets #314:comets-physical#
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II. The Zodiacal Light #318:zodiacal#
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III. THE STELLAR UNIVERSE #322:stellar#
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I. General Aspect of the Heavens #322:aspect#
II. The Stars #330:stars#
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The Constellations #330:constellations#
Clusters #350:clusters#
Double and Multiple Stars #355:double-stars#
New and Variable Stars #358:new-stars#
Distance of the Stars #364:star-distance#
Proper Motion of the Stars #365:star-proper#
Chemical and Physical Constitution of the Stars #371:star-chemical#
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III. Nebulæ #373:nebulae#
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Classification of Nebulæ #373:nebulae-classification#
Irregular Nebulæ #376:nebulae-irregular#
Spiral Nebulæ #384:nebulae-spiral#
The Nebular Hypothesis #391:nebular-hypothesis#
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IV. The Structure of the Stellar Universe #396:stellar-universe#
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.h2 id='celestial-sphere' title='I. The Celestial Sphere.'
I. | THE CELESTIAL SPHERE.
.sp 2
I. The Sphere.--A sphere is a solid figure bounded by
a surface which curves equally in all directions at every
point. The rate at which the surface curves is called the
curvature of the sphere. The smaller the sphere, the greater
is its curvature. Every point on the surface of a sphere is
equally distant from a point within, called the centre of
the sphere. The circumference of a sphere is the distance
around its centre. The diameter of a sphere is the distance
through its centre. The radius of a sphere is the
distance from the surface to the centre. The surfaces of
two spheres are to each other as the squares of their radii
or diameters; and the volumes of two spheres are to each
other as the cubes of their radii or diameters.
Distances on the surface of a sphere are usually denoted in
degrees. A degree is 1/360 of the circumference of the sphere.
The larger a sphere, the longer are the degrees on it.
A curve described about any point on the surface of a
sphere, with a radius of uniform length, will be a circle.
As the radius of a circle described on a sphere is a curved
line, its length is usually denoted in degrees. The circle
described on the surface of a sphere increases with the
length of the radius, until the radius becomes 90°, in which
case the circle is the largest that can possibly be described
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on the sphere. The largest circles that can be described on
the surface of a sphere are called great circles, and all other
circles small circles.
.pm letter-start
Any number of great circles may be described on the surface
of a sphere, since any point on the sphere may be used
for the centre of the circle. The plane of every great circle
passes through the centre of the sphere, while the planes of
all the small circles pass through the sphere away from the
centre. All great circles on the same sphere are of the same
size, while the small circles differ in size according to the distance
of their planes from the centre of the sphere. The farther
the plane of a circle is from the centre of the sphere, the
smaller is the circle.
By a section of a sphere we usually mean the figure of the
surface formed by the cutting; by a plane section we mean one
whose surface is plane. Every
plane section of a sphere is
a circle. When the section
passes through the centre of
the sphere, it is a great
circle; in every other case
the section is a small circle.
Thus, AN and SB (Fig. 1)
are small circles, and MM'
and SN are large circles.
.pm letter-end
.if h
.il fn=fig001.png w=50% alt='Circles'
.ca Fig. 1.
.if-
.if t
[Illustration: Fig. 1.]
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.pm letter-start
In a diagram representing
a sphere in section, all the
circles whose planes cut the
section are represented by
straight lines. Thus, in Fig. 2, we have a diagram representing
in section the sphere of Fig. 1. The straight lines AN, SB,
MM', and SN, represent the corresponding circles of Fig. 1.
.pm letter-end
The axis of a sphere is the diameter on which it rotates.
The poles of a sphere are the ends of its axis. Thus, supposing
the spheres of Figs. 1 and 2 to rotate on the diameter
PP', this line would be called the axis of the sphere,
and the points P and P' the poles of the sphere. A great
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circle, MM', situated half way between the poles of a
sphere, is called the equator of the sphere.
Every great circle of a sphere has two poles. These are
the two points on the surface
of the sphere which lie
90° away from the circle.
The poles of a sphere are
the poles of its equator.
.if h
.il fn=fig002.png w=50% alt='Circles'
.ca Fig. 2.
.if-
.if t
[Illustration: Fig. 2.]
.if-
2. The Celestial Sphere.--The
heavens appear to
have the form of a sphere,
whose centre is at the eye
of the observer; and all the
stars seem to lie on the surface
of this sphere. This
form of the heavens is a
mere matter of perspective. The stars are really at very
unequal distances from us; but they are all seen projected
upon the celestial
sphere in the direction
in which they
happen to lie. Thus,
suppose an observer
situated at C (Fig. 3),
stars situated at a, b,
d, e, f, and g, would
be projected upon the
sphere at A, B, D, E,
F, and G, and would
appear to lie on the
surface of the heavens.
.if h
.il fn=fig003.png w=50% alt='Circles'
.ca Fig. 3.
.if-
.if t
[Illustration: Fig. 3.]
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3. The Horizon.--Only
half of the celestial sphere is visible at a time. The
plane that separates the visible from the invisible portion is
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called the horizon. This plane is tangent to the earth at
the point of observation, and extends indefinitely into space
in every direction. In Fig. 4, E represents the earth, O the
point of observation, and SN
the horizon. The points on
the celestial sphere directly
above and below the observer
are the poles of the horizon.
They are called respectively
the zenith and the nadir. No
two observers in different
parts of the earth have the
same horizon; and as a person
moves over the earth he
carries his horizon with him.
.if h
.il fn=fig004.png w=50% alt='Circles'
.ca Fig. 4.
.if-
.if t
[Illustration: Fig. 4.]
.if-
The dome of the heavens appears to rest on the earth,
as shown in Fig. 5. This is because distant objects on
the earth appear projected
against the
heavens in the direction
of the horizon.
.if h
.il fn=fig005.png w=50% alt='Circles'
.ca Fig. 5.
.if-
.if t
[Illustration: Fig. 5.]
.if-
The sensible horizon
is a plane tangent
to the earth
at the point of observation.
The rational
horizon is a
plane parallel with
the sensible horizon,
and passing through
the centre of the
earth. As it cuts the celestial sphere through the centre,
it forms a great circle. SN (Fig. 6) represents
the sensible horizon, and S'N' the rational horizon.
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Although these two horizons are really four thousand miles
apart, they appear to meet at the distance of the celestial
sphere; a line four thousand miles long at the distance of
the celestial sphere becoming
a mere point, far too
small to be detected with
the most powerful telescope.
.if h
.il fn=fig006.png w=50% alt='Circles'
.ca Fig. 6.
.if-
.if t
[Illustration: Fig. 6.]
.if-
.if h
.il fn=fig007.png w=50% alt='Celestial Sphere'
.ca Fig. 7.
.if-
.if t
[Illustration: Fig. 7.]
.if-
4. Rotation of the Celestial
Sphere.--It is well
known that the sun and the
majority of the stars rise in
the east, and set in the west.
In our latitude there are
certain stars in the north which never disappear below the
horizon. These stars are called the circumpolar stars. A
close watch, however, reveals the fact that these all appear
to revolve around one of their number called the pole star,
in the direction indicated
by the arrows in
Fig. 7. In a word, the
whole heavens appear
to rotate once a day,
from east to west,
about an axis, which
is the prolongation of
the axis of the earth.
The ends of this axis
are called the poles
of the heavens; and
the great circle of the
heavens, midway between these poles, is called the celestial
equator, or the equinoctial. This rotation of the heavens
is apparent only, being due to the rotation of the earth
from west to east.
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5. Diurnal Circles.--In this rotation of the heavens, the
stars appear to describe circles which are perpendicular to
the celestial axis, and parallel with the celestial equator.
These circles are called diurnal circles. The position of
the poles in the heavens
and the direction of the
diurnal circles with reference
to the horizon, change with
the position of the observer
on the earth. This is owing
to the fact that the horizon
changes with the position of
the observer.
.if h
.il fn=fig008.png w=50% alt='Circles'
.ca Fig. 8.
.if-
.if t
[Illustration: Fig. 8.]
.if-
When the observer is north
of the equator, the north
pole of the heavens is elevated
above the horizon, and the south pole is depressed
below it, and the diurnal circles are oblique to the horizon,
leaning to the south. This case is represented in Fig. 8, in
which PP' represents the
celestial axis, EQ the celestial
equator, SN the horizon,
and ab, cN, de, fg, Sh,
kl, diurnal circles. O is the
point of observation, Z the
zenith, and Z' the nadir.
.if h
.il fn=fig009.png w=50% alt='Circles'
.ca Fig. 9.
.if-
.if t
[Illustration: Fig. 9.]
.if-
When the observer is south
of the equator, as at O in
Fig. 9, the south pole is
elevated, the north pole depressed,
and the diurnal circles
are oblique to the horizon, leaning to the north. When
the diurnal circles are oblique to the horizon, as in Figs. 8
and 9, the celestial sphere is called an oblique sphere.
When the observer is at the equator, as in Fig. 10, the
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poles of the heavens are on the horizon, and the diurnal
circles are perpendicular to the horizon.
When the observer is at one of the poles, as in Fig. 11,
the poles of the heavens
are in the zenith and the
nadir, and the diurnal circles
are parallel with the
horizon.
.if h
.il fn=fig010.png w=50% alt='Circles'
.ca Fig. 10.
.if-
.if t
[Illustration: Fig. 10.]
.if-
.if h
.il fn=fig011.png w=50% alt='Circles'
.ca Fig. 11.
.if-
.if t
[Illustration: Fig. 11.]
.if-
6. Elevation of the Pole
and of the Equinoctial.--At
the equator the poles
of the heavens lie on the
horizon, and the celestial
equator passes through the
zenith. As a person moves
north from the equator, his
zenith moves north from the celestial equator, and his horizon
moves down from the north pole, and up from the south
pole. The distance of the zenith from the equinoctial, and
of the horizon from the celestial
poles, will always be equal
to the distance of the observer
from the equator. In other
words, the elevation of the
pole is equal to the latitude
of the place. In Fig. 12, O
is the point of observation,
Z the zenith, and SN the
horizon. NP, the elevation
of the pole, is equal to ZE,
the distance of the zenith
from the equinoctial, and to
the distance of O from the equator, or the latitude of the
place.
Two angles, or two arcs, which together equal 90°, are
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said to be complements of each other. ZE and ES in
Fig. 12 are together equal to 90°: hence they are complements
of each other. ZE is equal to the latitude of the
place, and ES is the elevation
of the equinoctial above
the horizon: hence the elevation
of the equinoctial is
equal to the complement of
the latitude of the place.
.if h
.il fn=fig012.png w=50% alt='Circles'
.ca Fig. 12.
.if-
.if t
[Illustration: Fig. 12.]
.if-
Were the observer south
of the equator, the zenith
would be south of the equinoctial,
and the south pole
of the heavens would be the
elevated pole.
.if h
.il fn=fig013.png w=50% alt='Circles'
.ca Fig. 13.
.if-
.if t
[Illustration: Fig. 13.]
.if-
7. Four Sets of Stars.--At most points of observation
there are four sets of stars. These four sets are shown in
Fig. 13.
(1) The stars in
the neighborhood of
the elevated pole
never set. It will
be seen from Fig.
13, that if the distance
of a star from
the elevated pole
does not exceed the
elevation of the pole,
or the latitude of
the place, its diurnal
circle will be wholly
above the horizon.
As the observer approaches the equator, the elevation of
the pole becomes less and less, and the belt of circumpolar
stars becomes narrower and narrower: at the equator it
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disappears entirely. As the observer approaches the pole,
the elevation of the pole increases, and the belt of circumpolar
stars becomes broader and broader, until at the pole
it includes half of the heavens. At the poles, no stars rise
or set, and only half of the stars are ever seen at all.
(2) The stars in the neighborhood of the depressed pole
never rise. The breadth of this belt also increases as the
observer approaches the pole, and decreases as he approaches
the equator, to vanish entirely when he reaches the equator.
The distance from the depressed pole to the margin of this
belt is always equal to the latitude of the place.
(3) The stars in the neighborhood of the equinoctial, on
the side of the elevated pole, set, but are above the horizon
longer than they are below it. This belt of stars extends
from the equinoctial to a point whose distance from the
elevated pole is equal to the latitude of the place: in other
words, the breadth of this third belt of stars is equal to
the complement of the latitude of the place. Hence this
belt of stars becomes broader and broader as the observer
approaches the equator, and narrower and narrower as he
approaches the pole. However, as the observer approaches
the equator, the horizon comes nearer and nearer the celestial
axis, and the time a star is below the horizon becomes
more nearly equal to the time it is above it. As the observer
approaches the pole, the horizon moves farther and farther
from the axis, and the time any star of this belt is
below the horizon becomes more and more unequal to the
time it is above it. The farther any star of this belt is from
the equinoctial, the longer the time it is above the horizon,
and the shorter the time it is below it.
(4) The stars which are in the neighborhood of the
equinoctial, on the side of the depressed pole, rise, but are
below the horizon longer than they are above it. The width
of this belt is also equal to the complement of the latitude
of the place. The farther any star of this belt is from the
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equinoctial, the longer time it is below the horizon, and the
shorter time it is above it; and, the farther the place from
the equator, the longer every star of this belt is below the
horizon, and the shorter the time it is above it.
At the equator every star
is above the horizon just
half of the time; and any
star on the equinoctial is
above the horizon just half
of the time in every part of
the earth, since the equinoctial
and horizon, being great
circles, bisect each other.
8. Vertical Circles.--Great
circles perpendicular
to the horizon are called vertical
circles. All vertical circles pass through the zenith and
nadir. A number of these circles are shown in Fig. 14,
in which SENW represents the horizon, and Z the zenith.
.if h
.il fn=fig014.png w=50% alt='Circles'
.ca Fig. 14.
.if-
.if t
[Illustration: Fig. 14.]
.if-
The vertical circle which
passes through the north and
south points of the horizon
is called the meridian; and
the one which passes through
the east and west points, the
prime vertical. These two
circles are shown in Fig. 15;
SZN being the meridian,
and EZW the prime vertical.
These two circles are
at right angles to each other,
or 90° apart; and consequently they divide the horizon into
four quadrants.
.if h
.il fn=fig015.png w=50% alt='Circles'
.ca Fig. 15.
.if-
.if t
[Illustration: Fig. 15.]
.if-
9. Altitude and Zenith Distance.--The altitude of a
heavenly body is its distance above the horizon, and its
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zenith distance is its distance from the zenith. Both the
altitude and the zenith distance of a body are measured on
the vertical circle which passes through the body. The altitude
and zenith distance of a heavenly body are complements
of each other.
10. Azimuth and Amplitude.--Azimuth is distance measured
east or west from the meridian. When a heavenly
body lies north of the prime vertical, its azimuth is measured
from the meridian on the north; and, when it lies
south of the prime vertical, its azimuth is measured from the
meridian on the south. The azimuth of a body can, therefore,
never exceed 90°. The azimuth of a body is the angle
which the plane of the vertical circle passing through it
makes with that of the meridian.
The amplitude of a body is its distance measured north
or south from the prime vertical. The amplitude and azimuth
of a body are complements of each other.
11. Alt-azimuth Instrument.--An instrument for measuring
the altitude and azimuth of a heavenly body is called
an alt-azimuth instrument. One form of this instrument is
shown in Fig. 16. It consists essentially of a telescope
mounted on a vertical circle, and capable of turning on a
horizontal axis, which, in turn, is mounted on the vertical
axis of a horizontal circle. Both the horizontal and the
vertical circles are graduated, and the horizontal circle is
placed exactly parallel with the plane of the horizon.
When the instrument is properly adjusted, the axis of the
telescope will describe a vertical circle when the telescope
is turned on the horizontal axis, no matter to what part of
the heavens it has been pointed.
The horizontal and vertical axes carry each a pointer.
These pointers move over the graduated circles, and mark
how far each axis turns.
To find the azimuth of a star, the instrument is turned
on its vertical axis till its vertical circle is brought into the
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plane of the meridian, and the reading of the horizontal
circle noted. The telescope is then directed to the star by
turning it on both its vertical and horizontal axes. The
reading of the horizontal circle is again noted. The difference
between these two readings of the horizontal circle
will be the azimuth of the star.
.if h
.il fn=fig016.png w=50% alt='Telescope'
.ca Fig. 16.
.if-
.if t
[Illustration: Fig. 16.]
.if-
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To find the altitude of a star, the reading of the vertical
circle is first ascertained when the telescope is pointed horizontally,
and again when the telescope is pointed at the star.
The difference between these two readings of the vertical
circle will be the altitude of the star.
12. The Vernier.--To enable the observer to read the
fractions of the divisions on the circles, a device called a
vernier is often employed. It consists of a short, graduated
arc, attached to the end of an arm c (Fig. 17), which is
carried by the axis, and turns with the telescope. This arc
is of the length of nine divisions on the circle, and it is
divided into ten equal parts. If 0 of the vernier coincides
with any division, say 6, of the circle, 1 of the vernier will
be 1/10 of a division to
the left of 7, 2 will be
2/10 of a division to the
left of 8, 3 will be 3/10,
of a division to the left
of 9, etc. Hence, when
1 coincides with 7, 0
will be at 6-1/10; when 2
coincides with 8, 0 will
be at 6-2/10; when 3 coincides with 9, 0 will be at 6-3/10, etc.
.if h
.il fn=fig017.png w=50% alt='Vernier'
.ca Fig. 17.
.if-
.if t
[Illustration: Fig. 17.]
.if-
To ascertain the reading of the circle by means of the
vernier, we first notice the zero line. If it exactly coincides
with any division of the circle, the number of that
division will be the reading of the circle. If there is not
an exact coincidence of the zero line with any division of
the circle, we run the eye along the vernier, and note which
of its divisions does coincide with a division of the circle.
The reading of the circle will then be the number of the
first division on the circle behind the 0 of the vernier, and
a number of tenths equal to the number of the division of
the vernier, which coincides with a division of the circle.
For instance, suppose 0 of the vernier beyond 6 of the
// File: psp_016.png
.pn +1
circle, and 7 of the vernier to coincide with 13 of the
circle. The reading of the circle will then be 6-7/10.
13. Hour Circles.--Great circles perpendicular to the
celestial equator are called hour circles. These circles all
pass through the poles of
the heavens, as shown in
Fig. 18. EQ is the celestial
equator, and P and
P' are the poles of the
heavens.
The point A on the
equinoctial (Fig. 19) is
called the vernal equinox,
or the first point of Aries.
The hour circle, APP',
which passes through it, is
called the equinoctial colure.
.if h
.il fn=fig018.png w=50% alt='Circles'
.ca Fig. 18.
.if-
.if t
[Illustration: Fig. 18.]
.if-
14. Declination and Right Ascension.--The declination
of a heavenly body is its distance north or south of the
celestial equator. The polar
distance of a heavenly body
is its distance from the nearer
pole. Declination and polar
distance are measured on
hour circles, and for the same
heavenly body they are complements
of each other.
.if h
.il fn=fig019.png w=50% alt='Circles'
.ca Fig. 19.
.if-
.if t
[Illustration: Fig. 19.]
.if-
The right ascension of a
heavenly body is its distance
eastward from the first point
of Aries, measured from the
equinoctial colure. It is equal to the arc of the celestial
equator included between the first point of Aries and the
hour circle which passes through the heavenly body. As
right ascension is measured eastward entirely around the
// File: psp_017.png
.pn +1
celestial sphere, it may have any value from 0° up to 360°.
Right ascension corresponds to longitude on the earth, and
declination to latitude.
15. The Meridian Circle.--The right ascension and
declination of a heavenly body are ascertained by means of
an instrument called the meridian circle, or transit instrument.
A side-view of this instrument is shown in Fig. 20.
.if h
.il fn=fig020.png w=50% alt='Telescope'
.ca Fig. 20.
.if-
.if t
[Illustration: Fig. 20.]
.if-
It consists essentially of a telescope mounted between two
piers, so as to turn in the plane of the meridian, and carrying
a graduated circle. The readings of this circle are
ascertained by means of fixed microscopes, under which it
turns. A heavenly body can be observed with this instrument,
only when it is crossing the meridian. For this reason
it is often called the transit circle.
To find the declination of a star with this instrument, we
// File: psp_018.png
.pn +1
first ascertain the reading of the circle when the telescope
is pointed to the pole, and then the reading of the circle
when pointed to the star on its passage across the meridian.
The difference between these two readings will be the polar
distance of the star, and the complement of them the declination
of the star.
To ascertain the reading of the circle when the telescope
is pointed to the pole, we must select one of the circumpolar
stars near the pole, and then point the telescope to
it when it crosses the meridian, both above and below the
pole, and note the reading of the circle in each case. The
mean of these two readings will be the reading of the circle
when the telescope is pointed to the pole.
16. Astronomical Clock.--An astronomical clock, or sidereal
clock as it is often called, is a clock arranged so as
to mark hours from 1 to 24, instead of from 1 to 12, as in
the case of an ordinary clock, and so adjusted as to mark
0 when the vernal equinox, or first point of Aries, is on the
meridian.
As the first point of Aries makes a complete circuit of
the heavens in twenty-four hours, it must move at the rate
of 15° an hour, or of 1° in four minutes: hence, when the
astronomical clock marks 1, the first point of Aries must be
15° west of the meridian, and when it marks 2, 30° west of
the meridian, etc. That is to say, by observing an accurate
astronomical clock, one can always tell how far the meridian
at any time is from the first point of Aries.
17. How to find Right Ascension with the Meridian
Circle.--To find the right ascension of a heavenly body,
we have merely to ascertain the exact time, by the astronomical
clock, at which the body crosses the meridian. If
a star crosses the meridian at 1 hour 20 minutes by the
astronomical clock, its right ascension must be 19°; if at
20 hours, its right ascension must be 300°.
To enable the observer to ascertain with great exactness
// File: psp_019.png
.pn +1
the time at which a star crosses the meridian, a number of
equidistant and parallel spider-lines
are stretched across the
focus of the telescope, as shown in
Fig. 21. The observer notes the
time when the star crosses each
spider-line; and the mean of all
of these times will be the time
when the star crosses the meridian.
The mean of several observations
is likely to be more nearly
exact than any single observation.
.if h
.il fn=fig021.png w=50% alt='Telescope focus'
.ca Fig. 21.
.if-
.if t
[Illustration: Fig. 21.]
.if-
.if h
.il fn=fig022.png w=50% alt='Telescope'
.ca Fig. 22.
.if-
.if t
[Illustration: Fig. 22.]
.if-
18. The Equatorial Telescope.--The equatorial telescope
is mounted on
two axes,--one parallel
with the axis of
the earth, and the
other at right angles
to this, and therefore
parallel with the plane
of the earth's equator.
The former is called
the polar axis, and
the latter the declination
axis. Each axis
carries a graduated
circle. These circles
are called respectively
the hour circle and
the declination circle.
The telescope is attached
directly to
the declination axis.
When the telescope is
fixed in any declination, and then turned on its polar axis,
// File: psp_020.png
.pn +1
the line of sight will describe a diurnal circle; so that, when
the tube is once directed to a star, it can be made to follow
the star by simply turning the telescope on its polar axis.
In the case of large instruments of this class, the polar
axis is usually turned by clock-work at the rate at which the
heavens rotate; so that, when the telescope has once been
pointed to a planet or other heavenly body, it will continue to
follow the body and keep it steadily in the field of view without
further trouble on the part of the observer.
The great Washington Equatorial is shown in Fig. 22. Its
object-glass is 26 inches in diameter, and its focal length is
32-1/2 feet. It was constructed by Alvan Clark & Sons of Cambridge,
Mass. It is one of the three largest refracting telescopes
at present in use. The Newall refractor at Gateshead,
Eng., has an objective 25 inches in diameter, and a focal length
of 29 feet. The great refractor at Vienna has an objective
27 inches in diameter. There are several large refractors now
in process of construction.
.if h
.il fn=fig023.png w=70% alt='Wire Micrometer'
.ca Fig. 23.
.if-
.if t
[Illustration: Fig. 23.]
.if-
19. The Wire Micrometer.--Large arcs in the heavens
are measured by means of the graduated circles attached to
the axes of the
telescopes; but
small arcs within
the field of view
of the telescope
are measured by means of instruments called micrometers,
mounted in the focus of the telescope. One of the most
convenient of these micrometers is that known as the wire
micrometer, and shown in Fig. 23.
The frame AA covers two slides, C and D. These slides
are moved by the screws F and G. The wires E and B
are stretched across the ends of the slides so as to be
parallel to each other. On turning the screws F and G
one way, these wires are carried apart; and on turning them
the other way they are brought together again. Sometimes
two parallel wires, x and y, shown in the diagram at the
// File: psp_021.png
.pn +1
right, are stretched across the frame at right angles to the
wires E, B. We may call the wires x and y the longitudinal
wires of the micrometer, and E and B the transverse
wires. Many instruments have only one longitudinal wire,
which is stretched across the middle of the focus. The
longitudinal wires are just in front of the transverse wires,
but do not touch them.
To find the distance between any two points in the field
of view with a micrometer, with a single longitudinal wire,
turn the frame till the longitudinal wire passes through the
two points; then set the wires E and B one on each
point, turn one of the screws, known as the micrometer
screw, till the two wires are brought together, and note the
number of times the screw is turned. Having previously
ascertained over what arc one turn of the screw will move
the wire, the number of turns will enable us to find the
length of the arc between the two points.
The threads of the micrometer screw are cut with great
accuracy; and the screw is provided with a large head, which
is divided into a hundred or more equal parts.
These divisions, by means of a fixed pointer, enable us to
ascertain what fraction of a turn the screw has made over
and above its complete revolutions.
20. Reflecting Telescopes.--It is possible to construct
mirrors of much larger size than lenses: hence reflecting
telescopes have an advantage over refracting telescopes as
regards size of aperture and of light-gathering power. They
are, however, inferior as regards definition; and, in order
to prevent flexure, it is necessary to give the speculum, or
mirror, a massiveness which makes the telescope unwieldy.
It is also necessary frequently to repolish the speculum;
and this is an operation of great delicacy, as the slightest
change in the form of the surface impairs the definition of
the image. These defects have been remedied, to a certain
extent, by the introduction of silver-on-glass mirrors; that is,
// File: psp_022.png
.pn +1
glass mirrors covered in front with a thin coating of silver.
Glass is only one-third as heavy as speculum-metal, and silver
is much superior to that metal in reflecting power; and
when the silver becomes tarnished, it can be removed and
renewed without danger of changing the form of the glass.
The Herschelian Reflector.--In this form of telescope the
mirror is slightly tipped, so that the image, instead of being
formed in the centre of the tube, is formed near one side
of it, as in Fig. 24. The observer can then view it without
putting his head inside the tube, and therefore without
cutting off any material portion of the light. In observation,
he must stand at the upper or outer end of the tube,
and look into it, his back being turned towards the object.
From his looking
directly into the
mirror, it is also
sometimes called
the front-view
telescope. The
great disadvantage
of this arrangement
is, that the rays cannot be brought to an exact
focus when they are thrown so far to one side of the axis,
and the injury to the definition is so great that the front-view
plan is now entirely abandoned.
.if h
.il fn=fig024.png w=80% alt='Reflecting Telescope'
.ca Fig. 24.
.if-
.if t
[Illustration: Fig. 24.]
.if-
The Newtonian Reflector.--The plan proposed by Sir
Isaac Newton was to place a small plane mirror just inside
the focus, inclined to the telescope at an angle of 45°, so
as to throw the rays to the side of the tube, where they
come to a focus, and form the image. An opening is made
in the side of the tube, just below where the image is
formed; and in this opening the eye-piece is inserted. The
small mirror cuts off some of the light, but not enough to
be a serious defect. An improvement which lessens this
defect has been made by Professor Henry Draper. The
// File: psp_023.png
.pn +1
inclined mirror is replaced by a small rectangular prism
(Fig. 25), by reflection from which the image is formed
very near the prism. A pair of lenses are then inserted in
the course of the rays, by which a second image is formed
at the opening in the side of the tube; and this second
image is viewed by an ordinary eye-piece.
.if h
.il fn=fig025.png w=90% alt='Reflecting Telescope'
.ca Fig. 25.
.if-
.if t
[Illustration: Fig. 25.]
.if-
The Gregorian Reflector.--This is a form proposed by
James Gregory, who probably preceded Newton as an inventor
of the reflecting telescope. Behind the focus, F
(Fig. 26), a small concave mirror, R, is placed, by which
the light is reflected back again down the tube. The larger
mirror, M, has an opening through its centre; and the small
mirror, R, is so adjusted as to form a second image of the
object in this opening. This image is then viewed by an
eye-piece which is screwed into the opening.
.if h
.il fn=fig026.png w=90% alt='Reflecting Telescope'
.ca Fig. 26.
.if-
.if t
[Illustration: Fig. 26.]
.if-
The Cassegrainian Reflector.--In principle this is the
same with the Gregorian; but the small mirror, R, is convex,
and is placed inside the focus, F, so that the rays are
reflected from it before reaching the focus, and no image
is formed until they reach the opening in the large mirror.
This form has an advantage over the Gregorian, in that the
// File: psp_024.png
.pn +1
telescope may be made shorter, and the small mirror can
be more easily shaped to the required figure. It has, therefore,
entirely superseded the original Gregorian form.
.if h
.il fn=fig027.png w=80% alt='Reflecting Telescope'
.ca Fig. 27.
.if-
.if t
[Illustration: Fig. 27.]
.if-
Optically these forms of telescope are inferior to the
Newtonian; but the latter is subject to the inconvenience,
that the observer must be stationed at the upper end of
the telescope, where he looks into an eye-piece screwed
into the side of the tube.
// File: psp_025.png
.pn +1
On the other hand, the Cassegrainian Telescope is pointed
directly at the object to be viewed, like a refractor; and the
observer stands at the lower end, and looks in at the opening
through the large mirror. This is, therefore, the most
convenient form of all in management.
.if h
.il fn=fig028.png w=80% alt='Reflecting Telescope'
.ca Fig. 28.
.if-
.if t
[Illustration: Fig. 28.]
.if-
The largest reflecting telescope yet constructed is that of
Lord Rosse, at Parsonstown, Ireland. Its speculum is 6 feet
in diameter, and its focal length 55 feet. It is commonly used
as a Newtonian. This telescope is shown in Fig. 27.
The great telescope of the Melbourne Observatory, Australia,
is a Cassegranian reflector. Its speculum is 4 feet in
// File: psp_026.png
.pn +1
diameter, and its focal length is 32 feet. It is shown in
Fig. 28.
.if h
.il fn=fig029.png w=80% alt='Reflecting Telescope'
.ca Fig. 29.
.if-
.if t
[Illustration: Fig. 29.]
.if-
The great reflector of the Paris Observatory is a Newtonian
reflector. Its mirror of silvered glass is 4 feet in diameter,
and its focal length is 23 feet. This telescope is shown in
Fig. 29.
21. The Sun's Motion among the Stars.--If we notice
// File: psp_027.png
.pn +1
the stars at the same hour night after night, we shall find
that the constellations are steadily advancing towards the
west. New constellations are continually appearing in the
east, and old ones disappearing in the west. This continual
advancing of the heavens towards the west is due to the fact
that the sun's place among the stars is continually moving
towards the east. The sun completes the circuit of the
heavens in a year, and is therefore moving eastward at the
rate of about a degree a day.
.if h
.il fn=fig030.png w=50% alt='Orbits'
.ca Fig. 30.
.if-
.if t
[Illustration: Fig. 30.]
.if-
This motion of the sun's place among the stars is due
to the revolution of
the earth around the
sun, and not to any
real motion of the
sun. In Fig. 30 suppose
the inner circle
to represent the orbit
of the earth around
the sun, and the outer
circle to represent the
celestial sphere. When
the earth is at E, the
sun's place on the
celestial sphere is at
S'. As the earth moves in the direction EF, the sun's
place on the celestial sphere must move in the direction
S'T: hence the revolution of the earth around the sun
would cause the sun's place among the stars to move around
the heavens in the same direction that the earth is moving
around the sun.
22. The Ecliptic.--The circle described by the sun in
its apparent motion around the heavens is called the ecliptic.
The plane of this circle passes through the centre of the
earth, and therefore through the centre of the celestial
sphere; the earth being so small, compared with the celestial
// File: psp_028.png
.pn +1
sphere, that it practically makes no difference whether
we consider a point on its surface, or one at its centre, as
the centre of the celestial sphere. The ecliptic is, therefore,
a great circle.
The earth's orbit lies in the plane of the ecliptic; but it
extends only an inappreciable distance from the sun towards
the celestial sphere.
.if h
.il fn=fig031.png w=50% alt='Tub'
.ca Fig. 31.
.if-
.if t
[Illustration: Fig. 31.]
.if-
23. The Obliquity of the
Ecliptic.--The ecliptic is inclined
to the celestial equator
by an angle of about 23-1/2°.
This inclination is called the
obliquity of the ecliptic. The
obliquity of the ecliptic is due
to the deviation of the earth's axis from a perpendicular to
the plane of its orbit. The axis of a rotating body tends
to maintain the same direction; and, as the earth revolves
around the sun, its axis points all the time in nearly the
same direction. The earth's axis deviates about 23-1/2° from
the perpendicular to its orbit; and, as the earth's equator is
at right angles to its axis, it
will deviate about 23-1/2° from
the plane of the ecliptic.
The celestial equator has the
same direction as the terrestrial
equator, since the axis
of the heavens has the same
direction as the axis of the
earth.
.if h
.il fn=fig032.png w=50% alt='Tub'
.ca Fig. 32.
.if-
.if t
[Illustration: Fig. 32.]
.if-
Suppose the globe at the centre of the tub (Fig. 31) to
represent the sun, and the smaller globes to represent the
earth in various positions in its orbit. The surface of the
water will then represent the plane of the ecliptic, and
the rod projecting from the top of the earth will represent
the earth's axis, which is seen to point all the time in the
// File: psp_029.png
.pn +1
same direction, or to lean the same way. The leaning of
the axis from the perpendicular to the surface of the water
would cause the earth's equator to be inclined the same
amount to the surface of the water, half of the equator being
above, and half of it below,
the surface. Were the axis
of the earth perpendicular
to the surface of the water,
the earth's equator would
coincide with the surface, as
is evident from Fig. 32.
.if h
.il fn=fig033.png w=50% alt='Circles'
.ca Fig. 33.
.if-
.if t
[Illustration: Fig. 33.]
.if-
24. The Equinoxes and
Solstices.--The ecliptic and
celestial equator, being great
circles, bisect each other.
Half of the ecliptic is north,
and half of it is south, of the equator. The points at which
the two circles cross are called the equinoxes. The one at
which the sun crosses the equator from south to north is
called the vernal equinox, and the one at which it crosses
from north to south the
autumnal equinox. The
points on the ecliptic midway
between the equinoxes
are called the solstices. The
one north of the equator is
called the summer solstice,
and the one south of the
equator the winter solstice.
In Fig. 33, EQ is the celestial
equator, EcE'c' the
ecliptic, V the vernal equinox, A the autumnal equinox, Ec
the winter solstice, and E'c' the summer solstice.
.if h
.il fn=fig034.png w=50% alt='Circles'
.ca Fig. 34.
.if-
.if t
[Illustration: Fig. 34.]
.if-
25. The Inclination of the Ecliptic to the Horizon.--Since
the celestial equator is perpendicular to the axis of
// File: psp_030.png
.pn +1
the heavens, it makes the same angle with it on every side:
hence, at any place, the equator makes always the same
angle with the horizon, whatever part of it is above the horizon.
But, as the ecliptic is oblique to the equator, it makes
different angles with the celestial axis on different sides;
and hence, at any place, the angle which the ecliptic makes
with the horizon varies according to the part which is above
the horizon. The two extreme angles for a place more than
23-1/2° north of the equator are shown in Figs. 34 and 35.
The least angle is formed when the vernal equinox is on
the eastern horizon, the autumnal on the western horizon,
and the winter solstice on the meridian, as in Fig. 34. The
angle which the ecliptic then
makes with the horizon is
equal to the elevation of the
equinoctial minus 23-1/2°. In
the latitude of New York this
angle = 49° - 23-1/2° = 25-1/2°.
.if h
.il fn=fig035.png w=50% alt='Circles'
.ca Fig. 35.
.if-
.if t
[Illustration: Fig. 35.]
.if-
The greatest angle is
formed when the autumnal
equinox is on the eastern
horizon, the vernal on the
western horizon, and the
summer solstice is on the meridian (Fig. 35). The angle
between the ecliptic and the horizon is then equal to the
elevation of the equinoctial plus 23-1/2°. In the latitude of
New York this angle = 49° + 23-1/2° = 72-1/2°.
Of course the equinoxes, the solstices, and all other points
on the ecliptic, describe diurnal circles, like every other
point in the heavens: hence, in our latitude, these points
rise and set every day.
26. Celestial Latitude and Longitude.--Celestial latitude
is distance measured north or south from the ecliptic;
and celestial longitude is distance measured on the ecliptic
eastward from the vernal equinox, or the first point of
// File: psp_031.png
.pn +1
Aries. Great circles perpendicular to the ecliptic are called
celestial meridians. These circles all pass through the
poles of the ecliptic, which are some 23-1/2° from the poles of
the equinoctial. The latitude of a heavenly body is measured
by the arc of a celestial meridian included between
the body and the ecliptic. The longitude of a heavenly
body is measured by the arc of the ecliptic included between
the first point of Aries and the meridian which passes
through the body. There are, of course, always two arcs
included between the first point of Aries and the meridian,--one
on the east, and the other on the west, of the first
point of Aries. The one on the east is always taken as
the measure of the longitude.
27. The Precession of the
Equinoxes.--The equinoctial
points have a slow westward
motion along the ecliptic.
This motion is at the
rate of about 50'' a year, and
would cause the equinoxes
to make a complete circuit
of the heavens in a period
of about twenty-six thousand
years. It is called the precession of the equinoxes. This
westward motion of the equinoxes is due to the fact that
the axis of the earth has a slow gyratory motion, like the
handle of a spinning-top which has begun to wabble a little.
This gyratory motion causes the axis of the heavens to
describe a cone in about twenty-six thousand years, and the
pole of the heavens to describe a circle about the pole of
the ecliptic in the same time. The radius of this circle
is 23-1/2°.
.if h
.il fn=fig036.png w=50% alt='Circles'
.ca Fig. 36.
.if-
.if t
[Illustration: Fig. 36.]
.if-
.pm letter-start
28. Illustration of Precession.--The precession of the
equinoxes may be illustrated by means of the apparatus shown
in Fig. 36. The horizontal and stationary ring EC represents
// File: psp_032.png
.pn +1
the ecliptic; the oblique ring E'Q represents the equator;
V and A represent the equinoctial point, and E and C the
solstitial points; B represents the pole of the ecliptic, P the
pole of the equator, and PO the celestial axis. The ring E'Q
is supported on a pivot at O; and the rod BP, which connects
B and P, is jointed at each end so as to admit of the movement
of P and B.
On carrying P around B, we shall see that E'Q will always
preserve the same obliquity to EC, and that the points V and A
will move around the circle EC. The same will also be true
of the points E and C.
.pm letter-end
29. Effects of Precession.--One effect of precession, as
has already been stated, is the revolution of the pole of the
heavens around the pole of the ecliptic in a period of about
twenty-six thousand years. The circle described by the
pole of the heavens, and the position of the pole at various
dates, are shown in Fig. 37, where o indicates the position
of the pole at the birth of Christ. The numbers round
the circle to the left of o are dates A.D., and those to the
right of o are dates B.C. It will be seen that the star at
the end of the Little Bear's tail, which is now near the
north pole, will be exactly at the pole about the year 2000.
It will then recede farther and farther from the pole till the
year 15000 A.D., when it will be about forty-seven degrees
away from the pole. It will be noticed that one of the stars
of the Dragon was the pole star about 2800 years B.C.
There are reasons to suppose that this was about the time
of the building of the Great Pyramid.
A second effect of precession is the shifting of the signs
along the zodiac. The zodiac is a belt of the heavens
along the ecliptic, extending eight degrees from it on each
side. This belt is occupied by twelve constellations, known
as the zodiacal constellations. They are Aries, Taurus,
Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius,
Capricornus, Aquarius, and Pisces. The zodiac is also
// File: psp_033.png
.pn +1
divided into twelve equal parts of thirty degrees each, called
signs. These signs have the same names as the twelve
zodiacal constellations, and when they were first named,
each sign occupied the same part of the zodiac as the corresponding
constellation; that is to say, the sign Aries was
in the constellation Aries, and the sign Taurus in the constellation
Taurus, etc. Now the signs are always reckoned
as beginning at the vernal equinox, which is continually
shifting along the ecliptic; so that the signs are continually
moving along the zodiac, while the constellations remain
stationary: hence it has come about that the first point of
Aries (the sign) is no longer in the constellation Aries, but
in Pisces.
.if h
.il fn=fig037.png w=80% alt='Zodiac'
.ca Fig. 37.
.if-
.if t
[Illustration: Fig. 37.]
.if-
// File: psp_034.png
.pn +1
Fig. 38 shows the position of the vernal equinox 2170
B.C. It was then in Taurus, just south of the Pleiades.
It has since moved from Taurus, through Aries, and into
Pisces, as shown in Fig. 39.
.if h
.il fn=fig038.png w=80% alt='Zodiac'
.ca Fig. 38.
.if-
.if t
[Illustration: Fig. 38.]
.if-
.if h
.il fn=fig039.png w=80% alt='Zodiac'
.ca Fig. 39.
.if-
.if t
[Illustration: Fig. 39.]
.if-
Since celestial longitude and right ascension are both
measured from the first point of Aries, the longitude and
right ascension of the stars are slowly changing from year
to year. It will be seen, from Figs. 38 and 39, that the
declination is also slowly changing.
30. Nutation.--The gyratory motion of the earth's axis
is not perfectly regular and uniform. The earth's axis has
// File: psp_035.png
.pn +1
a slight tremulous motion, oscillating to and fro through a
short distance once in about nineteen years. This tremulous
motion of the axis causes the pole of the heavens to
describe an undulating curve, as shown in Fig. 40, and
gives a slight unevenness to the motion of the equinoxes
along the ecliptic. This nodding motion of the axis is
called nutation.
.if h
.il fn=fig040.png w=50% alt='Curve'
.ca Fig. 40.
.if-
.if t
[Illustration: Fig. 40.]
.if-
31. Refraction.--When a ray of light
from one of the heavenly bodies enters the
earth's atmosphere obliquely, it will be bent
towards a perpendicular to the surface of
the atmosphere, since it will be entering a
denser medium. As the ray traverses the
atmosphere, it will be continually passing into denser and
denser layers, and will therefore be bent more and more
towards the perpendicular. This bending of the ray is
shown in Fig. 41. A ray which started from A would enter
the eye at C, as if it came from I: hence a star at A would
appear to be at I.
.if h
.il fn=fig041.png w=60% alt='Rays'
.ca Fig. 41.
.if-
.if t
[Illustration: Fig. 41.]
.if-
Atmospheric refraction displaces all the heavenly bodies
from the horizon towards the
zenith. This is evident from
Fig. 42. OD is the horizon,
and Z the zenith, of an observer
at O. Refraction would make
a star at Q appear at P: in
other words, it would displace
it towards the zenith. A star in
the zenith is not displaced by
refraction, since the rays which reach the eye from it traverse
the atmosphere vertically. The farther a star is from the
zenith, the more it is displaced by refraction, since the
greater is the obliquity with which the rays from it enter
the atmosphere.
.if h
.il fn=fig042.png w=60% alt='Rays'
.ca Fig. 42.
.if-
.if t
[Illustration: Fig. 42.]
.if-
At the horizon the displacement by refraction is about
// File: psp_036.png
.pn +1
half a degree; but it varies considerably with the state of
the atmosphere. Refraction causes a heavenly body to
appear above the horizon longer than it really is above it,
since it makes it appear to be on the horizon when it is
really half a degree below it.
The increase of refraction towards the horizon often
makes the sun, when near the horizon, appear distorted,
// File: psp_037.png
.pn +1
the lower limb of the sun being raised more than the upper
limb. This distortion is shown in Fig. 43. The vertical
diameter of the sun appears to be considerably less than
the horizontal diameter.
.if h
.il fn=fig043.png w=70% alt='Horizon'
.ca Fig. 43.
.if-
.if t
[Illustration: Fig. 43.]
.if-
32. Parallax.--Parallax is the displacement of an
object caused by a change in the point of view from which
it is seen. Thus in Fig. 44, the top of the tower S would
be seen projected against the sky at a by an observer at A,
and at b by an observer at B. In passing from A to B,
the top of the tower is displaced from a to b, or by the
angle aSb. This angle is called the parallax of S, as seen
from B instead of A.
.if h
.il fn=fig044.png w=80% alt='Parallax'
.ca Fig. 44.
.if-
.if t
[Illustration: Fig. 44.]
.if-
The geocentric parallax of a heavenly body is its displacement
caused by its being seen from the surface of the
earth, instead of from the centre of the earth. In Fig. 45,
R is the centre of the earth, and O the point of observation
on the surface of the earth. Stars at S, S', and S'', would,
from the centre of the earth, appear at Q, Q', and Q'';
while from the point O on the surface of the earth, these
same stars would appear at P, P' and P'', being displaced
// File: psp_038.png
.pn +1
from their position, as seen from the centre of the earth, by
the angles QSP, Q'S'P', and Q''S''P''. It will be seen
that parallax displaces a body from the zenith towards the
horizon, and that the parallax of a body is greatest when it
is on the horizon. The parallax of a heavenly body when
on the horizon is called its horizontal parallax. A body
in the zenith is not displaced by parallax, since it would
be seen in the same direction from both the centre and
the surface of the
earth.
.if h
.il fn=fig045.png w=80% alt='Parallax'
.ca Fig. 45.
.if-
.if t
[Illustration: Fig. 45.]
.if-
The parallax of
a body at S''' is
Q'''S'''P, which is
seen to be greater
than QSP; that
is to say, the parallax
of a heavenly
body increases
with its nearness
to the earth. The
distance and parallax of a body are so related, that, either
being known, the other may be computed.
.pm letter-start
33. Aberration.--Aberration is a slight displacement of a
star, owing to an apparent change in the direction of the rays
of light which proceed from it, caused by the motion of the
earth in its orbit. If we walk rapidly in any direction in the
rain, when the drops are falling vertically, they will appear to
come into our faces from the direction in which we are walking.
Our own motion has apparently changed the direction in which
the drops are falling.
.if h
.il fn=fig046.png w=60% alt='Gun'
.ca Fig. 46.
.if-
.if t
[Illustration: Fig. 46.]
.if-
In Fig. 46 let A be a gun of a battery, from which a shot
is fired at a ship, DE, that is passing. Let ABC be the
course of the shot. The shot enters the ship's side at B, and
passes out at the other side at C; but in the mean time the
ship has moved from E to e, and the part B, where the shot
entered, has been carried to b. If a person on board the ship
// File: psp_039.png
.pn +1
could see the ball as it crossed the ship, he would see it cross
in the diagonal line bC; and he would at once say that the
cannon was in the direction of Cb. If the ship were moving
in the opposite direction, he would
say that the cannon was just as far
the other side of its true position.
Now, we see a star in the direction
in which the light coming from the
star appears to be moving. When
we examine a star with a telescope,
we are in the same condition as the
person who on shipboard saw the
cannon-ball cross the ship. The telescope
is carried along by the earth
at the rate of eighteen miles a second: hence the light will
appear to pass through the tube in a slightly different direction
from that in which it is really moving: just as the cannon-ball
appears to pass through the ship in a different direction
from that in which it is really moving. Thus in
Fig. 47, a ray of light coming in the direction SOT
would appear to traverse the tube OT of a telescope,
moving in the direction of the arrow, as if it were
coming in the direction S'O.
.pm letter-end
.if h
.il fn=fig047.png w=40% alt='Ray'
.ca Fig. 47.
.if-
.if t
[Illustration: Fig. 47.]
.if-
.pm letter-start
As light moves with enormous velocity, it passes
through the tube so quickly, that it is apparently
changed from its true direction only by a very slight
angle: but it is sufficient to displace the star. This
apparent change in the direction of light caused by
the motion of the earth is called aberration of light.
.pm letter-end
34. The Planets.--On watching the stars attentively
night after night, it will be found, that while
the majority of them appear fixed on the surface
of the celestial sphere, so as to maintain their relative
positions, there are a few that wander about
among the stars, alternately advancing towards the
east, halting, and retrograding towards the west. These wandering
stars are called planets.
Their motions appear quite irregular; but, on the whole,
// File: psp_040.png
.pn +1
their eastward motion is in excess of their westward, and in
a longer or shorter time they all complete the circuit of the
heavens. In almost every instance, their paths are found to
lie wholly in the belt of the zodiac.
.if h
.il fn=fig048.png w=60% alt='Planet Path'
.ca Fig. 48.
.if-
.if t
[Illustration: Fig. 48.]
.if-
Fig. 48 shows a portion of the apparent path of one of
the planets.
// File: psp_041.png
.pn +1
.h2 id='solar-system' title='II. The Solar System.'
II. | THE SOLAR SYSTEM.
.sp 2
.h3 id=theory
I. THEORY OF THE SOLAR SYSTEM.
.sp 2
35. Members of the Solar System.--The solar system
is composed of the sun, planets, moons, comets, and meteors.
Five planets, besides the earth, are readily distinguished by
the naked eye, and were known to the ancients: these are
Mercury, Venus, Mars, Jupiter, and Saturn. These, with
the sun and moon, made up the seven planets of the ancients,
from which the seven days of the week were named.
.sp 2
.h4 id='ptolemy'
The Ptolemaic System.
.sp 2
36. The Crystalline Spheres.--We have seen that all the
heavenly bodies appear to be situated on the surface of the
celestial sphere. The ancients assumed that the stars were
really fixed on the surface of a crystalline sphere, and that
they were carried around the earth daily by the rotation of
this sphere. They had, however, learned to distinguish the
planets from the stars, and they had come to the conclusion
that some of the planets were nearer the earth than others,
and that all of them were nearer the earth than the stars
are. This led them to imagine that the heavens were composed
of a number of crystalline spheres, one above another,
each carrying one of the planets, and all revolving around
the earth from east to west, but at different rates. This
structure of the heavens is shown in section in Fig. 49.
.if h
.il fn=fig049.png w=80% alt='Heavenly Spheres'
.ca Fig. 49.
.if-
.if t
[Illustration: Fig. 49.]
.if-
// File: psp_042.png
.pn +1
37. Cycles and Epicycles.--The ancients had also noticed
that, while all the planets move around the heavens from
west to east, their motion is not one of uniform advancement.
Mercury and Venus appear to oscillate to and fro
across the sun, while Jupiter and Saturn appear to oscillate
to and fro across a centre which is moving around the
earth, so as to describe a series of loops, as shown in
Fig. 50.
.if h
.il fn=fig050.png w=80% alt='Planetary Loops'
.ca Fig. 50.
.if-
.if t
[Illustration: Fig. 50.]
.if-
The ancients assumed that the planets moved in exact
circles, and, in fact, that all motion in the heavens was
circular, the circle being the simplest and most perfect
curve. To account for the loops described by the planets,
they imagined that each planet revolved in a circle around a
centre, which, in turn, revolved in a circle around the earth.
The circle described by this centre around the earth they
called the cycle, and the circle described by the planet
around this centre they called the epicycle.
// File: psp_043.png
.pn +1
38. The Eccentric.--The ancients assumed that the
planets moved at a uniform rate in describing the epicycle,
and also the centre in describing the cycle. They had,
however, discovered that the planets advance eastward more
rapidly in some parts of their orbits than in others. To
account for this they assumed that the cycles described by
the centre, around which the planets revolved, were eccentric;
that is to say, that the earth was not at the centre
of the cycle, but some distance away from it, as shown
in Fig. 51. E is the position of the earth, and C is the
// File: psp_044.png
.pn +1
centre of the cycle. The lines from E are drawn so as to
intercept equal arcs of the cycle. It will be seen at once
that the angle between any pair of lines is greatest at P, and
least at A; so that, were a
planet moving at the same rate
at P and A, it would seem to
be moving much faster at P.
The point P of the planet's
cycle was called its perigee, and
the point A its apogee.
.if h
.il fn=fig051.png w=60% alt='Orbits'
.ca Fig. 51.
.if-
.if t
[Illustration: Fig. 51.]
.if-
As the apparent motion of
the planets became more accurately
known, it was found
necessary to make the system
of cycles, epicycles, and eccentrics
exceedingly complicated to represent that motion.
.sp 2
.h4 id='copernicus'
The Copernican System.
.sp 2
39. Copernicus.--Copernicus simplified the Ptolemaic
system greatly by assuming that the earth and all the planets
revolved about the sun as a centre. He, however, still maintained
that all motion in the heavens was circular, and hence
he could not rid his system entirely of cycles and epicycles.
.sp 2
.h4 id='brahe'
Tycho Brahe's System.
.sp 2
40. Tycho Brahe.--Tycho Brahe was the greatest of the
early astronomical observers. He, however, rejected the system
of Copernicus, and adopted one of his own, which was
much more complicated. He held that all the planets but
the earth revolved around the sun, while the sun and moon
revolved around the earth. This system is shown in Fig. 52.
.if h
.il fn=fig052.png w=90% alt='System of Brahe'
.ca Fig. 52.
.if-
.if t
[Illustration: Fig. 52.]
.if-
.sp 2
.h4 id='kepler'
Kepler's System.
.sp 2
41. Kepler.--While Tycho Brahe devoted his life to the
observation of the planets. Kepler gave his to the study
// File: psp_045.png
.pn +1
of Tycho's observations, for the purpose of discovering the
true laws of planetary motion. He banished the complicated
system of cycles, epicycles, and eccentrics forever from
the heavens, and discovered the three laws of planetary
motion which have rendered his name immortal.
42. The Ellipse.--An ellipse is a closed curve which has
two points within it, the sum of whose distances from every
point on the curve is the same. These two points are called
the foci of the ellipse.
.if h
.il fn=fig053.png w=80% alt='Ellipse'
.ca Fig. 53.
.if-
.if t
[Illustration: Fig. 53.]
.if-
One method of describing an ellipse is shown in Fig. 53.
Two tacks, F and F', are stuck into a piece of paper, and
to these are fastened the two ends of a string which is longer
than the distance between the tacks. A pencil is then
placed against the string, and carried around, as shown in
the figure. The curve described by the pencil is an ellipse.
The two points F and F' are the foci of the ellipse: the
sum of the distances of these two points from every point
on the curve is equal to the length of the string. When
half of the ellipse has been described, the pencil must be
// File: psp_046.png
.pn +1
held against the other side of the string in the same way,
and carried around as before.
The point O, half way between F and F', is called the
centre of the ellipse; AA' is the major axis of the ellipse,
and CD is the minor axis.
43. The Eccentricity of the Ellipse.--The ratio of the
distance between the two foci to the major axis of the
ellipse is called the eccentricity of the ellipse. The greater
the distance between the two
foci, compared with the major
axis of the ellipse, the greater is
the eccentricity of the ellipse;
and the less the distance between
the foci, compared with
the length of the major axis,
the less the eccentricity of the
ellipse. The ellipse of Fig. 54
has an eccentricity of 1/8. This
ellipse scarcely differs in appearance
from a circle. The ellipse of Fig. 55 has an eccentricity
of 1/2, and that of Fig. 56 an eccentricity of 7/8.
.if h
.il fn=fig054.png w=60% alt='Eccentric Ellipse'
.ca Fig. 54.
.if-
.if t
[Illustration: Fig. 54.]
.if-
.if h
.il fn=fig055.png w=60% alt='Eccentric Ellipse'
.ca Fig. 55.
.if-
.if t
[Illustration: Fig. 55.]
.if-
.if h
.il fn=fig056.png w=60% alt='Eccentric Ellipse'
.ca Fig. 56.
.if-
.if t
[Illustration: Fig. 56.]
.if-
44. Kepler's First Law.--Kepler first discovered that
all the planets move from west to east in ellipses which have
// File: psp_047.png
.pn +1
the sun as a common focus. This law of planetary motion
is known as Kepler's First Law. The planets appear to
describe loops, because we view them from a moving point.
The ellipses described by the planets differ in eccentricity;
and, though they all have one focus at the sun, their major
axes have different directions. The eccentricity of the planetary
orbits is comparatively
small. The ellipse of Fig. 54
has seven times the eccentricity
of the earth's orbit, and twice
that of the orbit of any of the
larger planets except Mercury;
and its eccentricity is more
than half of that of the orbit
of Mercury. Owing to their
small eccentricity, the orbits of
the planets are usually represented by circles in astronomical
diagrams.
.if h
.il fn=fig057.png w=60% alt='Ellipse'
.ca Fig. 57.
.if-
.if t
[Illustration: Fig. 57.]
.if-
45. Kepler's Second Law.--Kepler next discovered that
a planet's rate of motion in the various parts of its orbit
is such that a line drawn from the planet to the sun would
always sweep over equal areas in equal times. Thus, in
Fig. 57, suppose the planet would move from P to P^1 in the
same time that it would move
from P^2 to P^3, or from P^4 to
P^5; then the dark spaces, which
would be swept over by a line
joining the sun and the planet,
in these equal times, would all
be equal.
A line drawn from the sun to a planet is called the
radius vector of the planet. The radius vector of a planet
is shortest when the planet is nearest the sun, or at perihelion,
and longest when the planet is farthest from the sun,
or at aphelion: hence, in order to have the areas equal, it
// File: psp_048.png
.pn +1
follows that a planet must move fastest when at perihelion,
and slowest at aphelion.
Kepler's Second Law of planetary motion is usually
stated as follows: The radius vector of a planet describes
equal areas in equal times in every part of the planet's
orbit.
46. Kepler's Third Law.--Kepler finally discovered that
the periodic times
of the planets bear
the following relation
to the distances
of the planets
from the sun: The
squares of the periodic
times of the
planets are to each
other as the cubes
of their mean distances
from the sun. This is known as Kepler's Third Law
of planetary motion. By periodic time is meant the time it
takes a planet to revolve around the sun.
These three laws of Kepler's are the foundation of modern
physical astronomy.
.sp 2
.h4 id='newton'
The Newtonian System.
.sp 2
47. Newton's Discovery.--Newton followed Kepler, and
by means of his three laws of planetary motion made his
own immortal discovery of the law of gravitation. This
law is as follows: Every portion of matter in the universe
attracts every other portion with a force varying directly as
the product of the masses acted upon, and inversely as the
square of the distances between them.
48. The Conic Sections.--The conic sections are the
figures formed by the various plane sections of a right cone.
There are four classes of figures formed by these sections,
// File: psp_049.png
.pn +1
according to the angle which the plane of the section
makes with the axis of the cone.
OPQ, Fig. 58, is a right cone, and ON is its axis.
Any section, AB, of this cone, whose plane is perpendicular
to the axis of the cone, is a circle.
.if h
.il fn=fig058.png w=60% alt='Cone'
.ca Fig. 58.
.if-
.if t
[Illustration: Fig. 58.]
.if-
Any section, CD, of this cone, whose plane is oblique
to the axis, but forms with it an angle greater than NOP,
is an ellipse. The less the angle which the plane of the
section makes with the
axis, the more elongated
is the ellipse.
Any section, EF, of
this cone, whose plane
makes with the axis an
angle equal to NOP, is
a parabola. It will be
seen, that, by changing the
obliquity of the plane CD
to the axis NO, we may
pass uninterruptedly from
the circle through ellipses
of greater and greater
elongation to the parabola.
Any section, GH, of
this cone, whose plane
makes with the axis ON
an angle less than NOP, is a hyperbola.
.if h
.il fn=fig059.png w=70% alt='Sections'
.ca Fig. 59.
.if-
.if t
[Illustration: Fig. 59.]
.if-
It will be seen from Fig. 59, in which comparative views
of the four conic sections are given, that the circle and
the ellipse are closed curves, or curves which return into
themselves. The parabola and the hyperbola are, on the
contrary, open curves, or curves which do not return into
themselves.
.pm letter-start
49. A Revolving Body is continually Falling towards its
Centre of Revolution.--In Fig. 60 let M represent the moon,
// File: psp_050.png
.pn +1
and E the earth around which the moon is revolving in the
direction MN. It will be seen that the moon, in moving from
M to N, falls towards the earth a distance equal to mN. It
is kept from falling into the earth by its orbital motion.
.pm letter-end
.if h
.il fn=fig060.png w=50% alt='Orbits'
.ca Fig. 60.
.if-
.if t
[Illustration: Fig. 60.]
.if-
.pm letter-start
The fact that a
body might be projected
forward fast
enough to keep it
from falling into the
earth is evident from
Fig. 61. AB represents
the level surface
of the ocean,
C a mountain from
the summit of which
a cannon-ball is supposed
to be fired in
the direction CE.
AD is a line parallel
with CE; DB is a
line equal to the distance
between the two
parallel lines AD and
CE. This distance is equal to that over which gravity would
pull a ball towards the centre of the earth in a minute. No
matter, then, with what velocity the ball C is fired, at the end
of a minute it will be somewhere on the line AD. Suppose
it were fired fast enough to reach the point D in a minute:
it would be on the line AD at the end of the
minute, but still just as far from the surface of
the water as when it started. It will be seen,
that, although it has all the while been falling
towards the earth, it has all the while kept at
exactly the same distance from the surface.
The same thing would of course be true during
each succeeding minute, till the ball came
round to C again, and the ball would continue to revolve in a
circle around the earth.
.pm letter-end
.if h
.il fn=fig061.png w=90% alt='Ships on Earth'
.ca Fig. 61.
.if-
.if t
[Illustration: Fig. 61.]
.if-
50. The Form of a Body's Orbit depends upon the Rate of
// File: psp_051.png
.pn +1
its Forward Motion.--If the ball C were fired fast enough to
reach the line AD beyond the point D, it would be farther
from the surface at the end of the second than when it
started. Its orbit would no longer be circular, but elliptical.
If the speed of projection were gradually augmented,
the orbit would become a more and more elongated ellipse.
At a certain rate of projection, the orbit would become a
parabola; at a still greater rate,
a hyperbola.
51. The Moon held in her
Orbit by Gravity.--Newton compared
the distance mN that the
moon is drawn to the earth in
a given time, with the distance
a body near the surface of the
earth would be pulled toward
the earth in the same time; and
he found that these distances
are to each other inversely as
the squares of the distances of
the two bodies from the centre
of the earth. He therefore concluded
that the moon is drawn
to the earth by gravity, and that the intensity of gravity
decreases as the square of the distance increases.
.if h
.il fn=fig062.png w=50% alt='Orbits'
.ca Fig. 62.
.if-
.if t
[Illustration: Fig. 62.]
.if-
52. Any Body whose Orbit is a Conic Section, and which
moves according to Kepler's Second Law, is acted upon by a
// File: psp_052.png
.pn +1
Force varying inversely as the Square of the Distance.--Newton
compared the distance which any body,
moving in an ellipse, according to Kepler's
Second Law, is drawn towards the sun in the
same time in different parts of its orbit. He
found these distances in all cases to vary
inversely as the square of the distance of
the planet from the sun. Thus, in Fig. 62,
suppose a planet would move from K to B
in the same time that it would move from k
to b in another part of its orbit. In the first
instance the planet would be drawn towards
the sun the distance AB, and in the second
instance the distance ab. Newton found that
AB : ab = (SK)^2 : (Sk)^2. He also found that
the same would be true when the body moved
in a parabola or a hyperbola: hence he concluded
that every body that moves around the
sun in an ellipse, a parabola, or a hyperbola,
is moving under the influence of gravity.
[Transcriber's Note: In Newton's equation above, (SK)^2 means to group S and K
together and square their product. In the original book, instead of using parentheses,
there was a vinculum, a horizontal bar,
drawn over the S and the K to express the same grouping.]
.if h
.il fn=fig063.png w=50% alt='Orbits'
.ca Fig. 63.
.if-
.if t
[Illustration: Fig. 63.]
.if-
53. The Force that draws the Different
Planets to the Sun Varies inversely as the Squares of the
Distances of the Planets from the Sun.--Newton compared
the distances jK and eF,
over which two planets are
drawn towards the sun in
the same time, and found
these distances to vary
inversely as the squares
of the distances of the
planets from the sun:
hence he concluded that
all the planets are held
in their orbits by gravity.
He also showed that this
would be true of any two
bodies that were revolving
around the sun's centre,
according to Kepler's Third Law.
// File: psp_053.png
.pn +1
54. The Copernican System.--The theory of the solar
system which originated with Copernicus, and which was
developed and completed by Kepler and Newton, is commonly
known as the Copernican System. This system is
shown in Fig. 64.
.if h
.il fn=fig064.png w=80% alt='Copernican System'
.ca Fig. 64.
.if-
.if t
[Illustration: Fig. 64.]
.if-
.sp 2
.h3 id='sun-planets'
II. THE SUN AND PLANETS.
.sp 2
.h4 id='earth'
I. THE EARTH.
.sp 2
.h4 id='earth-form'
Form and Size.
.sp 2
55. Form of the Earth.--In ordinary language the
term horizon denotes the line that bounds the portion of
the earth's surface that is visible at any point.
(1) It is well known that the horizon of a plain presents
the form of a circle surrounding the observer. If the
latter moves, the circle moves also; but its form remains the
same, and is modified only when mountains or other obstacles
limit the view. Out at sea, the circular form of the
horizon is still more decided, and changes only near the
coasts, the outline of which breaks the regularity.
Here, then, we obtain a first notion of the rotundity of
the earth, since a sphere is the only body which is presented
always to us under the form of a circle, from whatever point
on its surface it is viewed.
(2) Moreover, it cannot be maintained that the horizon
is the vanishing point of distinct vision, and that it is this
which causes the appearance of a circular boundary, because
the horizon is enlarged when we mount above the surface
of the plain. This will be evident from Fig. 65, in which a
mountain is depicted in the middle of a plain, whose uniform
curvature is that of a sphere. From the foot of the
mountain the spectator will have but a very limited horizon.
Let him ascend half way, his visual radius extends, is inclined
below the first horizon, and reveals a more extended circular
// File: psp_054.png
.pn +1
area. At the summit of the mountain the horizon still
increases; and, if the atmosphere is pure, the spectator will
see numerous objects where from the lower stations the sky
alone was visible.
.if h
.il fn=fig065.png w=80% alt='Mountain'
.ca Fig. 65.
.if-
.if t
[Illustration: Fig. 65.]
.if-
This extension of the horizon would be inexplicable if
the earth had the form of an extended plane.
// File: psp_055.png
.pn +1
(3) The curvature of the surface of the sea manifests
itself in a still more striking manner. If we are on the
coast at the summit of a hill, and a vessel appears on the
horizon (Fig. 66), we see only the tops of the masts and
the highest sails; the lower sails and the hull are invisible.
As the vessel approaches, its lower part comes into view
above the horizon, and soon it appears entire.
.if h
.il fn=fig066.png w=80% alt='Ocean'
.ca Fig. 66.
.if-
.if t
[Illustration: Fig. 66.]
.if-
In the same manner the sailors from the ship see the
different parts of objects on the land appear successively,
beginning with the highest. The reason of this will be
evident from Fig. 67, where the course of a vessel, seen in
profile, is figured on the convex surface of the sea.
.if h
.il fn=fig067.png w=90% alt='Ocean'
.ca Fig. 67.
.if-
.if t
[Illustration: Fig. 67.]
.if-
As the curvature of the ocean is the same in every direction,
it follows that the surface of the ocean is spherical.
The same is true of the surface of the land, allowance being
made for the various inequalities of the surface. From
these and various other indications, we conclude that the
earth is a sphere.
56. Size of the Earth.--The size of the earth is ascertained
by measuring the length of a degree of a meridian,
and multiplying this by three hundred and sixty. This gives
the circumference of the earth as about twenty-five thousand
miles, and its diameter as about eight thousand miles. We
know that the two stations between which we measure are
one degree apart when the elevation of the pole at one
station is one degree greater than at the other.
57. The Earth Flattened at the Poles.--Degrees on the
meridian have been measured in various parts of the earth,
and it has been found that they invariably increase in length
// File: psp_056.png
.pn +1
as we proceed from the equator towards the pole: hence
the earth must curve less and less rapidly as we approach the
poles; for the less the curvature of a circle, the larger the
degrees on it.
.if h
.il fn=fig068.png w=90% alt='Earth'
.ca Fig. 68.
.if-
.if t
[Illustration: Fig. 68.]
.if-
58. The Earth in Space.--In Fig. 68 we have a view
of the earth suspended in space. The side of the earth
turned towards the sun is illumined, and the other side is in
darkness. As the planet rotates on its axis, successive portions
of it will be turned towards the sun. As viewed from
a point in space between it and the sun, it will present
light and dark portions, which will assume different forms
according to the portion which is illumined. These different
appearances are shown in Fig. 69.
.if h
.il fn=fig069.png w=70% alt='Earth'
.ca Fig. 69.
.if-
.if t
[Illustration: Fig. 69.]
.if-
// File: psp_057.png
.pn +1
.sp 2
.h4 id='day-night'
Day and Night.
.sp 2
59. Day and Night.--The succession of day and night
is due to the rotation of the earth on its axis, by which a
place on the surface of the earth is carried alternately into
the sunshine and out of it. As the sun moves around the
// File: psp_058.png
.pn +1
heavens on the ecliptic, it will be on the celestial equator
when at the equinoxes, and 23-1/2° north of the equator when at
the summer solstice, and 23-1/2° south of the equator when
at the winter solstice.
60. Day and Night
when the Sun is at the
Equinoxes.--When the
sun is at either equinox,
the diurnal circle described
by the sun will
coincide with the celestial
equator; and therefore
half of this diurnal
circle will be above the
horizon at every point on
the surface of the globe.
At these times day and night will be equal in every part of
the earth.
.if h
.il fn=fig070.png w=70% alt='Earth'
.ca Fig. 70.
.if-
.if t
[Illustration: Fig. 70.]
.if-
.if h
.il fn=fig071.png w=70% alt='Earth'
.ca Fig. 71.
.if-
.if t
[Illustration: Fig. 71.]
.if-
.pm letter-start
The equality of days and nights when the sun is on the
celestial equator is also
evident from the following
considerations: one-half of
the earth is in sunshine all
of the time; when the sun
is on the celestial equator,
it is directly over the equator
of the earth, and the
illumination extends from
pole to pole, as is evident
from Figs. 70 and 71, in
the former of which the
sun is represented as on
the eastern horizon at a
place along the central line
of the figure, and in the latter as on the meridian along
the same line. In each diagram it is seen that the illumination
// File: psp_059.png
.pn +1
extends from pole to pole: hence, as the earth rotates on its
axis, every place on the surface will be in the sunshine and
out of it just half of the time.
.pm letter-end
61. Day and Night when the Sun is at the Summer
Solstice.--When the sun is
at the summer solstice, it
will be 23-1/2° north of the
celestial equator. The diurnal
circle described by the
sun will then be 23-1/2° north
of the celestial equator; and
more than half of this diurnal
circle will be above the
horizon at all places north
of the equator, and less
than half of it at places
south of the equator: hence the days will be longer than the
nights at places north of the equator, and shorter than the
nights at places south of
the equator. At places
within 23-1/2° of the north
pole, the entire diurnal
circle described by the
sun will be above the
horizon, so that the sun
will not set. At places
within 23-1/2° of the south
pole of the earth, the entire
diurnal circle will be
below the horizon, so that
the sun will not rise.
.if h
.il fn=fig072.png w=70% alt='Earth'
.ca Fig. 72.
.if-
.if t
[Illustration: Fig. 72.]
.if-
.if h
.il fn=fig073.png w=70% alt='Earth'
.ca Fig. 73.
.if-
.if t
[Illustration: Fig. 73.]
.if-
.pm letter-start
The illumination of the earth at this time is shown in
Figs. 72 and 73. In Fig. 72 the sun is represented as on the
western horizon along the middle line of the figure, and in
Fig. 73 as on the meridian. It is seen at once that the illumination
// File: psp_060.png
.pn +1
extends 23-1/2° beyond the north pole, and falls 23-1/2°
short of the south pole. As the earth rotates on its axis,
places near the north pole will be in the sunshine all the time,
while places near the south pole will be out of the sunshine
all the time. All places north of the equator will be in the
sunshine longer than they are out of it, while all places south
of the equator will be out of the sunshine longer than they
are in it.
.pm letter-end
62. Day and Night when the Sun is at the Winter Solstice.--When
the sun is at the winter solstice, it is 23-1/2°
south of the celestial equator. The diurnal circle described
by the sun is then 23-1/2° south of the celestial equator. More
than half of this diurnal circle will therefore be above the
horizon at all places south of the equator, and less than
half of it at all places north of the equator: hence the days
will be longer than the nights south of the equator, and
shorter than the nights at places north of the equator. At
places within 23-1/2° of the south pole, the diurnal circle described
by the sun will be entirely above the horizon, and
the sun will therefore not set. At places within 23-1/2° of the
north pole, the diurnal circle described by the sun will be
wholly below the horizon, and therefore the sun will not rise.
.pm letter-start
The illumination of the earth at this time is shown in
Figs. 74 and 75, and is seen to be the reverse of that shown
in Figs. 72 and 73.
.pm letter-end
.if h
.il fn=fig074.png w=70% alt='Earth'
.ca Fig. 74.
.if-
.if t
[Illustration: Fig. 74.]
.if-
.if h
.il fn=fig075.png w=70% alt='Earth'
.ca Fig. 75.
.if-
.if t
[Illustration: Fig. 75.]
.if-
63. Variation in the Length of Day and Night.--As
long as the sun is north of the equinoctial, the nights will
be longer than the days south of the equator, and shorter
than the days north of the equator. It is just the reverse
when the sun is south of the equator.
The farther the sun is from the equator, the greater is the
inequality of the days and nights.
The farther the place is from the equator, the greater the
inequality of its days and nights.
When the distance of a place from the north pole is less
// File: psp_061.png
.pn +1
than the distance of the sun north of the equinoctial, it
will have continuous day without night, since the whole of
the sun's diurnal circle will be above the horizon. A place
within the same distance of
the south pole will have
continuous night.
When the distance of a
place from the north pole is
less than the distance of the
sun south of the equinoctial,
it will have continuous
night, since the whole of
the sun's diurnal circle will
then be below the horizon.
A place within the same
distance of the south pole will then have continuous day.
At the equator the days and nights are always equal;
since, no matter where the sun is in the heavens, half of
all the diurnal circles described by it will be above the
horizon, and half of them
below it.
64. The Zones.--It
will be seen, from what
has been stated above,
that the sun will at some
time during the year be
directly overhead at every
place within 23-1/2° of the
equator on either side.
This belt of the earth is
called the torrid zone.
The torrid zone is bounded
by circles called the tropics; that of Cancer on the
north, and that of Capricorn on the south.
It will also be seen, that, at every place within 23-1/2° of
// File: psp_062.png
.pn +1
either pole, there will be, some time during the year, a
day during which the sun will not rise, or on which it will
not set. These two belts of the earth's surface are called
the frigid zones. These zones are bounded by the arctic
circles. The nearer a place is to the poles, the greater the
number of days on which the sun does not rise or set.
Between the frigid zones and the torrid zones, there are
two belts on the earth which are called the temperate zones.
The sun is never overhead at any place in these two zones,
but it rises and sets every day at every place within their
limits.
65. The Width of the Zones.--The distance the frigid
zones extend from the poles, and the torrid zones from the
equator, is exactly equal to the obliquity of the ecliptic, or the
deviation of the axis of the earth from the perpendicular to
the plane of its orbit. Were this deviation forty-five degrees,
the obliquity of the ecliptic would be forty-five degrees, the
torrid zone would extend forty-five degrees from the equator,
and the frigid zones forty-five degrees from the poles. In
this case there would be no temperate zones. Were this
deviation fifty degrees, the torrid and frigid zones would
overlap ten degrees, and there would be two belts of ten
degrees on the earth, which would experience alternately
during the year a torrid and a frigid climate.
Were the axis of the earth perpendicular to the plane
of the earth's orbit, there would be no zones on the earth,
and no variation in the length of day and night.
66. Twilight.--Were it not for the atmosphere, the
darkness of midnight would begin the moment the sun
sank below the horizon, and would continue till he rose
again above the horizon in the east, when the darkness of
the night would be suddenly succeeded by the full light
of day. The gradual transition from the light of day to
the darkness of the night, and from the darkness of the
night to the light of day, is called twilight, and is due to
// File: psp_063.png
.pn +1
the diffusion of light from the upper layers of the atmosphere
after the sun has ceased to shine on the lower layers
at night, or before it has begun to shine on them in the
morning.
.if h
.il fn=fig076.png w=80% alt='Earth'
.ca Fig. 76.
.if-
.if t
[Illustration: Fig. 76.]
.if-
Let ABCD (Fig. 76) represent a portion of the earth,
A a point on its surface where the sun S is setting; and let
SAH be a ray of light just grazing the earth at A, and
leaving the atmosphere at the point H. The point A is
illuminated by the whole reflective atmosphere HGFE.
The point B, to which the sun has set, receives no direct
solar light, nor any reflected from that part of the atmosphere
which is below ALH; but it receives a twilight from
the portion HLF, which lies above the visible horizon BF.
The point C receives a twilight only from the small portion
of the atmosphere; while at D the twilight has
ceased altogether.
67. Duration of Twilight.--The astronomical limit of
twilight is generally understood to be the instant when stars
of the sixth magnitude begin to be visible in the zenith at
evening, or disappear in the morning.
.pm letter-start
Twilight is usually reckoned to last until the sun's depression
below the horizon amounts to eighteen degrees: this, however,
varies; in the tropics a depression of sixteen or seventeen
degrees being sufficient to put an end to the phenomenon,
while in England a depression of seventeen to twenty-one
degrees is required. The duration of twilight differs in different
// File: psp_064.png
.pn +1
latitudes; it varies also in the same latitude at different
seasons of the year, and depends, in some measure, on the
meteorological condition of the atmosphere. When the sky
is of a pale color, indicating the presence of an unusual
amount of condensed vapor, twilight is of longer duration.
This happens habitually in the polar regions. On the contrary,
within the tropics, where the air is pure and dry, twilight sometimes
lasts only fifteen minutes. Strictly speaking, in the latitude
of Greenwich there is no true night from May 22 to
July 21, but constant twilight from sunset to sunrise. Twilight
reaches its minimum three weeks before the vernal equinox,
and three weeks after the autumnal equinox, when its duration
is an hour and fifty minutes. At midwinter it is longer by
about seventeen minutes; but the augmentation is frequently
not perceptible, owing to the greater prevalence of clouds and
haze at that season of the year, which intercept the light, and
hinder it from reaching the earth. The duration is least at
the equator (an hour and twelve minutes), and increases as
we approach the poles; for at the former there are two twilights
every twenty-four hours, but at the latter only two in a
year, each lasting about fifty days. At the north pole the sun
is below the horizon for six months, but from Jan. 29 to the
vernal equinox, and from the autumnal equinox to Nov. 12,
the sun is less than eighteen degrees below the horizon; so
that there is twilight during the whole of these intervals, and
thus the length of the actual night is reduced to two months
and a half. The length of the day in these regions is about
six months, during the whole of which time the sun is constantly
above the horizon. The general rule is, that to the
inhabitants of an oblique sphere the twilight is longer in proportion
as the place is nearer the elevated pole, and the sun is
farther from the equator on the side of the elevated pole.
.pm letter-end
.sp 2
.h4 id='seasons'
The Seasons.
.sp 2
68. The Seasons.--While the sun is north of the celestial
equator, places north of the equator are receiving heat
from the sun by day longer than they are losing it by radiation
at night, while places south of the equator are losing
// File: psp_065.png
.pn +1
heat by radiation at night longer than they are receiving it
from the sun by day. When, therefore, the sun passes north
of the equator, the temperature begins to rise at places
north of the equator, and to fall at places south of it. The
rise of temperature is most rapid north of the equator when
the sun is at the summer solstice; but, for some time after
this, the earth continues to receive more heat by day than it
loses by night, and therefore the temperature continues to
rise. For this reason, the heat is more excessive after the
sun passes the summer solstice than before it reaches it.
69. The Duration of the Seasons.--Summer is counted
as beginning in June, when the sun is at the summer solstice,
and as continuing until the sun reaches the autumnal
equinox, in September. Autumn then begins, and continues
until the sun is at the winter solstice, in December. Winter
follows, continuing until the sun comes to the vernal equinox,
in March, when spring
begins, and continues
to the summer solstice.
In popular
reckoning the seasons
begin with the
first day of June,
September, December,
and March.
The reason why
winter is counted as
occurring after the
winter solstice is similar
to the reason why
the summer is placed
after the summer solstice. The earth north of the equator
is losing heat most rapidly at the time of the winter solstice;
but for some time after this it loses more heat by night than
it receives by day: hence for some time the temperature
// File: psp_066.png
.pn +1
continues to fall, and the cold is more intense after the
winter solstice than before it.
.if h
.il fn=fig077.png w=70% alt='Seasons'
.ca Fig. 77.
.if-
.if t
[Illustration: Fig. 77.]
.if-
Of course, when it is summer in the northern hemisphere,
it is winter in the southern hemisphere, and the reverse.
// File: psp_067.png
.pn +1
Fig. 77 shows the portion of the earth's orbit included in
each season. It will be seen that the earth is at perihelion
in the winter season for
places north of the
equator, and at aphelion
in the summer season.
This tends to mitigate
somewhat the extreme
temperatures of our
winters and summers.
.if h
.il fn=fig078.png w=70% alt='Seasons'
.ca Fig. 78.
.if-
.if t
[Illustration: Fig. 78.]
.if-
70. The Illumination
of the Earth at the
different Seasons.--Fig.
78 shows the earth
as it would appear to
an observer at the sun
during each of the four seasons; that is to say, the portion
of the earth that is receiving the sun's rays. Figs. 79,
80, 81, and 82 are enlarged views of the earth, as seen
from the sun at the time
of the summer solstice, of
the autumnal equinox, of
the winter solstice, and
of the vernal equinox.
.if h
.il fn=fig079.png w=70% alt='Seasons'
.ca Fig. 79.
.if-
.if t
[Illustration: Fig. 79.]
.if-
.if h
.il fn=fig080.png w=70% alt='Seasons'
.ca Fig. 80.
.if-
.if t
[Illustration: Fig. 80.]
.if-
.if h
.il fn=fig081.png w=70% alt='Seasons'
.ca Fig. 81.
.if-
.if t
[Illustration: Fig. 81.]
.if-
.if h
.il fn=fig082.png w=70% alt='Seasons'
.ca Fig. 82.
.if-
.if t
[Illustration: Fig. 82.]
.if-
.if h
.il fn=fig083.png w=70% alt='Seasons'
.ca Fig. 83.
.if-
.if t
[Illustration: Fig. 83.]
.if-
Fig. 83 is, so to speak,
a side view of the earth,
showing the limit of sunshine
on the earth when
the sun is at the summer
solstice; and Fig. 84,
showing the limit of sunshine
when the sun is at
the autumnal equinox.
.if h
.il fn=fig084.png w=70% alt='Seasons'
.ca Fig. 84.
.if-
.if t
[Illustration: Fig. 84.]
.if-
71. Cause of the Change of Seasons.--Variety in the
length of day and night, and diversity in the seasons, depend
// File: psp_068.png
.pn +1
upon the obliquity of the ecliptic. Were there no obliquity
of the ecliptic, there
would be no inequality
in the length of day
and night, and but
slight diversity of seasons.
The greater the
obliquity of the ecliptic,
the greater would
be the variation in the
length of the days and
nights, and the more
extreme the changes
of the seasons.
.sp 2
.h4 id='tides'
Tides.
.sp 2
72. Tides.--The alternate rise and fall of the surface of
the sea twice in the course of a lunar day, or of twenty-four
hours and fifty-one minutes, is known as the tides. When
the water is rising, it is said to be flood tide; and when
it is falling, ebb tide.
When the water is at its
greatest height, it is said
to be high water; and
when at its least height,
low water.
.pm letter-start
73. Cause of the Tides.--It
has been known to
seafaring nations from a
remote antiquity that there
is a singular connection
between the ebb and flow
of the tides and the diurnal
motion of the moon.
.pm letter-end
.if h
.il fn=fig085.png w=70% alt='Tides'
.ca Fig. 85.
.if-
.if t
[Illustration: Fig. 85.]
.if-
.pm letter-start
This tidal movement in seeming obedience to the moon was
a mystery until the study of the law of gravitation showed it
// File: psp_069.png
.pn +1
to be due to the attraction of the moon on the waters of the
ocean. The reason why there are two tides a day will appear
from Fig. 85. Let M be the moon, E the earth, and EM the
line joining their centres. Now, strictly speaking, the moon
does not revolve around the earth any more than the earth
around the moon; but the centre of each body moves around
the common centre of gravity of the two bodies. The earth
// File: psp_070.png
.pn +1
being eighty times as heavy as the moon, this centre is situated
within the former, about three-quarters of the way from its
centre to its surface, at the point G. The body of the earth
itself being solid, every part of it, in consequence of the
moon's attraction, may be considered as describing a circle
once in a month, with a radius equal to EG. The centrifugal
force caused by this rotation is just balanced by the mean
attraction of the moon upon the earth. If this attraction were
the same on every part of the earth, there would be everywhere
an exact balance between it and the centrifugal force.
But as we pass from E to D the attraction of the moon diminishes,
owing to the increased distance: hence at D the centrifugal
force predominates, and the water therefore tends to move
away from the centre E. As we pass from E towards C, the
attraction of the moon increases, and therefore exceeds the centrifugal
force: consequently at C there is a tendency to draw
the water towards the moon, but still away from the centre E.
At A and B the attraction of the moon increases the gravity
of the water, owing to the convergence of the lines BM and
AM, along which it acts: hence the action of the moon tends
to make the waters rise at D and C, and to fall at A and B,
causing two tides to each apparent diurnal revolution of the
moon.
74. The Lagging of the Tides.--If the waters everywhere
yielded immediately to the attractive force of the moon, it would
always be high water when the moon was on the meridian, low
water when she was rising or setting, and high water again
when she was on the meridian below the horizon. But, owing
to the inertia of the water, some time is necessary for so slight
a force to set it in motion; and, once in motion, it continues
so after the force has ceased, and until it has acted some time
in the opposite direction. Therefore, if the motion of the
// File: psp_071.png
.pn +1
water were unimpeded, it would not be high water until some
hours after the moon had passed the meridian. The free
motion of the water is also impeded by the islands and continents.
These deflect the tidal wave from its course in such a
way that it may, in some cases, be many hours, or even a
whole day, behind its time. Sometimes two waves meet each
other, and raise a very high tide. In some places the tides
run up a long bay, where the motion of a large mass of water
will cause an enormous tide to be raised. In the Bay of
Fundy both of these causes are combined. A tidal wave coming
up the Atlantic coast meets the ocean wave from the
east, and, entering the bay with their combined force, they
// File: psp_072.png
.pn +1
raise the water at the head of it to the height of sixty or
seventy feet.
.pm letter-end
75. Spring-Tides and Neap-Tides.--The sun produces
a tide as well as the moon; but the tide-producing force
of the sun is only about four-tenths of that of the moon.
At new and full moon the two bodies unite their forces,
the ebb and flow become greater than the average, and
we have the spring-tides. When the moon is in her first
or third quarter, the two forces act against each other;
the tide-producing force is the difference of the two; the
ebb and flow are less than the average; and we have the
neap-tides.
.if h
.il fn=fig086.png w=70% alt='Tides'
.ca Fig. 86.
.if-
.if t
[Illustration: Fig. 86.]
.if-
.if h
.il fn=fig087.png w=70% alt='Tides'
.ca Fig. 87.
.if-
.if t
[Illustration: Fig. 87.]
.if-
.if h
.il fn=fig088.png w=70% alt='Tides'
.ca Fig. 88.
.if-
.if t
[Illustration: Fig. 88.]
.if-
Fig. 86 shows the tide that would be produced by the
moon alone; Fig. 87, the tide produced by the combined
action of the sun and moon; and Fig. 88, by the sun and
moon acting at right angles to each other.
The tide is affected by the distance of the moon from
// File: psp_073.png
.pn +1
the earth, being highest near the time when the moon is in
perigee, and lowest near the time when she is in apogee.
When the moon is in perigee, at or near the time of a new
or full moon, unusually high tides occur.
.pm letter-start
76. Diurnal Inequality of Tides.--The height of the tide
at a given place is influenced by the declination of the moon.
When the moon has no declination, the highest tides should
occur along the equator, and the heights should diminish from
thence toward the north and south; but the two daily tides at
any place should have the same height. When the moon has
north declination, as shown in Fig. 89, the highest tides on
the side of the earth next the moon will be at places having
a corresponding
north latitude,
as at B, and on
the opposite
side at those
which have an
equal south latitude.
Of the
two daily tides
at any place, that
which occurs
when the moon
is nearest the
zenith should be the greatest: hence, when the moon's declination
is north, the height of the tide at a place in north latitude
should be greater when the moon is above the horizon than
when she is below it. At the same time, places south of the
equator have the highest tides when the moon is below the
horizon, and the least when she is above it. This is called
the diurnal inequality, because its cycle is one day; but it
varies greatly in amount at different places.
.pm letter-end
.if h
.il fn=fig089.png w=70% alt='Tides'
.ca Fig. 89.
.if-
.if t
[Illustration: Fig. 89.]
.if-
77. Height of Tides.--At small islands in mid-ocean
the tides never rise to a great height, sometimes even less
than one foot; and the average height of the tides for the
islands of the Atlantic and Pacific Oceans is only three feet
// File: psp_074.png
.pn +1
and a half. Upon approaching an extensive coast where
the water is shallow, the height of the tide is increased; so
that, while in mid-ocean the average height does not exceed
three feet and a half, the average in the neighborhood of
continents is not less than four or five feet.
.sp 2
.h4 id='day-time'
The Day and Time.
.sp 2
78. The Day.--By the term day we sometimes denote
the period of sunshine as contrasted with that of the absence
of sunshine, which we call night, and sometimes the period
of the earth's rotation on its axis. It is with the latter
signification that the term is used in this section. As the
earth rotates on its axis, it carries the meridian of a place
with it; so that, during each complete rotation of the earth,
the portion of the meridian which passes overhead from
pole to pole sweeps past every star in the heavens from west
to east. The interval between two successive passages of
this portion of the meridian across the same star is the
exact period of the complete rotation of the earth. This
period is called a sidereal day. The sidereal day may also
be defined as the interval between two successive passages
of the same star across the meridian; the passage of the
meridian across the star, and the passage or transit of the
star across the meridian, being the same thing looked at
from a different point of view. The interval between two
successive passages of the meridian across the sun, or of the
sun across the meridian, is called a solar day.
79. Length of the Solar Day.--The solar day is a little
longer than the sidereal day. This is owing to the sun's
eastward motion among the stars. We have already seen
that the sun's apparent position among the stars is continually
shifting towards the east at a rate which causes it to
make a complete circuit of the heavens in a year, or three
hundred and sixty-five days. This is at the rate of about
one degree a day: hence, were the sun and a star on the
// File: psp_075.png
.pn +1
meridian together to-day, when the meridian again came
around to the star, the sun would
appear about one degree to the
eastward: hence the meridian must
be carried about one degree farther
in order to come up to the
sun. The solar day must therefore
be about four minutes longer
than the sidereal day.
.if h
.il fn=fig090.png w=70% alt='Day'
.ca Fig. 90.
.if-
.if t
[Illustration: Fig. 90.]
.if-
.if h
.il fn=fig091.png w=70% alt='Day'
.ca Fig. 91.
.if-
.if t
[Illustration: Fig. 91.]
.if-
The fact that the earth must
make more than a complete rotation
is also evident from Figs. 90
and 91. In Fig. 90, ba represents
the plane of the meridian, and the
small arrows indicate the direction
the earth is rotating on its axis,
and revolving in its orbit. When
the earth is at 1, the sun is on the meridian at a. When
the earth has moved to 2, it has made a complete rotation,
as is shown by the fact that the plane of the meridian is
// File: psp_076.png
.pn +1
parallel with its position at 1; but it is evident that the
meridian has not yet come up with the sun. In Fig. 91,
OA represents the plane of the meridian, and OS the
direction of the sun. The small arrows indicate the direction
of the rotation and
revolution of the earth. In
passing from the first position
to the second the
earth makes a complete
rotation, but the meridian
is not brought up to the
sun.
80. Inequality in the
Length of Solar Days.--The
sidereal days are all
of the same length; but the
solar days differ somewhat in length. This difference is due
to the fact that the sun's apparent position moves eastward,
or away from the meridian, at a variable rate.
.pm letter-start
There are three reasons why this rate is variable:--
(1) The sun's eastward motion is
due to the revolution of the earth
in its orbit. Now, the earth's orbital
motion is not uniform, being fastest
when the earth is at perihelion,
and slowest when the earth is at
aphelion: hence, other things being
equal, solar days will be longest
when the earth is at perihelion,
and shortest when the earth is at
aphelion.
.pm letter-end
.if h
.il fn=fig092.png w=70% alt='Day'
.ca Fig. 92.
.if-
.if t
[Illustration: Fig. 92.]
.if-
.if h
.il fn=fig093.png w=70% alt='Day'
.ca Fig. 93.
.if-
.if t
[Illustration: Fig. 93.]
.if-
.pm letter-start
(2) The sun's eastward motion
is along the ecliptic. Now, from Figs. 92 and 93, it will be
seen, that, when the sun is at one of the equinoxes, it will
be moving away from the meridian obliquely; and, from Figs.
94 and 95, that, when the sun is at one of the solstices, it will
// File: psp_077.png
.pn +1
be moving away from the meridian perpendicularly: hence,
other things being equal, the sun would move away from the
meridian fastest, and the days be longest, when the sun is at
the solstices; while it would move away from the meridian
slowest, and the days be shortest, when the sun is at the equinoxes.
That a body moving
along the ecliptic must be
moving at a variable angle to
the meridian becomes very
evident on turning a celestial
globe so as to bring each
portion of the ecliptic under
the meridian in turn.
.pm letter-end
.if h
.il fn=fig094.png w=60% alt='Solstice'
.ca Fig. 94.
.if-
.if t
[Illustration: Fig. 94.]
.if-
.if h
.il fn=fig095.png w=60% alt='Solstice'
.ca Fig. 95.
.if-
.if t
[Illustration: Fig. 95.]
.if-
.pm letter-start
(3) The sun, moving along
the ecliptic, always moves in
a great circle, while the point
of the meridian which is to
overtake the sun moves in a
diurnal circle, which is sometimes
a great circle and sometimes a small circle. When the
sun is at the equinoxes, the point of the meridian which is to
overtake it moves in a great circle. As the sun passes from
the equinoxes to the solstices,
the point of the meridian which
is to overtake it moves on a
smaller and smaller circle: hence,
as we pass away from the celestial
equator, the points of the
meridian move slower and slower.
Therefore, other things being
equal, the meridian will gain
upon the sun most rapidly, and
the days be shortest, when the
sun is at the equinoxes; while it
will gain on the sun least rapidly, and the days will be longest,
when the sun is at the solstices.
.pm letter-end
The ordinary or civil day is the mean of all the solar
days in a year.
// File: psp_078.png
.pn +1
81. Sun Time and Clock Time.--It is noon by the sun
when the sun is on the meridian, and by the clock at the
middle of the civil day. Now, as the civil days are all of
the same length, while solar days are of variable length, it
seldom happens that the middles of these two days coincide,
or that sun time and clock time agree. The difference
between sun time and clock time, or what is often called
apparent solar time and mean solar time, is called the equation
of time. The sun is said to be slow when it crosses the
meridian after noon by the clock, and fast when it crosses
the meridian before noon by the clock. Sun time and clock
time coincide four times a year; during two intermediate
seasons the clock time is ahead, and during two it is behind.
.tb
The following are the dates of coincidence and of maximum
deviation, which vary but slightly from year to year:--
.pm verse-start
February 10 True sun fifteen minutes slow.
April 15 True sun correct.
May 14 True sun four minutes fast.
June 14 True sun correct.
July 25 True sun six minutes slow.
August 31 True sun correct.
November 2 True sun sixteen minutes fast.
December 24 True sun correct.
.pm verse-end
One of the effects of the equation of time which is frequently
misunderstood is, that the interval from sunrise until
noon, as given in the almanacs, is not the same as that between
noon and sunset. The forenoon could not be longer or shorter
than the afternoon, if by "noon" we meant the passage of the
sun across the meridian; but the noon of our clocks being
sometimes fifteen minutes before or after noon by the sun, the
former may be half an hour nearer to sunrise than to sunset,
or vice versa.
.sp 2
.h4 id='year'
The Year.
.sp 2
82. The Year.--The year is the time it takes the earth
to revolve around the sun, or, what amounts to the same
thing, the time it takes the sun to pass around the ecliptic.
// File: psp_079.png
.pn +1
(1) The time it takes the sun to pass from a star around
to the same star again is called a sidereal year. This is,
of course, the exact time it takes the earth to make a complete
revolution around the sun.
.if h
.il fn=fig096.png w=60% alt='Year'
.ca Fig. 96.
.if-
.if t
[Illustration: Fig. 96.]
.if-
(2) The time it takes the sun to pass around from the
vernal equinox, or the first
point of Aries, to the vernal
equinox again, is called the
tropical year. This is a little
shorter than the sidereal year,
owing to the precession of
the equinoxes. This will be
evident from Fig. 96. The
circle represents the ecliptic,
S the sun, and E the vernal
equinox. The sun moves
around the ecliptic eastward, as indicated by the long arrow,
while the equinox moves slowly westward, as indicated by
the short arrow. The sun will therefore meet the equinox
before it has quite completed the circuit of the heavens.
The exact lengths of these respective years are:--
.pm verse-start
Sidereal year 365.25636=365 days 6 hours 9 min 9 sec
Tropical year 365.24220=365 days 5 hours 48 min 46 sec
.pm verse-end
Since the recurrence of the seasons depends on the tropical
year, the latter is the one to be used in forming the
calendar and for the purposes of civil life generally. Its
true length is eleven minutes and fourteen seconds less than
three hundred and sixty-five days and a fourth.
It will be seen that the tropical year is about twenty minutes
shorter than the sidereal year.
.pm letter-start
(3) The time it takes the earth to pass from its perihelion
point around to the perihelion point again is called the anomalistic
year. This year is about four minutes longer than the
sidereal year. This is owing to the fact that the major axis of
// File: psp_080.png
.pn +1
the earth's orbit is slowly moving around to the east at the
rate of about ten seconds a year. This causes the perihelion
point P (Fig. 97) to move eastward at that rate, as indicated
by the short arrow. The earth E is also moving eastward, as
indicated by the long arrow. Hence the earth, on starting at
the perihelion, has to make a little more than a complete circuit
to reach the perihelion point again.
.pm letter-end
.if h
.il fn=fig097.png w=60% alt='Year'
.ca Fig. 97.
.if-
.if t
[Illustration: Fig. 97.]
.if-
.pm letter-start
83. The Calendar.--The solar year, or the interval between
two successive passages of the same equinox by the sun,
is 365 days, 5 hours, 48 minutes,
46 seconds. If, then,
we reckon only 365 days to
a common or civil year, the
sun will come to the equinox
5 hours, 48 minutes, 46 seconds,
or nearly a quarter of
a day, later each year; so
that, if the sun entered Aries
on the 20th of March one
year, he would enter it on
the 21st four years after, on
the 22d eight years after, and so on. Thus in a comparatively
short time the spring months would come in the winter, and
the summer months in the spring.
Among different ancient nations different methods of computing
the year were in use. Some reckoned it by the revolution
of the moon, some by that of the sun; but none, so
far as we know, made proper allowances for deficiencies and
excesses. Twelve moons fell short of the true year, thirteen
exceeded it; 365 days were not enough, 366 were too many.
To prevent the confusion resulting from these errors, Julius
Cæsar reformed the calendar by making the year consist of
365 days, 6 hours (which is hence called a Julian year), and
made every fourth year consist of 366 days. This method of
reckoning is called Old Style.
But as this made the year somewhat too long, and the error
in 1582 amounted to ten days, Pope Gregory XIII., in order
to bring the vernal equinox back to the 21st of March again,
ordered ten days to be struck out of that year, calling the next
// File: psp_081.png
.pn +1
day after the 4th of October the 15th; and, to prevent similar
confusion in the future, he decreed that three leap-years should
be omitted in the course of every four hundred years. This
way of reckoning time is called New Style. It was immediately
adopted by most of the European nations, but was not
accepted by the English until the year 1752. The error then
amounted to eleven days, which were taken from the month of
September by calling the 3d of that month the 14th. The Old
Style is still retained by Russia.
According to the Gregorian calendar, every year whose number
is divisible by four is a leap-year, except, that, in the case
of the years whose numbers are exact hundreds, those only are
leap-years which are divisible by four after cutting off the last
two figures. Thus the years 1600, 2000, 2400, etc., are leap-years;
1700, 1800, 1900, 2100, 2200, etc., are not. The error
will not amount to a day in over three thousand years.
84. The Dominical Letter.--The dominical letter for any
year is that which we often see placed against Sunday in the
almanacs, and is always one of the first seven in the alphabet.
Since a common year consists of 365 days, if this number is
divided by seven (the number of days in a week), there will be
a remainder of one: hence a year commonly begins one day
later in the week than the preceding one did. If a year of
365 days begins on Sunday, the next will begin on Monday;
if it begins on Thursday, the next will begin on Friday; and
so on. If Sunday falls on the 1st of January, the first letter
of the alphabet, or A, is the dominical letter. If Sunday falls
on the 7th of January (as it will the next year, unless the first
is leap-year), the seventh letter, G, is the dominical letter. If
Sunday falls on the 6th of January (as it will the third year,
unless the first or second is leap-year), the sixth letter, F, will
be the dominical letter. Thus, if there were no leap-years, the
dominical letters would regularly follow a retrograde order,
G, F, E, D, C, B, A.
But leap-years have 366 days, which, divided by seven,
leaves two remainder: hence the years following leap-years will
begin two days later in the week than the leap-years did. To
prevent the interruption which would hence occur in the order
of the dominical letters, leap-years have two dominical letters,
// File: psp_082.png
.pn +1
one indicating Sunday till the 29th of February, and the other
for the rest of the year.
.pm letter-end
By Table I. below, the dominical letter for any year (New
Style) for four thousand years from the beginning of the Christian
Era may be found; and it will be readily seen how the
// File: psp_083.png---\stygiania\gajeff47\macaw89\robotnyk\Adair\-------
.pn +1
Table could be extended indefinitely by continuing the centuries
at the top in the same order.
To find the dominical letter by this table, look for the hundreds
of years at the top, and for the years below a hundred,
at the left hand.
Thus the letter for 1882 will be opposite the number 82,
and in the column having 1800 at the top; that is, it will be A.
In the same way, the letters for 1884, which is a leap-year, will
be found to be FE.
Having the dominical letter of any year, Table II. shows
what days of every month of the year will be Sundays.
To find the Sundays of any month in the year by this table,
look in the column, under the dominical letter, opposite the
name of the month given at the left.
From the Sundays the date of any other day of the week
can be readily found.
Thus, if we wish to know on what day of the week Christmas
falls in 1889, we look opposite December, under the letter
F (which we have found to be the dominical letter for the year),
and find that the 22d of the month is a Sunday; the 25th, or
Christmas, will then be Wednesday.
In the same way we may find the day of the week corresponding
to any date (New Style) in history. For instance, the
17th of June, 1775, the day of the fight at Bunker Hill, is found
to have been a Saturday.
These two tables then serve as a perpetual almanac.
Table I.
.ta r:2 r:2 r:2 r:2 r:5 r:4 r:4 r:4
| | | | 100 | 200| 300| 400
| | | | 500| 600| 700| 800
| | | | 900| 1000| 1100| 1200
| | | | 1300| 1400| 1500| 1600
| | | | 1700| 1800| 1900| 2000
| | | | 2100| 2200| 2300| 2400
| | | |---|---|---|----
| | | | C| E| G| BA
| | | | | | |
1 | 29 | 57 | 85 | B | D | F | G
2 | 30 | 58 | 86 | A | C | E | F
3 | 31 | 59 | 87 | G | B | D | E
4 | 32 | 60 | 88 | FE | AG | CB | DC
5 | 33 | 61 | 89 | D | F | A | B
6 | 34 | 62 | 90 | C | E | G | A
7 | 35 | 63 | 91 | B | D | F | G
8 | 36 | 64 | 92 | AG | CB | ED | FE
9 | 37 | 65 | 93 | F | A | C | D
10 | 38 | 66 | 94 | E | G | B | C
11 | 39 | 67 | 95 | D | F | A | B
12 | 40 | 68 | 96 | CB | ED | GF | AG
13 | 41 | 69 | 97 | A | C | E | F
14 | 42 | 70 | 98 | G | B | D | E
15 | 43 | 71 | 99 | F | A | C | D
16 | 44 | 72 | .. | ED | GF | BA | CB
17 | 45 | 73 | .. | C | E | G | A
18 | 46 | 74 | .. | B | D | F | G
19 | 47 | 75 | .. | A | C | E | F
20 | 48 | 76 | .. | GF | BA | DC | ED
21 | 49 | 77 | .. | E | G | B | C
22 | 50 | 78 | .. | D | F | A | B
23 | 51 | 79 | .. | C | E | G | A
24 | 52 | 80 | .. | BA | DC | FE | GF
25 | 53 | 81 | .. | G | B | D | E
26 | 54 | 82 | .. | F | A | C | D
27 | 55 | 83 | .. | E | G | B | C
28 | 56 | 84 | .. | DC | FE | AG | BA
.ta-
Table II.
.ta l:12 r:2 r:2 r:2 r:2 r:2 r:2 r:2
| | A | B | C | D | E | F | G
| | | | | | |
| 1 | 2 | 3 | 4 | 5 | 6 | 7
Jan. 31. | 8 | 9 | 10 | 11 | 12 | 13 | 14
| 15 | 16 | 17 | 18 | 19 | 20 | 21
Oct. 31. | 22 | 23 | 24 | 25 | 26 | 27 | 28
| 29 | 30 | 31 | .. | .. | .. | ..
| | | | | | |
Feb. 28-29. | .. | .. | .. | 1 | 2 | 3 | 4
| 5 | 6 | 7 | 8 | 9 | 10 | 11
March 31. | 12 | 13 | 14 | 15 | 16 | 17 | 18
| 19 | 20 | 21 | 22 | 23 | 24 | 25
Nov. 30. | 26 | 27 | 28 | 29 | 30 | 31 | ..
| | | | | | |
| .. | .. | .. | .. | .. | .. | 1
April 30. | 2 | 3 | 4 | 5 | 6 | 7 | 8
| 9 | 10 | 11 | 12 | 13 | 14 | 15
July 31 | 16 | 17 | 18 | 19 | 20 | 21 | 22
| 23 | 24 | 25 | 26 | 27 | 28 | 29
| 30 | 31 | .. | .. | .. | .. | ..
| | | | | | |
| .. | .. | 1 | 2 | 3 | 4 | 5
| 6 | 7 | 8 | 9 | 10 | 11 | 12
Aug. 31. | 13 | 14 | 15 | 16 | 17 | 18 | 19
| 20 | 21 | 22 | 23 | 24 | 25 | 26
| 27 | 28 | 29 | 30 | 31 | .. | ..
| | | | | | |
| .. | .. | .. | .. | .. | 1 | 2
Sept. 30. | 3 | 4 | 5 | 6 | 7 | 8 | 9
| 10 | 11 | 12 | 13 | 14 | 15 | 16
| 17 | 18 | 19 | 20 | 21 | 22 | 23
Dec. 31. | 24 | 25 | 26 | 27 | 28 | 29 | 30
| 31 | .. | .. | .. | .. | .. | ..
| | | | | | |
| .. | 1 | 2 | 3 | 4 | 5 | 6
| 7 | 8 | 9 | 10 | 11 | 12 | 13
May. 31. | 14 | 15 | 16 | 17 | 18 | 19 | 20
| 21 | 22 | 23 | 24 | 25 | 26 | 27
| 28 | 29 | 30 | 31 | .. | .. | ..
| | | | | | |
| .. | .. | .. | .. | 1 | 2 | 3
| 4 | 5 | 6 | 7 | 8 | 9 | 10
June 30. | 11 | 12 | 13 | 14 | 15 | 16 | 17
| 18 | 19 | 20 | 21 | 22 | 23 | 24
| 25 | 26 | 27 | 28 | 29 | 30 | ..
.ta-
.sp 2
.h4 id='earth-weight'
Weight of the Earth and Precession.
.sp 2
85. The Weight of the Earth.--There are several methods
of ascertaining the weight and mass of the earth. The simplest,
and perhaps the most trustworthy method is to compare
the pull of the earth upon a ball of lead with that of a known
mass of lead upon it. The pull of a known mass of lead upon
the ball may be measured by means of a torsion balance. One
form of the balance employed for this purpose is shown in
Figs. 98 and 99. Two small balls of lead, b and b, are fastened
to the ends of a light rod e, which is suspended from the point
F by means of the thread FE. Two large balls of lead, W
and W, are placed on a turn-table, so that one of them shall
// File: psp_084.png
.pn +1
be just in front of one of the small balls, and the other just
behind the other small ball. The pull of the large balls turns
the rod around a little so as to bring the small balls nearer the
large ones. The small balls move towards the large ones till
they are stopped by the torsion of the thread, which is then
equal to the pull of the large balls. The deflection of the rod
is carefully measured. The table is then turned into the position
indicated by the dotted lines in Fig. 99, so as to reverse
the position of the large balls with reference to the small ones.
The rod is now deflected in the opposite direction, and the
amount of deflection is again carefully measured. The second
measurement is made as a check upon the accuracy of the first.
The force required to twist the thread as much as it was
// File: psp_085.png
.pn +1
twisted by the deflection of the rod is ascertained by measurement.
This gives the pull of the two large balls upon the two
small ones. We next calculate what this pull would be were
the balls as far apart as the small balls are from the centre of
the earth. We can then form the following proportion: the
pull of the large balls upon the small ones is to the pull of the
earth upon the small ones as the mass of the large balls is to
the mass of the earth, or as the weight of the large balls is
to the weight of the earth. Of course, the pull of the earth
upon the small balls is the weight of the small balls. In this
way it has been ascertained that the mass of the earth is about
5.6 times that of a globe of water of the same size. In other
words, the mean density of the earth is about 5.6.
.if h
.il fn=fig098.png w=90% alt='Weight Measurement'
.ca Fig. 99.
.if-
.if t
[Illustration: Fig. 98.]
.if-
.if h
.il fn=fig099.png w=80% alt='Weight Measurement'
.ca Fig. 99.
.if-
.if t
[Illustration: Fig. 99.]
.if-
The weight of the earth in pounds may be found by multiplying
the number of cubic feet in it by 62-1/2 (the weight, in
pounds, of one cubic foot of water), and this product by 5.6.
.if h
.il fn=fig100.png w=80% alt='Precession'
.ca Fig. 100.
.if-
.if t
[Illustration: Fig. 100.]
.if-
86. Cause of Precession.--We have seen that the earth is
flattened at the poles: in other words, the earth has the form
of a sphere, with a protuberant ring around its equator. This
equatorial ring is inclined to the plane of the ecliptic at an angle
of about 23-1/2°. In Fig. 100 this ring is represented as detached
from the enclosed sphere. S represents the sun, and Sc the
ecliptic. As the point A of the ring is nearer the sun than the
point B is, the sun's pull upon A is greater than upon B:
hence the sun tends to pull the ring over into the plane of the
ecliptic; but the rotation of the earth tends to keep the ring in
the same plane. The struggle between these two tendencies
causes the earth, to which the ring is attached, to wabble like
a spinning-top, whose rotation tends to keep it erect, while
gravity tends to pull it over. The handle of the top has a
gyratory motion, which causes it to describe a curve. The axis
of the heavens corresponds to the handle of the top.
// File: psp_086.png
.pn +1
.sp 2
.h4 id='moon'
II. THE MOON.
.sp 2
.h4 id='moon-distance'
Distance, Size, and Motions.
.sp 2
87. The Distance of the Moon.--The moon is the nearest
of the heavenly bodies. Its distance from the centre of
the earth is only about sixty times the radius of the earth,
or, in round numbers, two hundred and forty thousand miles.
.pm letter-start
The ordinary method of finding the distance of one of the
nearer heavenly bodies is first to ascertain its horizontal parallax.
This enables us to form a right-angled triangle, the
lengths of whose sides are easily computed, and the length of
whose hypothenuse is the distance of the body from the centre
of the earth.
.pm letter-end
.if h
.il fn=fig101.png w=60% alt='Parallax'
.ca Fig. 101.
.if-
.if t
[Illustration: Fig. 101.]
.if-
.pm letter-start
Horizontal parallax has already been defined (32) as the displacement
of a heavenly
body when on the horizon,
caused by its being seen
from the surface, instead of
the centre, of the earth.
This displacement is due
to the fact that the body is seen in a different direction from
the surface of the earth from that in which it would be seen
from the centre. Horizontal parallax might be defined as the
difference in the directions in which a body on the horizon
would be seen from the surface and from the centre of the
earth. Thus, in Fig. 101, C is the centre of the earth, A a
point on the surface, and B a body on the horizon of A. AB
is the direction in which the body would be seen from A, and
CB the direction in which it would be seen from C. The difference
of these directions, or the angle ABC, is the parallax
of the body.
The triangle BAC is right-angled at A; the side AC is the
radius of the earth, and the hypothenuse is the distance of the
body from the centre of the earth. When the parallax ABC
is known, the length of CB can easily by found by trigonometrical
computation.
We have seen (32) that the parallax of a heavenly body
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grows less and less as the body passes from the horizon
towards the zenith. The parallax of a body and its altitude
are, however, so related, that, when we know the parallax at
any altitude, we can readily compute the horizontal parallax.
The usual method of finding the parallax of one of the
nearer heavenly bodies is first to find its parallax when on the
meridian, as seen from two places on the earth which differ
considerably in latitude: then to calculate what would be the
parallax of the body as seen from one of these places and the
centre of the earth: and then finally to calculate what would
be the parallax were the body on the horizon.
.pm letter-end
.if h
.il fn=fig102.png w=70% alt='Parallax'
.ca Fig. 102.
.if-
.if t
[Illustration: Fig. 102.]
.if-
.pm letter-start
Thus, we should ascertain the parallax of the body B (Fig.
102) as seen from A and D, or the angle ABD. We should
then calculate its parallax as seen from A and C, or the angle
ABC. Finally we should calculate what its parallax would be
were the body on the horizon, or the angle AB'C.
The simplest method of
finding the parallax of a
body B (Fig. 102) as seen
from the two points A and
D is to compare its direction
at each point with that
of the same fixed star near
the body. The star is so
distant, that it will be seen
in the same direction from
both points: hence, if the
direction of the body differs from that of the star 2° as seen
from one point, and 2° 6' as seen from the other point, the two
lines AB and DB must differ in direction by 6'; in other
words, the angle ABD would be 6'.
The method just described is the usual method of finding
the parallax of the moon.
.pm letter-end
88. The Apparent Size of the Moon.--The apparent size
of a body is the visual angle subtended by it; that is, the
angle formed by two lines drawn from the eye to two opposite
points on the outline of the body. The apparent size
of a body depends upon both its magnitude and its distance.
// File: psp_088.png
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The apparent size, or angular diameter, of the moon is
about thirty-one minutes. This is ascertained by means of
the wire micrometer already described (19). The instrument
is adjusted so that its longitudinal wire shall pass
through the centre of the moon, and its transverse wires
shall be tangent to the limbs of the
moon on each side, at the point
where they are cut by the longitudinal
wire. The micrometer screw
is then turned till the wires are
brought together. The number of
turns of the screw needed to accomplish
this will indicate the arc between
the wires, or the angular
diameter of the moon.
.if h
.il fn=fig103.png w=60% alt='Moon'
.ca Fig. 103.
.if-
.if t
[Illustration: Fig. 103.]
.if-
In order to be certain that the longitudinal wire shall pass
through the centre of the moon, it is best to take the moon
when its disc is in the form of a crescent, and to place the
longitudinal wire against the points, or cusps, of the crescent,
as shown in Fig. 103.
.if h
.il fn=fig104.png w=70% alt='Moon'
.ca Fig. 104.
.if-
.if t
[Illustration: Fig. 104.]
.if-
89. The Real Size of the Moon.--The real diameter of
the moon is a little over one-fourth of that of the earth, or
a little more than two thousand miles. The comparative
sizes of the earth and moon are shown in Fig. 104.
// File: psp_089.png
.pn +1
.if h
.il fn=fig105.png w=70% alt='Moon'
.ca Fig. 105.
.if-
.if t
[Illustration: Fig. 105.]
.if-
.pm letter-start
The distance and apparent size of the moon being known,
her real diameter is found by means of a triangle formed as
shown in Fig. 105. C represents the centre of the moon, CB
the distance of the moon from the earth, and CA the radius of
the moon. BAC is a triangle, right-angled at A. The angle
ABC is half the apparent
diameter of the moon.
With the angles A and B,
and the side CB known, it
is easy to find the length
of AC by trigonometrical
computation. Twice AC will be the diameter of the moon.
.pm letter-end
The volume of the moon is about one-fiftieth of that of
the earth.
90. Apparent Size of the Moon on the Horizon and in
the Zenith..--The moon is nearly four thousand miles farther
from the observer when she is on the horizon than
when she is in the zenith. This is evident from Fig. 106.
C is the centre of the earth, M the moon on the horizon,
M' the moon in the
zenith, and O the point of
observation. OM is the
distance of the moon when
she is on the horizon, and
OM' the distance of the
moon from the observer
when she is in the zenith.
CM is equal to CM', and
OM is about the length of CM; but OM' is about four
thousand miles shorter than CM': hence OM' is about
four thousand miles shorter than OM.
.if h
.il fn=fig106.png w=70% alt='Moon'
.ca Fig. 106.
.if-
.if t
[Illustration: Fig. 106.]
.if-
.pm letter-start
Notwithstanding the moon is much nearer when at the
zenith than at the horizon, it seems to us much larger at the
horizon.
This is a pure illusion, as we become convinced when we
measure the disc with accurate instruments, so as to make the
// File: psp_090.png
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result independent of our ordinary way of judging. When the
moon is near the horizon, it seems placed beyond all the objects
on the surface of the earth in that direction, and therefore farther
off than at the zenith, where no intervening objects enable
us to judge of its distance. In any case, an object which keeps
the same apparent magnitude seems to us, through the instinctive
habits of the eye, the larger in proportion as we judge it
to be more distant.
91. The Apparent Size of the Moon increased by Irradiation.--In
the case of the moon, the word apparent means much
more than it does in the case of other celestial bodies. Indeed,
its brightness causes our eyes to play us false. As is well
known, the crescent of the new moon seems part of a much
larger sphere than that which it has been said, time out of mind,
to "hold in its arms." The bright portion of the moon as seen
with our measuring instruments, as well as when seen with the
naked eye, covers a larger space in the field of the telescope
than it would if it were not so bright. This effect of irradiation,
as it is called, must be allowed for in exact measurements
of the diameter of the moon.
.pm letter-end
.if h
.il fn=fig107.png w=90% alt='Moon Images'
.ca Fig. 107.
.if-
.if t
[Illustration: Fig. 107.]
.if-
92. Apparent Size of the Moon in Different Parts of
her Orbit.--Owing to the eccentricity of the moon's orbit,
her distance from the earth varies somewhat from time to
time. This variation causes a corresponding variation in
her apparent size, which is illustrated in Fig. 107.
93. The Mass of the Moon.--The moon is considerably
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less dense than the earth, its mass being only about one-eightieth
of that of the earth; that is, while it would take
only about fifty moons to make the bulk of the earth, it
would take about eighty to make the mass of the earth.
.pm letter-start
One method of finding the mass of the moon is to compare
her effect in producing the tides with that of the sun. We
first calculate what would be the moon's effect in producing the
tides, were she as far off as the sun. We then form the following
proportion: as the sun's effect in producing the tides is to
the moon's effect at the same distance, so is the mass of the
sun to the mass of the moon.
The method of finding the mass of the sun will be given
farther on.
.pm letter-end
94. The Orbital Motion of the Moon.--If we watch
the moon from night to night, we see that she moves eastward
quite rapidly among the stars. When the new moon
is first visible, it appears near the horizon in the west, just
after sunset. A week later the moon will be on the meridian
at the same hour, and about a week later still on the eastern
horizon. The moon completes the circuit of the heavens
in a period of about thirty days, moving eastward at the
rate of about twelve degrees a day. This eastward motion
of the moon is due to the fact that she is revolving around
the earth from west to east.
.if h
.il fn=fig108.png w=70% alt='Moon Aspects'
.ca Fig. 108.
.if-
.if t
[Illustration: Fig. 108.]
.if-
95. The Aspects of the Moon.--As the moon revolves
around the earth, she comes into different positions with
reference to the earth and sun. These different positions of
the moon are called the aspects of the moon. The four
chief aspects of the moon are shown in Fig. 108. When
the moon is at M, she appears in the opposite part of the
heavens to the sun, and is said to be in opposition; when at
M' and at M''', she appears ninety degrees away from the
sun, and is said to be in quadrature; when at M'', she
appears in the same part of the heavens as the sun, and is
said to be in conjunction.
// File: psp_092.png
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96. The Sidereal and Synodical Periods of the Moon.--The
sidereal period of the moon is the time it takes her to
pass around from a star to that star again, or the time it
takes her to make
a complete revolution
around the earth.
This is a period
of about twenty-seven
days and a third. It
is sometimes called
the sidereal month.
The synodical period
of the moon is the
time that it takes the
moon to pass from
one aspect around to the same aspect again. This is a
period of about twenty-nine days and a half, and it is sometimes
called the synodical month.
.if h
.il fn=fig109.png w=80% alt='Moon'
.ca Fig. 109.
.if-
.if t
[Illustration: Fig. 109.]
.if-
The reason why the synodical period is longer than the
sidereal period will appear from Fig. 109. S represents the
position of the sun, E that of the earth, and the small
// File: psp_093.png
.pn +1
circle the orbit of the moon around the earth. The arrow
in the small circle represents the direction the moon is
revolving around the earth, and the arrow in the arc between
E and E' indicates the direction of the earth's motion in
its orbit. When the moon is at M_{1}, she is in conjunction.
As the moon revolves around the earth, the earth moves
forward in its orbit. When the moon has come round to
m_{1}, so that m_{3}m_{1} is parallel with M_{3}M_{1}, she will have made
a complete or sidereal revolution around the earth; but
she will not be in conjunction again till she has come round
to M, so as again to be between the earth and sun. That
is to say, the moon must make more than a complete revolution
in a synodical period.
.if h
.il fn=fig110.png w=90% alt='Moon'
.ca Fig. 110.
.if-
.if t
[Illustration: Fig. 110.]
.if-
.pm letter-start
The greater length of the synodical period is also evident
from Fig. 110. T represents the earth, and L the moon. The
arrows indicate the direction in which each is moving. When
the earth is at T, and the moon at L, the latter is in conjunction.
When the earth has reached T', and the moon L', the
latter has made a sidereal revolution; but she will not be
in conjunction again till the earth has reached T'', and the
moon L''.
.pm letter-end
97. The Phases of the Moon.--When the new moon
appears in the west, it has the form of a crescent, with its
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convex side towards the sun, and its horns towards the east.
As the moon advances towards quadrature, the crescent
grows thicker and thicker, till it becomes a half-circle at
// File: psp_095.png
.pn +1
first quarter. When it passes quadrature, it begins to become
convex also on the side away from the sun, or gibbous in
form. As it approaches opposition, it becomes more and
more nearly circular, until at opposition it is a full circle.
From full moon to last quarter it is again gibbous, and at
last quarter a half-circle. From last quarter to new moon
it is again crescent; but the horns of the crescent are now
turned towards the west. The successive phases of the
moon are shown in Fig. 111.
.if h
.il fn=fig111.png w=70% alt='Moon Phases'
.ca Fig. 111.
.if-
.if t
[Illustration: Fig. 111.]
.if-
98. Cause of the Phases of the Moon.--Take a globe,
half of which is colored white and the other half black in
such a way that the line which separates the white and black
portions shall be a great circle which passes through the
poles of the globe, and rotate the globe slowly, so as to
bring the white half gradually into view. When the white
part first comes into view, the line of separation between
it and the black part, which we may call the terminator,
appears concave, and its projection on a plane perpendicular
to the line of vision is a concave line. As more and more
of the white portion comes into view, the projection of the
terminator becomes less and less concave. When half of
the white portion comes into view, the terminator is projected
as a straight line. When more than half of the white
portion comes into view, the terminator begins to appear as
a convex line, and this line becomes more and more convex
till the whole of the white half comes into view, when the
terminator becomes circular.
.if h
.il fn=fig112.png w=70% alt='Moon Phases'
.ca Fig. 112.
.if-
.if t
[Illustration: Fig. 112.]
.if-
The moon is of itself a dark, opaque globe; but the half
that is towards the sun is always bright, as shown in Fig. 112.
This bright half of the moon corresponds to the white half
of the globe in the preceding illustration. As the moon
revolves around the earth, different portions of this illumined
half are turned towards the earth. At new moon, when the
moon is in conjunction, the bright half is turned entirely
away from the earth, and the disc of the moon is black and
// File: psp_096.png
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invisible. Between new moon and first quarter, less than
half of the illumined side is turned towards the earth, and
we see this illumined portion projected as a crescent. At
first quarter, just half of the illumined side is turned towards
the earth, and we see this half projected as a half-circle.
Between first quarter and full, more than half of the illumined
side is turned towards the earth, and we see it as
gibbous. At full, the whole of the illumined side is turned
towards us, and we see it as a full circle. From full to new
moon again, the phases occur in the reverse order.
// File: psp_097.png
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99. The Form of the Moon's Orbit.--The orbit of the
moon around the earth is an ellipse of slight eccentricity.
The form of this ellipse is shown in Fig. 113. C is the
centre of the ellipse, and E the position of the earth at one
of its foci. The eccentricity of the ellipse is only about
one-eighteenth. It is impossible for the eye to distinguish
such an ellipse from a circle.
.if h
.il fn=fig113.png w=70% alt='Moon Orbit'
.ca Fig. 113.
.if-
.if t
[Illustration: Fig. 113.]
.if-
100. The Inclination of the Moon's Orbit.--The plane
of the moon's orbit is inclined to the ecliptic by an angle
of about five degrees. The two points where the moon's
orbit cuts the ecliptic are called her nodes. The moon's
nodes have a westward motion corresponding to that of the
equinoxes, but much more rapid. They complete the circuit
of the ecliptic in about nineteen years.
The moon's latitude ranges from 5° north to 5° south;
and since, owing to the motion of her nodes, the moon is,
// File: psp_098.png
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during a period of nineteen years, 5° north and 5° south of
every part of the ecliptic, her declination will range from
23-1/2° + 5° = 28-1/2° north to 23-1/2° + 5° = 28-1/2° south.
101. The Meridian Altitude of the Moon.--The meridian
altitude of any body is its altitude when on the meridian.
In our latitude, the meridian altitude of any point on the
equinoctial is forty-nine degrees. The meridian altitude of
the summer solstice is 49° + 23-1/2° = 72-1/2°, and that of the
winter solstice is 49° - 23-1/2° = 25-1/2°. The greatest meridian
altitude of the moon is 72-1/2° + 5° = 77-1/2°, and its least
meridian altitude, 25-1/2° - 5° = 20-1/2°.
When the moon's meridian altitude is greater than the
elevation of the equinoctial, it is said to run high, and when
less, to run low. The full moon runs high when the sun is
south of the equinoctial, and low when the sun is north of
the equinoctial. This is because the full moon is always in
the opposite part of the heavens to the sun.
.pm letter-start
102. Wet and Dry Moon.--At the time of new moon, the
cusps of the crescent sometimes lie in a line which is nearly
perpendicular with the horizon, and sometimes in a line which
is nearly parallel with the horizon. In the former case the
moon is popularly described as a wet moon, and in the latter
case as a dry moon.
.pm letter-end
.if h
.il fn=fig114.png w=80% alt='Moon Orbit'
.ca Fig. 114.
.if-
.if t
[Illustration: Fig. 114.]
.if-
.pm letter-start
The great circle
which passes through
the centre of the sun
and moon will pass
through the centre of
the crescent, and be
perpendicular to the
line joining the cusps.
Now the ecliptic makes
the least angle with the horizon when the vernal equinox is
on the eastern horizon and the autumnal equinox is on the
western. In our latitude, as we have seen, this angle is 25-1/2°:
hence in our latitude, if the moon were at new on the ecliptic
// File: psp_099.png
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when the sun is at the autumnal equinox, as shown at M_{3}
(Fig. 114), the great circle passing through the centre of the
sun and moon would be the ecliptic, and at New York would
be inclined to the horizon at an angle of 25-1/2°. If the moon
happened to be 5° south of the ecliptic at this time, as at
M_{4}, the great circle passing
through the centre of
the sun and moon would
make an angle of only
20-1/2° with the horizon.
In either of these cases
the line joining the cusps
would be nearly perpendicular
to the horizon.
.pm letter-end
.if h
.il fn=fig115.png w=70% alt='Moon Orbit'
.ca Fig. 115.
.if-
.if t
[Illustration: Fig. 115.]
.if-
.pm letter-start
If the moon were at
new on the ecliptic when
the sun is near the vernal
equinox, as shown at M_{1}
(Fig. 115), the great circle
passing through the centres of the sun and moon would make
an angle of 72-1/2° with the horizon at New York; and were the
moon 5° north of the ecliptic at that time, as shown at M_{2}, this
great circle would make an angle of 77-1/2° with the horizon. In
either of these cases, the line joining the cusps would be nearly
parallel with the horizon.
At different times, the line joining the cusps may have every
possible inclination to the horizon between the extreme cases
shown in Figs. 114 and 115.
.pm letter-end
103. Daily Retardation of the Moon's Rising.--The
moon rises, on the average, about fifty minutes later each
day. This is owing to her eastward motion. As the moon
makes a complete revolution around the earth in about
twenty-seven days, she moves eastward at the rate of about
thirteen degrees a day, or about twelve degrees a day faster
than the sun. Were the moon, therefore, on the horizon
at any hour to-day, she would be some twelve degrees below
the horizon at the same hour to-morrow. Now, as the horizon
// File: psp_100.png
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moves at the rate of one degree in four minutes, it
would take it some fifty minutes to come up to the moon so
as to bring her upon the horizon. Hence the daily retardation
of the moon's rising is about fifty minutes; but it
varies considerably in different
parts of her orbit.
.pm letter-start
There are two reasons for
this variation in the daily
retardation:--
(1) The moon moves at a
varying rate in her orbit; her
speed being greatest at perigee,
and least at apogee: hence,
other things being equal, the
retardation is greatest when
the moon is at perigee, and least when she is at apogee.
.pm letter-end
.if h
.il fn=fig116.png w=70% alt='Moon Orbit'
.ca Fig. 116.
.if-
.if t
[Illustration: Fig. 116.]
.if-
.if h
.il fn=fig117.png w=70% alt='Moon Orbit'
.ca Fig. 117.
.if-
.if t
[Illustration: Fig. 117.]
.if-
.pm letter-start
(2) The moon moves at a varying angle to the horizon.
The moon moves nearly in the plane of the ecliptic, and of
course she passes both equinoxes every lunation. When she
is near the autumnal equinox, her path makes the greatest
angle with the eastern horizon, and when she is near the
vernal equinox, the least angle:
hence the moon moves away
from the horizon fastest when
she is near the autumnal equinox,
and slowest when she is
near the vernal equinox. This
will be evident from Figs. 116
and 117. In each figure, SN
represents a portion of the
eastern horizon, and Ec, E'c',
a portion of the ecliptic. AE,
in Fig. 116, represents the autumnal equinox, and AEM the
daily motion of the moon. VE, in Fig. 117, represents the
vernal equinox, and VEM' the motion of the moon for one
day. In the first case this motion would carry the moon away
from the horizon the distance AM, and in the second case the
// File: psp_101.png
.pn +1
distance A'M'. Now, it is evident that AM is greater than
A'M': hence, other things being equal, the greatest retardation
of the moon's rising will be when the moon is near the autumnal
equinox, and the least retardation when the moon is near
the vernal equinox.
.pm letter-end
The least retardation at New York is twenty-three minutes,
and the greatest an hour and seventeen minutes. The greatest
and least retardations vary somewhat from month to
month; since they depend not only upon the position of the
moon in her orbit with reference to the equinoxes, but also
upon the latitude of the moon, and upon her nearness to
the earth.
.if h
.il fn=fig118.png w=70% alt='Moon Orbit'
.ca Fig. 118.
.if-
.if t
[Illustration: Fig. 118.]
.if-
The direction of the moon's motion with reference to
the ecliptic is shown in Fig. 118, which shows the moon's
motion for one day in July, 1876.
104. The Harvest Moon--The long and short retardations
in the rising of the moon, though they occur every
month, are not likely to attract attention unless they occur
at the time of full moon. The long retardations for full
moon occur when the moon is near the autumnal equinox
at full. As the full moon is always opposite to the sun, the
// File: psp_102.png
.pn +1
sun must in this case be near the vernal equinox: hence
the long retardations for full moon occur in the spring, the
greatest retardation being in March.
The least retardations for full moon occur when the moon
is near the vernal equinox at full: the sun must then be
near the autumnal equinox. Hence the least retardations for
full moon occur in the months of August, September, and
October. The retardation is, of course, least for September;
and the full moon of this month rises night after night less
than half an hour later than the previous night. The full
moon of September is called the "Harvest Moon," and that
of October the "Hunter's Moon."
105. The Rotation of the Moon.--A careful examination
of the spots on the disc of the moon reveals the fact
that she always presents the same side to the earth. In
order to do this, she must rotate on her axis while making a
revolution around the earth, or in about twenty-seven days.
106. Librations of the Moon.--The moon appears to
rock slowly to and fro, so as to allow us to see alternately a
little farther around to the right and the left, or above and
below, than we otherwise could. This apparent rocking of
the moon is called libration. The moon has three librations:--
(1) Libration in Latitude.--This libration enables us
to see alternately a little way around on the northern and
southern limbs of the moon.
.pm letter-start
This libration is due to the fact that the axis of the moon
is not quite perpendicular to the plane of her orbit. The
deviation from the perpendicular is six degrees and a half. As
the axis of the moon, like that of the earth, maintains the same
direction, the poles of the moon will be turned alternately six
degrees and a half toward and from the earth.
.pm letter-end
(2) Libration in Longitude.--This libration enables us
to see alternately a little farther around on the eastern and
western limbs of the moon.
// File: psp_103.png
.pn +1
.if h
.il fn=fig119.png w=70% alt='Moon Orbit'
.ca Fig. 119.
.if-
.if t
[Illustration: Fig. 119.]
.if-
.pm letter-start
It is due to the fact that the moon's axial motion is uniform,
while her orbital motion is not. At perigee her orbital motion
will be in advance of her axial motion, while at apogee the
axial motion will be in advance of the orbital. In Fig. 119,
E represents the earth, M the moon, the large arrow the
direction of the moon's motion in her orbit, and the small
arrow the direction of her motion of rotation. When the
moon is at M, the line AB, drawn perpendicular to EM,
represents the circle which divides the visible from the invisible
portion of the moon. While the moon is passing from M to
M', the moon performs less than a quarter of a rotation, so
that AB is no longer perpendicular to EM'. An observer on
the earth can now see
somewhat beyond A on
the western limb of
the moon, and not quite
up to B on the eastern
limb. While the moon
is passing from M' to
M'', her axial motion
again overtakes her orbital
motion, so that the
line AB again becomes
perpendicular to the
line joining the centre
of the moon to the
centre of the earth. Exactly the same side is now turned
towards the earth as when the moon was at M. While the
moon passes from M'' to M''', her axial motion gets in advance
of her orbital motion, so that AB is again inclined to the line
joining the centres of the earth and moon. A portion of the
eastern limb of the moon beyond B is now brought into view
to the earth, and a portion of the western limb at A is carried
out of view. While the moon is passing from M''' to M, the
orbital motion again overtakes the axial motion, and AB is
again perpendicular to ME.
.pm letter-end
(3) Parallactic Libration.--While an observer at the
centre of the earth would get the same view of the moon,
// File: psp_104.png
.pn +1
whether she were on the eastern horizon, in the zenith,
or on the western horizon, an observer on the surface of
the earth does not get exactly the same view in these
three cases. When the moon is on the eastern horizon,
an observer on the surface of the earth would see a little
farther around on the western limb of the moon than when
she is in the zenith, and not quite so far around on the eastern
limb. On the contrary, when the moon is on the
western horizon, an observer on the surface of the earth
sees a little farther around on the eastern limb of the moon
than when she is in the zenith, and not quite so far around
on her western limb.
.if h
.il fn=fig120.png w=70% alt='Moon Orbit'
.ca Fig. 120.
.if-
.if t
[Illustration: Fig. 120.]
.if-
.pm letter-start
This will be evident from Fig. 120. E is the centre of
the earth, and O a
point on its surface.
AB is a line drawn
through the centre
of the moon, perpendicular
to a line
joining the centres
of the moon and
the earth. This line
marks off the part
of the moon turned
towards the centre
of the earth, and remains
essentially the same during the day. CD is a line drawn
through the centre of the moon perpendicular to a line joining
the centre of the moon and the point of observation. This
line marks off the part of the moon turned towards O. When
the moon is in the zenith, CD coincides with AB; but, when
the moon is on the horizon, CD is inclined to AB. When the
moon is on the eastern horizon, an observer at O sees a little
beyond B, and not quite to A; and, when she is on the western
horizon, he sees a little beyond A, and not quite to B. B is
on the western limb of the moon, and A on her eastern limb.
Since this libration is due to the point from which the moon
// File: psp_105.png
.pn +1
is viewed, it is called parallactic libration; and, since it occurs
daily, it is called diurnal libration.
.pm letter-end
.if h
.il fn=fig121.png w=70% alt='Moon Phases'
.ca Fig. 121.
.if-
.if t
[Illustration: Fig. 121.]
.if-
.pm letter-start
107. Portion of the Lunar Surface brought into View by
Libration.--The area brought into view by the first two librations
is between one-twelfth and one-thirteenth of the whole
lunar surface, or nearly one-sixth of the hemisphere of the moon
which is turned away from the earth when the moon is at her
state of mean libration. Of course a precisely equal portion
of the hemisphere turned towards us during mean libration is
carried out of view by the lunar librations.
If we add to each of these areas a fringe about one degree
wide, due to the diurnal libration, and which we may call the
parallactic fringe, we shall find that the total area brought into
view is almost exactly one-eleventh part of the whole surface
of the moon. A similar area is carried out of view; so that
the whole region thus swayed out of and into view amounts
to two-elevenths of the moon's surface. This area is shown
in Fig. 121, which is a side view of the moon.
.pm letter-end
// File: psp_106.png
.pn +1
.if h
.il fn=fig122.png w=90% alt='Moon Orbit'
.ca Fig. 122.
.if-
.if t
[Illustration: Fig. 122.]
.if-
.pm letter-start
108. The Moon's Path through Space.--Were the earth
stationary, the moon would describe an ellipse around it similar
to that of Fig. 113; but, as the earth moves forward in her
orbit at the same time that the moon revolves around it, the
moon is made to describe a sinuous path, as shown by the
continuous line in Fig. 122. This feature of the moon's path is
greatly exaggerated in the upper portion of the diagram. The
form of her path is given with a greater degree of accuracy in
the lower part of the figure (the broken line represents the path
// File: psp_107.png
.pn +1
of the earth); but even here there is considerable exaggeration.
The complete serpentine path of the moon around the sun is
shown, greatly exaggerated, in Fig. 123, the broken line being
the path of the earth.
.pm letter-end
.if h
.il fn=fig123.png w=60% alt='Moon Orbit'
.ca Fig. 123.
.if-
.if t
[Illustration: Fig. 123.]
.if-
.pm letter-start
The path described by the moon through space is much the
same as that described by a point on the circumference of a
wheel which is rolled over another wheel. If we place a circular
disk against the wall, and carefully roll along its edge
another circular disk (to which a piece of lead pencil has been
fastened so as to mark upon the wall), the curve described will
somewhat resemble that described by the moon. This curve
is called an epicycloid, and it will be seen that at every point
it is concave towards the centre of the larger disk. In the
same way the moon's orbit is at every point concave towards
the sun.
.pm letter-end
.if h
.il fn=fig124.png w=70% alt='Moon Orbit'
.ca Fig. 124.
.if-
.if t
[Illustration: Fig. 124.]
.if-
.pm letter-start
The exaggeration of the sinuosity in Fig. 123 will be more
evident when it is stated, that, on the scale of Fig. 124, the
// File: psp_108.png
.pn +1
whole of the serpentine curve would lie within the breadth of
the fine circular line MM'.
.pm letter-end
109. The Lunar Day.--The lunar day is twenty-nine
times and a half as long as the terrestrial day. Near the
moon's equator the sun shines without intermission nearly
fifteen of our days, and is absent for the same length of
time. Consequently, the vicissitudes of temperature to
which the surface is exposed must be very great. During
the long lunar night the temperature of a body on the
moon's surface would probably fall lower than is ever known
on the earth, while during the day it must rise higher than
anywhere on our planet.
.if h
.il fn=fig125.png w=90% alt='Moon Orbit'
.ca Fig. 125.
.if-
.if t
[Illustration: Fig. 125.]
.if-
.pm letter-start
It might seem, that, since the moon rotates on her axis in
about twenty-seven days, the lunar day ought to be twenty-seven
days long, instead of twenty-nine. There is, however,
a solar, as well as a sidereal, day at the moon, as on the earth;
and the solar day at the moon is longer than the sidereal day,
for the same reason as on the earth. During the solar day the
moon must make both a synodical rotation and a synodical
revolution. This will be evident from Fig. 125, in which is
shown the path of the moon during one complete lunation.
E, E', E'', etc., are the successive positions of the earth; and
1, 2, 3, 4, 5, the successive positions of the moon. The small
arrows indicate the direction of the moon's rotation. The
moon is full at 1 and 5. At 1, A, at the centre of the moon's
// File: psp_109.png
.pn +1
disk, will have the sun, which lies in the direction AS, upon the
meridian. Before A will again have the sun on the meridian,
the moon must have made a synodical revolution; and, as will
be seen by the dotted lines, she must have made more than a
complete rotation. The rotation which brings the point A into
the same relation to the earth and sun is called a synodical
rotation.
It will also be evident from this diagram that the moon must
make a synodical rotation during a synodical revolution, in
order always to present the same side to the earth.
.pm letter-end
110. The Earth as seen from the Moon.--To an observer
on the moon, the earth would be an immense moon,
going through the same phases that the moon does to us;
but, instead of rising and setting, it would only oscillate to
and fro through a few degrees. On the other side of the
moon it would never be seen at all. The peculiarities of
the moon's motions which cause the librations, and make a
spot on the moon's disk seem to an observer on the earth
to oscillate to and fro, would cause the earth as a whole to
appear to a lunar observer to oscillate to and fro in the
heavens in a similar manner.
It is a well-known fact, that, at the time of new moon, the
dark part of the moon's surface is partially illumined, so
that it becomes visible to the naked eye. This must be due
to the light reflected to the moon from the earth. Since at
new moon the moon is between the earth and sun, it follows,
that, when it is new moon at the earth, it must be full earth
at the moon: hence, while the bright crescent is enjoying
full sunlight, the dark part of its surface is enjoying the
light of the full earth. Fig. 126 represents the full earth as
seen from the moon.
.if h
.il fn=fig126.png w=70% alt='Moon View'
.ca Fig. 126.
.if-
.if t
[Illustration: Fig. 126.]
.if-
.sp 2
.h4 id='moon-atmosphere'
The Atmosphere of the Moon.
.sp 2
111. The Moon has no Appreciable Atmosphere.--There
are several reasons for believing that the moon has little or
no atmosphere.
// File: psp_110.png
.pn +1
(1) Had the moon an atmosphere, it would be indicated
at the time of a solar eclipse, when the moon passes over
the disk of the sun. If the atmosphere were of any considerable
// File: psp_111.png
.pn +1
density, it would absorb a part of the sun's rays,
so as to produce a dusky border in front of the moon's disk,
as shown in Fig. 127. In reality no such dusky border is
ever seen; but the limb of the moon appears sharp, and
clearly defined, as in
Fig. 128.
.if h
.il fn=fig127.png w=70% alt='Moon Eclipse'
.ca Fig. 127.
.if-
.if t
[Illustration: Fig. 127.]
.if-
.if h
.il fn=fig128.png w=70% alt='Moon Eclipse'
.ca Fig. 128.
.if-
.if t
[Illustration: Fig. 128.]
.if-
If the atmosphere
were not dense enough
to produce this dusky
border, its refraction
would be sufficient
to distort the delicate
cusps of the
sun's crescent in the
manner shown at
the top of Fig. 125;
but no such distortion is ever observed. The cusps always
appear clear and sharp, as shown at the bottom of the figure:
hence it would seem that there can be no atmosphere of
appreciable density at the
moon.
(2) The absence of an
atmosphere from the moon
is also shown by the absence
of twilight and of
diffused daylight.
Upon the earth, twilight
continues until the sun is
eighteen degrees below the
horizon; that is, day and
night are separated by a
belt twelve hundred miles in breadth, in which the transition
from light to darkness is gradual. We have seen (66) that
this twilight results from the refraction and reflection of
light by our atmosphere; and, if the moon had an atmosphere,
// File: psp_112.png
.pn +1
we should notice a similar gradual transition from
the bright to the dark portions of her surface. Such, however,
is not the case. The boundary between the light and
darkness, though irregular, is sharply defined. Close to this
boundary the unillumined portion of the moon appears just
as dark as at any distance from it.
The shadows on the moon are also pitchy black, without
a trace of diffused daylight.
.if h
.il fn=fig129.png w=70% alt='Moon Atmosphere'
.ca Fig. 129.
.if-
.if t
[Illustration: Fig. 129.]
.if-
.pm letter-start
(3) The absence of an atmosphere is also proved by the
absence of refraction when the moon passes between us and
the stars. Let AB (Fig. 129) represent the disk of the moon,
and CD an atmosphere supposed to surround it. Let SAE
represent a straight line from the earth, touching the moon at
A, and let S be a star situated in the direction of this line. If
the moon had no
atmosphere, this
star would appear
to touch the edge
of the moon at
A; but, if the
moon had an atmosphere,
a star behind the edge of the moon, at S', would
be visible at the earth; for the ray S'A would be bent by the
atmosphere into the direction AE'. So, also, on the opposite
side of the moon, a star might be seen at the earth, although
really behind the edge of the moon: hence, if the moon had an
atmosphere, the time during which a star would be concealed
by the moon would be less than if it had no atmosphere, and
the amount of this effect must be proportional to the density
of the atmosphere.
The moon, in her orbital course across the heavens, is continually
passing before, or occulting, some of the stars that so
thickly stud her apparent path; and when we see a star thus
pass behind the lunar disk on one side, and come out again on
the other side, we are virtually observing the setting and rising
of that star upon the moon. The moon's apparent diameter
has been measured over and over again, and is known with
// File: psp_113.png
.pn +1
great accuracy; the rate of her motion across the sky is also
known with perfect accuracy: hence it is easy to calculate how
long the moon will take to travel across a part of the sky
exactly equal in length to her own diameter. Supposing, then,
that we observe a star pass behind the moon, and out again, it
is clear, that, if there is no atmosphere, the interval of time
during which it remains occulted ought to be exactly equal to
the computed time which the moon would take to pass over the
star. If, however, from the existence of a lunar atmosphere,
the star disappears too late, and re-appears too soon, as we
have seen it would, these two intervals will not agree; the computed
time will be greater than the observed time, and the
difference will represent the amount of refraction the star's
light has sustained or suffered, and hence the extent of atmosphere
it has had to pass through.
Comparisons of these two intervals of time have been
repeatedly made, the most extensive being executed under the
direction of the Astronomer Royal of England, several years
ago, and based upon no less than two hundred and ninety-six
occultation observations. In this determination the measured
or telescopic diameter of the moon was compared with
the diameter deduced from the occultations; and it was found
that the telescopic diameter was greater than the occultation
diameter by two seconds of angular measurement, or by about
a thousandth part of the whole diameter of the moon. This
discrepancy is probably due, in part at least, to irradiation (91),
which augments the apparent size of the moon, as seen in the
telescope as well as with the naked eye; but, if the whole two
seconds were caused by atmospheric refraction, this would
imply a horizontal refraction of one second, which is only one
two-thousandth of the earth's horizontal refraction. It is possible
that an atmosphere competent to produce this refraction
would not make itself visible in any other way.
But an atmosphere two thousand times rarer than our air
can scarcely be regarded as an atmosphere at all. The contents
of an air-pump receiver can seldom be rarefied to a
greater extent than to about a thousandth of the density of air
at the earth's surface; and the lunar atmosphere, if it exists
at all, is thus proved to be twice as attenuated as what we
commonly call a vacuum.
.pm letter-end
// File: psp_114.png
.pn +1
.sp 2
.h4 id='moon-surface'
The Surface of the Moon.
.sp 2
.if h
.il fn=fig130.png w=90% alt='Moon Surface'
.ca Fig. 130.
.if-
.if t
[Illustration: Fig. 130.]
.if-
112. Dusky Patches on the Disk of the Moon.--With
the naked eye, large dusky patches are seen on the moon,
in which popular fancy has detected a resemblance to a
human face. With a telescope of low power, these dark
patches appear as smooth as water, and they were once
supposed to be seas. This theory was the origin of the
name mare (Latin for sea), which is still applied to the
larger of these plains; but, if there were water on the surface
of the moon, it could not fail to manifest its presence
by its vapor, which would form an appreciable atmosphere.
Moreover, with a high telescopic power, these plains present
// File: psp_115.png
.pn +1
a more or less uneven surface; and, as the elevations and
depressions are found to be permanent, they cannot, of
course, belong to the surface of water.
.pm letter-start
The chief of these plains are shown in Fig. 130. They are
Mare Crisium, Mare Foecunditatis, Mare Nectaris, Mare Tranquillitatis,
Mare Serenitatis, Mare Imbrium, Mare Frigoris,
and Oceanus Procellarum. All these plains can easily be recognized
on the surface of
the full moon with the unaided
eye.
.pm letter-end
113. The Terminator
of the Moon.--The
terminator of the moon
is the line which separates
the bright and dark
portions of its disk.
When viewed with a
telescope of even moderate
power, the terminator
is seen to be very
irregular and uneven.
Many bright points are
seen just outside of the
terminator in the dark
portion of the disk, while
all along in the neighborhood
of the terminator
are bright patches
and dense shadows. These appearances are shown in Figs.
131 and 132, which represent the moon near the first and
last quarters. They indicate that the surface of the moon
is very rough and uneven.
.if h
.il fn=fig131.png w=90% alt='Moon Surface'
.ca Fig. 131.
.if-
.if t
[Illustration: Fig. 131.]
.if-
.if h
.il fn=fig132.png w=90% alt='Moon Surface'
.ca Fig. 132.
.if-
.if t
[Illustration: Fig. 132.]
.if-
As it is always either sunrise or sunset along the terminator,
the bright spots outside of it are clearly the tops of
mountains, which catch the rays of the sun while their bases
// File: psp_116.png
.pn +1
are in the shade. The bright patches in the neighborhood
of the terminator are the sides of hills and mountains which
are receiving the full light of the sun, while the dense
shadows near by are cast by these elevations.
114. Height of the Lunar Mountains.--There are two
methods of finding the height of lunar mountains:--
(1) We may measure the length of the shadows, and
then calculate the height of the mountains that would cast
such shadows with the sun at the required height above the
horizon.
.pm letter-start
The length of a shadow may be obtained by the following
method: the longitudinal wire of the micrometer (19) is adjusted
so as to pass through the shadow whose length is to be measured,
// File: psp_117.png
.pn +1
and the transverse wires are placed one at each end of
the shadow, as shown in Fig. 133. The micrometer screw is
then turned till the wires are brought together, so as to ascertain
the length of the arc between them. We may then form
the proportion: the number of seconds in the semi-diameter
of the moon is to the number of seconds in the length of the
shadow, as the length of the moon's radius in miles to the
length of the shadow in miles.
.pm letter-end
.if h
.il fn=fig133.png w=90% alt='Moon Surface'
.ca Fig. 133.
.if-
.if t
[Illustration: Fig. 133.]
.if-
.pm letter-start
The height of the sun above the horizon is ascertained
by measuring the angular distance of the mountain from the
terminator.
.pm letter-end
(2) We may measure the distance of a bright point from
the terminator, and then construct a right-angled triangle,
as shown in Fig. 134. A solution of this triangle will enable
us to ascertain the height of the mountain whose top is just
catching the level rays of the sun.
.if h
.il fn=fig134.png w=70% alt='Moon Surface'
.ca Fig. 134.
.if-
.if t
[Illustration: Fig. 134.]
.if-
.pm letter-start
B is the centre of the moon, M the top of the mountain,
// File: psp_118.png
.pn +1
and SAM a ray of sunlight which just grazes the terminator
at A, and then strikes the top of the mountain at M. The
triangle BAM is right-angled at A. BA is the radius of the
moon, and AM is known by measurement; BM, the hypothenuse,
may then be found by computation. BM is evidently
equal to the radius of the moon plus the height of the mountain.
.pm letter-end
By one or the other of these methods, the heights of
the lunar mountains have been found with a great degree
of accuracy. It is claimed that the heights of the lunar
mountains are more accurately known than those of the
mountains on the earth. Compared with the size of the
moon, lunar mountains attain a greater height than those
on the earth.
115. General Aspect of the Lunar Surface.--A cursory
examination of the moon with a low power is sufficient to
show the prevalence of crater-like inequalities and the general
tendency to circular shape which is apparent in nearly
all the surface markings; for even the large "seas" and
the smaller patches of the same character repeat in their
outlines the round form of the craters. It is along the
terminator that we see these crater-like spots to the best
advantage; as it is there that the rising or setting sun casts
// File: psp_119.png
.pn +1
long shadows over the lunar landscape, and brings elevations
into bold relief. They vary greatly in size; some being
so large as to bear a sensible proportion to the moon's
diameter, while the smallest are so minute as to need the
most powerful telescopes and the finest conditions of atmosphere
to perceive them.
.if h
.il fn=fig135.png w=90% alt='Moon Surface'
.ca Fig. 135.
.if-
.if t
[Illustration: Fig. 135.]
.if-
The prevalence of ring-shaped mountains and plains will
be evident from Fig. 135, which is from a photograph of a
model of the moon constructed by Nasmyth.
This same feature is nearly as marked in Figs. 131 and
132, which are copies of Rutherfurd's photographs of the
moon.
116. Lunar Craters.--The smaller saucer-shaped formations
on the surface of the moon are called craters. They
// File: psp_120.png
.pn +1
are of all sizes, from a mile to a hundred and fifty miles in
diameter; and they are supposed to be of volcanic origin.
A high telescopic power shows that these craters vary remarkably,
not only in size, but also in structure and arrangement.
Some are considerably elevated above the surrounding
surface, others are basins hollowed out of that surface, and
with low surrounding ramparts; some are like walled plains,
while the majority have their lowest depression considerably
below the surrounding surface; some are isolated upon the
plains, others are thickly crowded together, overlapping and
intruding upon each other; some have elevated peaks or
cones in their centres, and some are without these central
cones, while others, again, contain several minute craters
instead; some have their ramparts whole and perfect, others
have them broken or deformed, and many have them
divided into terraces, especially on their inner sides.
A typical lunar crater is shown in Fig. 136.
.if h
.il fn=fig136.png w=90% alt='Moon Surface'
.ca Fig. 136.
.if-
.if t
[Illustration: Fig. 136.]
.if-
It is not generally believed that any active volcanoes exist
on the moon at the present time, though some observers
have thought they discerned indications of such volcanoes.
.if h
.il fn=fig137.png w=90% alt='Copernicus Crater'
.ca Fig. 137.
.if-
.if t
[Illustration: Fig. 137.]
.if-
117. Copernicus.--This is one of the grandest of lunar
craters (Fig. 137). Although its diameter (forty-six miles)
is exceeded by others, yet, taken as a whole, it forms one of
the most impressive and interesting objects of its class.
Its situation, near the centre of the lunar disk, renders all
its wonderful details conspicuous, as well as those of objects
immediately surrounding it. Its vast rampart rises to upwards
of twelve thousand feet above the level of the plateau,
nearly in the centre of which stands a magnificent group of
cones, three of which attain a height of more than twenty-four
hundred feet.
Many ridges, or spurs, may be observed leading away from
the outer banks of the great rampart. Around the crater,
extending to a distance of more than a hundred miles on
every side, there is a complex network of bright streaks,
// File: psp_121.png
.pn +1
which diverge in all directions. These streaks do not
appear in the figure, nor are they seen upon the moon,
// File: psp_122.png
.pn +1
except at and near the full phase. They show conspicuously,
however, by their united lustre on the full moon.
This crater is seen just to the south-west of the large
dusky plain in the upper part of Fig. 132. This plain is
Mare Imbrium, and the mountain-chain seen a little to the
right of Copernicus is named the Apennines. Copernicus
is also seen in Fig. 135, a little to the left of the same
range.
Under circumstances specially favorable, myriads of comparatively
minute but perfectly formed craters may be observed
for more than seventy miles on all sides around
Copernicus. The district on the south-east side is specially
rich in these thickly scattered craters, which we have reason
to suppose stand over or upon the bright streaks.
// File: psp_123.png
.pn +1
118. Dark Chasms.--Dark cracks, or chasms, have been
observed on various parts of the moon's surface. They
sometimes occur singly, and sometimes in groups. They are
often seen to radiate from some central cone, and they
appear to be of volcanic origin. They have been called
canals and rills.
.if h
.il fn=fig138.png w=90% alt='Chasm'
.ca Fig. 138.
.if-
.if t
[Illustration: Fig. 138.]
.if-
One of the most remarkable groups of these chasms is
that to the west of the crater named Triesneker. The
crater and the chasms are shown in Fig. 138. Several of
these great cracks obviously diverge from a small crater near
the west bank of the great one, and they subdivide as they
extend from the apparent point of divergence, while they
are crossed by others. These cracks, or chasms, are nearly
// File: psp_124.png
.pn +1
a mile broad at the widest part, and, after extending full a
hundred miles, taper away till they become invisible.
.if h
.il fn=fig139.png w=90% alt='Mountains'
.ca Fig. 139.
.if-
.if t
[Illustration: Fig. 139.]
.if-
119. Mountain-Ranges.--There are comparatively few
mountain-ranges on the moon. The three most conspicuous
are those which partially enclose Mare Imbrium; namely,
the Apennines on the south, and the Caucasus and the
Alps on the east and north-east. The Apennines are the
most extended of these, having a length of about four hundred
// File: psp_125.png
.pn +1
and fifty miles. They rise gradually, from a comparatively
level surface towards the south-west, in the form of
innumerable small elevations, which increase in number and
height towards the north-east, where they culminate in a
range of peaks whose altitude and rugged aspect must form
one of the most terribly grand and romantic scenes which
// File: psp_126.png
.pn +1
imagination can conceive. The north-east face of the range
terminates abruptly in an almost vertical precipice; while
over the plain beneath, intensely black spire-like shadows
are cast, some of which at sunrise extend full ninety miles,
till they lose themselves in the general shading due to the
curvature of the lunar surface. Many of the peaks rise to
heights of from eighteen thousand to twenty thousand feet
above the plain at their north-east base (Fig. 139).
.if h
.il fn=fig140.png w=90% alt='Mountains'
.ca Fig. 140.
.if-
.if t
[Illustration: Fig. 140.]
.if-
Fig. 140 represents an ideal lunar landscape near the base
of such a lunar range. Owing to the absence of an atmosphere,
the stars will be visible in full daylight.
.if h
.il fn=fig141.png w=90% alt='Mountains'
.ca Fig. 141.
.if-
.if t
[Illustration: Fig. 141.]
.if-
120. The Valley of the Alps.--The range of the Alps
is shown in Fig. 141. The great crater at the north end of
this range is named Plato. It is seventy miles in diameter.
// File: psp_127.png
.pn +1
The most remarkable feature of the Alps is the valley
near the centre of the range. It is more than seventy-five
miles long, and about six miles wide at the broadest part.
When examined under favorable circumstances, with a high
magnifying power, it is seen to be a vast flat-bottomed
valley, bordered by gigantic mountains, some of which
attain heights of ten thousand feet or more.
.if h
.il fn=fig142.png w=90% alt='Mountains'
.ca Fig. 142.
.if-
.if t
[Illustration: Fig. 142.]
.if-
121. Isolated Peaks.--There are comparatively few
isolated peaks to be found on the surface of the moon.
One of the most remarkable of these is that known as Pico,
and shown in Fig. 142. Its height exceeds eight thousand
feet, and it is about three times as long at the base as it is
broad. The summit is cleft into three peaks, as is shown
by the three-peaked shadow it casts on the plain.
122. Bright Rays.--About the time of full moon, with
a telescope of moderate power, a number of bright lines
may be seen radiating from several of the lunar craters,
extending often to the distance of hundreds of miles.
These streaks do not arise from any perceptible difference
of level of the surface, they have no very definite outline,
// File: psp_128.png
.pn +1
and they do not present any sloping sides to catch more
sunlight, and thus shine brighter, than the general surface.
Indeed, one great peculiarity of them is, that they come out
most forcibly when the sun is shining perpendicularly upon
them: hence they are best seen when the moon is at full,
and they are not visible at all at those regions upon which
the sun is rising or setting. They are not diverted by elevations
in their path, but traverse in their course craters,
mountains, and plains alike, giving a slight additional brightness
to all objects over which they pass, but producing no
// File: psp_129.png
.pn +1
other effect upon them. "They look as if, after the whole
surface of the moon had assumed its final configuration, a
vast brush charged with a whitish pigment had been drawn
over the globe in straight lines, radiating from a central
point, leaving its trail upon every thing it touched, but
obscuring nothing."
.if h
.il fn=fig143.png w=90% alt='Mountains'
.ca Fig. 143.
.if-
.if t
[Illustration: Fig. 143.]
.if-
The three most conspicuous craters from which these
lines radiate are Tycho, Copernicus, and Kepler. Tycho is
seen at the bottom of Figs. 143 and 130. Kepler is a little
to the left of Copernicus in the same figures.
It has been thought that these bright streaks are chasms
which have been filled with molten lava, which, on cooling,
would afford a smooth reflecting surface on the top.
123. Tycho.--This crater is fifty-four miles in diameter,
and about sixteen thousand feet deep, from the highest ridge
of the rampart to the surface of the plateau, whence rises a
central cone five thousand feet high. It is one of the most
conspicuous of all the lunar craters; not so much on
account of its dimensions as from its being the centre from
whence diverge those remarkable bright streaks, many of
which may be traced over a thousand miles of the moon's
surface (Fig. 143). Tycho appears to be an instance of a
vast disruptive action which rent the solid crust of the moon
into radiating fissures, which were subsequently filled with
molten matter, whose superior luminosity marks the course
of the cracks in all directions from the crater as their
common centre. So numerous are these bright streaks
when examined by the aid of the telescope, and they give
to this region of the moon's surface such increased luminosity,
that, when viewed as a whole, the locality can be
distinctly seen at full moon by the unassisted eye, as a
bright patch of light on the southern portion of the disk.
// File: psp_130.png
.pn +1
.sp 2
.h3 id='planets'
III. INFERIOR AND SUPERIOR PLANETS.
.sp 2
.h4 id='inferior-planets'
Inferior Planets.
.sp 2
124. The Inferior Planets.--The inferior planets are
those which lie between
the earth and
the sun, and whose
orbits are included
by that of the earth.
They are Mercury
and Venus.
.if h
.il fn=fig144.png w=70% alt='Inferior Planets'
.ca Fig. 144.
.if-
.if t
[Illustration: Fig. 144.]
.if-
125. Aspects of an
Inferior Planet.--The
four chief aspects
of an inferior planet
as seen from the
earth are shown in
Fig. 144, in which S
represents the sun, P the planet, and E the earth.
When the planet is
between the earth and
the sun, as at P, it
is said to be in inferior
conjunction.
When it is in the
same direction as the
sun, but beyond it,
as at P'', it is said to
be in superior conjunction.
When the planet is
at such a point in
its orbit that a line
drawn from the earth to it would be tangent to the orbit,
as at P' and P''', it is said to be at its greatest elongation.
// File: psp_131.png
.pn +1
.if h
.il fn=fig145.png w=70% alt='Inferior Planet Motion'
.ca Fig. 145.
.if-
.if t
[Illustration: Fig. 145.]
.if-
126. Apparent Motion of an Inferior Planet.--When the
planet is at P, if it could be seen at all, it would appear
in the heavens at A. As it moves from P to P', it will
appear to move in the heavens from A to B. Then, as it
moves from P' to P'', it will appear to move back again
from B to A. While it moves from P'' to P''', it will appear
to move from A to C; and, while moving from P''' to P,
it will appear to move back again from C to A. Thus the
planet will appear to oscillate to and fro across the sun from
B to C, never getting farther from the sun than B on the
west, or C on the east: hence, when at these points, it is
said to be at its greatest western and eastern elongations.
This oscillating motion of an inferior planet across the sun,
combined with the sun's motion among the stars, causes the
// File: psp_132.png
.pn +1
planet to describe a path among the stars similar to that
shown in Fig. 145.
.if h
.il fn=fig146.png w=70% alt='Inferior Planet Motion'
.ca Fig. 146.
.if-
.if t
[Illustration: Fig. 146.]
.if-
127. Phases of an Inferior Planet.--An inferior planet,
when viewed with a telescope, is found to present a succession
of phases similar to those of the moon. The reason
of this is evident from Fig. 146. As an inferior planet
passes around the sun, it presents sometimes more and
sometimes less of its bright hemisphere to the earth. When
the earth is at T, and Venus at superior conjunction, the
planet turns the whole of its bright hemisphere towards the
earth, and appears full; it then becomes gibbous, half, and
crescent. When it comes into inferior conjunction, it turns
its dark hemisphere towards
the earth: it then becomes
crescent, half, gibbous, and
full again.
128. The Sidereal and
Synodical Periods of an Inferior
Planet.--The time it
takes a planet to make a
complete revolution around
the sun is called the sidereal period of the planet;
and the time it takes it to pass from one aspect around to
the same aspect again, its synodical period.
.if h
.il fn=fig147.png w=70% alt='Inferior Planet Motion'
.ca Fig. 147.
.if-
.if t
[Illustration: Fig. 147.]
.if-
The synodical period of an inferior planet is longer than
its sidereal period. This will be evident from an examination
of Fig. 147. S is the position of the sun, E that of
the earth, and P that of the planet at inferior conjunction.
Before the planet can be in inferior conjunction
again, it must pass entirely around its orbit, and overtake
the earth, which has in the mean time passed on in its orbit
to E'.
While the earth is passing from E to E', the planet
passes entirely around its orbit, and from P to P' in addition.
// File: psp_133.png
.pn +1
Now the arc PP' is just equal to the arc EE': hence the
planet has to pass over the same arc that the earth does,
and 360° more. In other words, the planet has to gain
360° on the earth.
The synodical period of the planet is found by direct
observation.
.pm letter-start
129. The Length of the Sidereal Period.--The length of
the sidereal period of an inferior planet may be found by the
following computation:--
.pm verse-start
Let a denote the synodical period of the planet,
Let b denote the sidereal period of the earth,
Let x denote the sidereal period of the planet.
Then 360°/b = the daily motion of the earth,
And 360°/x = the daily motion of the planet,
And 360°/x - 360°/b = the daily gain of the planet:
Also 360°/a = the daily gain of the planet:
Hence 360°/x - 360°/b = 360°/a.
Dividing by 360°, we have 1/x - 1/b = 1/a;
Clearing of fractions, we have ab - ax = bx:
Transposing and collecting, we have (a + b)x = ab:
Therefore x = ab/a+b.
.pm verse-end
130. The Relative Distance of an Inferior Planet.--By the
relative distance of a planet, we mean its distance from the sun
compared with the earth's distance from the sun. The relative
distance of an inferior planet may be found by the following
method:--
.pm letter-end
.if h
.il fn=fig148.png w=70% alt='Inferior Planet Motion'
.ca Fig. 148.
.if-
.if t
[Illustration: Fig. 148.]
.if-
.pm letter-start
Let V, in Fig. 148, represent the position of Venus at its
greatest elongation from the sun, S the position of the sun,
and E that of the earth. The line EV will evidently be tangent
to a circle described about the sun with a radius equal
to the distance of Venus from the sun at the time of this greatest
// File: psp_134.png
.pn +1
elongation. Draw the radius SV and the line SE. Since
SV is a radius, the angle at V is a right angle. The angle
at E is known by measurement, and the angle at S is equal to
90°- the angle E. In the right-angled triangle EVS, we then
know the three angles, and we wish to find the ratio of the
side SV to the side SE.
The ratio of these lines may be found by trigonometrical
computation as follows:--
.pm verse-start
VS : ES = sin SEV : 1.
.pm verse-end
Substitute the value of the sine of SEV, and we have
.pm verse-start
VS : ES = .723 : 1.
.pm verse-end
Hence the relative distances of Venus and of the earth from
the sun are .723 and 1.
.pm letter-end
.sp 2
.h4 id='superior-planets'
Superior Planets.
.sp 2
131. The Superior Planets.--The superior planets are
those which lie beyond the earth. They are Mars, the
Asteroids, Jupiter, Saturn, Uranus, and Neptune.
.if h
.il fn=fig149.png w=70% alt='Superior Planet Motion'
.ca Fig. 149.
.if-
.if t
[Illustration: Fig. 149.]
.if-
132. Apparent Motion of a Superior Planet.--In order to
deduce the apparent motion of a superior
planet from the real motions of the earth
and planet, let S (Fig. 149) be the place
of the sun; 1, 2, 3, etc., the orbit of the
earth; a, b, c, etc., the orbit of Mars; and
CGL a part of the starry firmament. Let
the orbit of the earth be divided into
twelve equal parts, each described in one
month; and let ab, bc, cd, etc., be the
spaces described by Mars in the same
time. Suppose the earth to be at the
point 1 when Mars is at the point a, Mars
will then appear in the heavens in the
direction of 1 a. When the earth is at
3, and Mars at c, he will appear in the
heavens at C. When the earth arrives
at 4, Mars will arrive at d, and will appear in the heavens at D.
While the earth moves from 4 to 5 and from 5 to 6, Mars will
// File: psp_135.png
.pn +1
appear to have advanced among the stars from D to E and
from E to F,
in the direction
from west to
east. During
the motion of
the earth from 6
to 7 and from 7
to 8, Mars will
appear to go
backward from
F to G and
from G to H,
in the direction
from east to
west. During
the motion of
the earth from 8
to 9 and from 9
to 10, Mars will
appear to advance from H to I and from I to K, in the direction
from west to east,
and the motion will
continue in the
same direction until
near the succeeding
opposition.
The apparent motion
of a superior
planet projected on
the heavens is thus
seen to be similar
to that of an inferior
planet, except
that, in the latter
case, the retrogression
takes place near
inferior conjunction, and in the former it takes place near
opposition.
// File: psp_136.png
.pn +1
.if h
.il fn=fig150.png w=60% alt='Superior Planet Motion'
.ca Fig. 150.
.if-
.if t
[Illustration: Fig. 150.]
.if-
133. Aspects of a Superior Planet.--The four aspects of
a superior planet are shown in Fig. 150, in which S is the
position of the sun, E that of the earth, and P that of the
planet.
When the planet is on the opposite side of the earth to
the sun, as at P, it is said to be in opposition. The sun
and the planet will then appear in opposite parts of the
heavens, the sun appearing at C, and the planet at A.
When the planet is on the opposite side of the sun to
the earth, as at P'', it is said to be in superior conjunction.
It will then appear in the same part of the heavens as the
sun, both appearing at C.
When the planet is at P' and P''', so that a line drawn
from the earth through the planet will make a right angle
with a line drawn from the earth to the sun, it is said to be
// File: psp_137.png
.pn +1
in quadrature. At P' it is in its western quadrature, and at
P''' in its eastern quadrature.
.if h
.il fn=fig151.png w=60% alt='Superior Planet Motion'
.ca Fig. 151.
.if-
.if t
[Illustration: Fig. 151.]
.if-
134. Phases of a Superior Planet.--Mars is the only
one of the superior planets that has appreciable phases.
At quadrature, as will appear from Fig. 151, Mars does not
present quite the same side to the earth as to the sun:
hence, near these parts of its orbit, the planet appears slightly
gibbous. Elsewhere in its orbit, the planet appears full.
All the other superior planets are so far away from the
sun and earth, that the sides which they turn towards the
sun and the earth in every part of their orbit are so nearly
the same, that no change in the form of their disks can be
detected.
135. The Synodical Period
of a Superior Planet.--During
a synodical period of a
superior planet the earth must
gain one revolution, or 360°,
on the planet, as will be evident
from an examination of
Fig. 152, in which S represents
the sun, E the earth,
and P the planet at opposition.
Before the planet can be in opposition again, the
earth must make a complete revolution, and overtake the
planet, which has in the mean time passed on from P to P'.
.if h
.il fn=fig152.png w=60% alt='Superior Planet Motion'
.ca Fig. 152.
.if-
.if t
[Illustration: Fig. 152.]
.if-
In the case of most of the superior planets the synodical
period is shorter than the sidereal period; but in the case
of Mars it is longer, since Mars makes more than a complete
revolution before the earth overtakes it.
The synodical period of a superior planet is found by
direct observation.
.pm letter-start
136. The Sidereal Period of a Superior Planet.--The
sidereal period of a superior planet is found by a method of
computation similar to that for finding the sidereal period of
an inferior planet:--
// File: psp_138.png
.pn +1
.pm verse-start
Let a denote the synodical period of the planet,
Let b denote the sidereal period of the earth,
Let x denote the sidereal period of the planet.
Then will 360°/b = daily motion of the earth,
And 360°/x = daily motion of the planet;
Also 360°/b - 360°/x = daily gain of the earth.
But 360°/a = daily gain of the earth:
Hence 360°/b - 360°/x = 360°/a
1/b - 1/x = 1/a
ax - ab = bx
(a-b)x = ab
x = ab/(a-b).
.pm verse-end
.pm letter-end
.if h
.il fn=fig153.png w=60% alt='Superior Planet Distance'
.ca Fig. 153.
.if-
.if t
[Illustration: Fig. 153.]
.if-
.pm letter-start
137. The Relative Distance of a Superior Planet.--Let
S, e, and m, in Fig. 153, represent the relative positions of the
sun, the earth, and Mars, when the latter planet is in opposition.
Let E and M represent the relative positions of the
earth and Mars the day after opposition. At the first observation
Mars will be seen in the direction emA, and at the second
observation in the direction EMA.
But the fixed stars are so distant, that if a line, eA, were
drawn to a fixed star at the first observation, and a line, EB,
drawn from the earth to the same fixed star at the second
// File: psp_139.png
.pn +1
observation, these two lines would be sensibly parallel; that is,
the fixed star would be seen in the direction of the line eA at
the first observation, and in the direction of the line EB, parallel
to eA, at the second observation. But if Mars were seen
in the direction of the fixed star at the first observation, it would
appear back, or west, of that star at the second observation by
the angular distance BEA; that is, the planet would have
retrograded that angular distance. Now, this retrogression of
Mars during one day, at the time of opposition, can be measured
directly by observation. This measurement gives us the
value of the angle BEA; but we know the rate at which both
the earth and Mars are moving in their orbits, and from this
we can easily find the angular distance passed over by each in
one day. This gives us the angles ESA and MSA. We can
now find the relative length of the lines MS and ES (which
represent the distances of Mars and of the earth from the sun),
both by construction and by trigonometrical computation.
Since EB and eA are parallel, the angle EAS is equal
to BEA.
.pm verse-start
SEA = 180° - (ESA + EAS)
ESM = ESA - MSA
EMS = 180° - (SEA + ESM).
.pm verse-end
We have then
.pm verse-start
MS : ES = sin SEA : sin EMS.
.pm verse-end
Substituting the values of the sines, and reducing the ratio
to its lowest terms, we have
.pm verse-start
MS : ES = 1.524 : 1.
.pm verse-end
Thus we find that the relative distances of Mars and the
earth from the sun are 1.524 and 1. By the simple observation
of its greatest elongation, we are able to determine the relative
distances of an inferior planet and the earth from the sun;
and, by the equally simple observation of the daily retrogression
of a superior planet, we can find the relative distances of
such a planet and the earth from the sun.
.pm letter-end
// File: psp_140.png
.pn +1
.sp 2
.h3 id='sun'
IV. THE SUN.
.sp 2
.h4 id='sun-magnitude'
I. MAGNITUDE AND DISTANCE OF THE SUN.
.sp 2
.if h
.il fn=fig154.png w=60% alt='Solar System'
.ca Fig. 154.
.if-
.if t
[Illustration: Fig. 154.]
.if-
138. The Volume of the Sun.--The apparent diameter
of the sun is about 32', being a little greater than that of
the moon. The real diameter of the sun is 866,400 miles,
or about a hundred and nine times that of the earth.
As the diameter of the moon's orbit is only about 480,000
miles, or some
sixty times the
diameter of the
earth, it follows
that the diameter
of the sun is
nearly double that
of the moon's
orbit: hence, were
the centre of the
sun placed at the
centre of the earth,
the sun would
completely fill the
moon's orbit, and
reach nearly as
far beyond it in every direction as it is from the earth to
the moon. The circumference of the sun as compared with
the moon's orbit is shown in Fig. 154.
The volume of the sun is 1,305,000 times that of the
earth.
139. The Mass of the Sun.--The sun is much less
dense than the earth. The mass of the sun is only 330,000
times that of the earth, and its density only about a fourth
that of the earth.
.pm letter-start
To find the mass of the sun, we first ascertain the distance
// File: psp_141.png
.pn +1
the earth would draw the moon towards itself in a given time,
were the moon at the distance of the sun, and then form the
proportion: as the distance the earth would draw the moon
towards itself is to the distance that the sun draws the earth
towards itself in the same time, so is the mass of the earth to
the mass of the sun.
.pm letter-end
Although the mass of the sun is over three hundred thousand
times that of the earth, the pull of gravity at the surface
of the sun is only about twenty-eight times as great as at
the surface of the earth. This is because the distance from
the surface of the sun to its centre is much greater than
from the surface to the centre of the earth.
.if h
.il fn=fig155.png w=80% alt='Sizes of Sun and Planets'
.ca Fig. 155.
.if-
.if t
[Illustration: Fig. 155.]
.if-
140. Size of the Sun Compared with that of the Planets.--The
size of the sun compared with that of the larger
// File: psp_142.png
.pn +1
planets is shown in Fig. 155. The mass of the sun is more
than seven hundred and fifty times that of all of the planets
and moons in the solar system. In Fig. 156 is shown the
apparent size of the sun as seen from the different planets.
The apparent diameter of the sun decreases as the distance
from it increases, and the disk of the sun decreases as the
square of the distance from it increases.
.if h
.il fn=fig156.png w=60% alt='Sizes of Sun and Planets'
.ca Fig. 156.
.if-
.if t
[Illustration: Fig. 156.]
.if-
141. The Distance of the Sun.--The mean distance
// File: psp_143.png
.pn +1
of the sun from the earth is about 92,800,000 miles. Owing
to the eccentricity of the earth's orbit, the distance of the
sun varies somewhat; being about 3,000,000 miles less in
January, when the earth is at perihelion, than in June, when
the earth is at aphelion.
.pm letter-start
"But, though the distance of the sun can easily be stated in
figures, it is not possible to give any real idea of a space so
enormous: it is quite beyond our power of conception. If one
were to try to walk such a distance, supposing that he could
walk four miles an hour, and keep it up for ten hours every day,
it would take sixty-eight years and a half to make a single
million of miles, and more than sixty-three hundred years to
traverse the whole.
"If some celestial railway could be imagined, the journey to
the sun, even if our trains ran sixty miles an hour day and night
and without a stop, would require over a hundred and seventy-five
years. Sensation, even, would not travel so far in a human
lifetime. To borrow the curious illustration of Professor Mendenhall,
if we could imagine an infant with an arm long enough
to enable him to touch the sun and burn himself, he would die
of old age before the pain could reach him; since, according
to the experiments of Helmholtz and others, a nervous shock
is communicated only at the rate of about a hundred feet per
second, or 1,637 miles a day, and would need more than a hundred
and fifty years to make the journey. Sound would do it
in about fourteen years, if it could be transmitted through celestial
space; and a cannon-ball in about nine, if it were to move
uniformly with the same speed as when it left the muzzle of
the gun. If the earth could be suddenly stopped in her orbit,
and allowed to fall unobstructed toward the sun, under the
accelerating influence of his attraction, she would reach the
centre in about four months. I have said if she could be
stopped; but such is the compass of her orbit, that, to make
its circuit in a year, she has to move nearly nineteen miles a
second, or more than fifty times faster than the swiftest rifle-ball;
and, in moving twenty miles, her path deviates from perfect
straightness by less than an eighth of an inch. And yet,
over all the circumference of this tremendous orbit, the sun
// File: psp_144.png
.pn +1
exercises his dominion, and every pulsation of his surface
receives its response from the subject earth."
(Professor C. A. Young: The Sun.)
142. Method of Finding the Sun's Distance.--There are
several methods of finding the sun's distance. The simplest
method is that of finding the actual distance of one of the
nearer planets by observing its displacement in the sky as seen
from widely separated points on the earth. As the relative
distances of the planets from each other and from the sun are
well known, we can easily deduce the actual distance of the
sun if we can find that of any of the planets. The two planets
usually chosen for this method are Mars and Venus.
(1) The displacement of Mars in the sky, as seen from two
observatories which differ considerably in latitude, is, of course,
greatest when Mars is nearest the earth. Now, it is evident
than Mars will be nearer the earth when in opposition than
when in any other part of its orbit; and the planet will be least
distant from the earth when it is at its perihelion point, and the
earth is at its aphelion point, at the time of opposition. This
method, then, can be used to the best advantage, when, at the
time of opposition, Mars is near its perihelion, and the earth
near its aphelion. These favorable oppositions occur about
once in fifteen years, and the last one was in 1877.
.pm letter-end
.if h
.il fn=fig157.png w=90% alt='Distance of Sun'
.ca Fig. 157.
.if-
.if t
[Illustration: Fig. 157.]
.if-
.pm letter-start
Suppose two observers situated at N' and S' (Fig. 157), near
the poles of the earth. The one at N' would see Mars in the
sky at N, and the one at S' would see it at S. The displacement
would be the angle NMS. Each observer measures
carefully the distance of Mars from the same fixed star near
it. The difference of these distances gives the displacement
of the planet, or the angle NMS. These observations were
made with the greatest care in 1877.
// File: psp_145.png
.pn +1
(2) Venus is nearest the earth at the time of inferior conjunction;
but it can then be seen only in the daytime. It is,
therefore, impossible to ascertain the displacement of Venus,
as seen from different stations, by comparing her distances
from a fixed star. Occasionally, at the time of inferior conjunction,
Venus passes directly across the sun's disk. The
last of these transits of Venus occurred in 1874, and the next
will occur in 1882. It will then be over a hundred years before
another will occur.
.pm letter-end
.if h
.il fn=fig158.png w=90% alt='Distance of Sun'
.ca Fig. 158.
.if-
.if t
[Illustration: Fig. 158.]
.if-
.pm letter-start
Suppose two observers, A and B (Fig. 158), near the poles
of the earth at the time of a transit of Venus. The observer
at A would see Venus crossing the sun at V_{2}, and the one at
B would see it crossing at V_{1}. Any observation made upon
Venus, which would give the distance and direction of Venus
from the centre of the sun, as seen from each station, would
enable us to calculate the angular distance between the two
chords described across the sun. This, of course, would give
the displacement of Venus on the sun's disk. This method was
first employed at the last transits of Venus which occurred
before 1874; namely, those of 1761 and 1769.
There are three methods of observation employed to ascertain
the apparent direction and distance of Venus from the
centre of the sun, called respectively the contact method, the
micrometric method, and the photographic method.
(a) In the contact method, the observation consists in noting
the exact time when Venus crosses the sun's limb. To ascertain
// File: psp_146.png
.pn +1
this it is necessary to observe the exact time of external
and internal contact. This observation, though apparently
simple, is really very difficult. With reference to this method
Professor Young says,--
"The difficulties depend in part upon the imperfections of
optical instruments and the human eye, partly upon the essential
nature of light leading to what is known as diffraction, and
partly upon the action of the planet's atmosphere. The two
first-named causes produce what is called irradiation, and operate
to make the apparent diameter of the planet, as seen on the
solar disk, smaller than it really is; smaller, too, by an amount
which varies with the size of the telescope, the perfection of
its lenses, and the tint and brightness of the sun's image.
The edge of the planet's
image is also rendered
slightly hazy and
indistinct.
.pm letter-end
.if h
.il fn=fig159.png w=90% alt='Distance of Sun'
.ca Fig. 159.
.if-
.if t
[Illustration: Fig. 159.]
.if-
.pm letter-start
"The planet's atmosphere
also causes
its disk to be surrounded
by a narrow
ring of light, which
becomes visible long
before the planet touches the sun, and, at the moment of internal
contact, produces an appearance, of which the accompanying
figure is intended to give an idea, though on an exaggerated
scale. The planet moves so slowly as to occupy more than
twenty minutes in crossing the sun's limb; so that even if the
planet's edge were perfectly sharp and definite, and the sun's
limb undistorted, it would be very difficult to determine the
precise second at which contact occurs. But, as things are,
observers with precisely similar telescopes, and side by side,
often differ from each other five or six seconds; and, where the
telescopes are not similar, the differences and uncertainties are
much greater.... Astronomers, therefore, at present are
pretty much agreed that such observations can be of little value
in removing the remaining uncertainty of the parallax, and are
disposed to put more reliance upon the micrometric and photographic
methods, which are free from these peculiar difficulties,
// File: psp_147.png
.pn +1
though, of course, beset with others, which, however, it is
hoped will prove less formidable."
(b) Of the micrometric method, as employed at the last
transit, Professor Young speaks as follows:--
"The micrometric method requires the use of a heliometer,--an
instrument common only in Germany, and requiring much
skill and practice in its use in order to obtain with it accurate
measures. At the late transit, a single English party, two or
three of the Russian parties, and all five of the German, were
equipped with these instruments; and at some of the stations
extensive series of measures were made. None of the results,
however, have appeared as yet; so that it is impossible to say
how greatly, if at all, this method will have the advantage in
precision over the contact observations."
(c) The following observations, with reference to the photographic
method, are also taken from Professor Young:--
"The Americans and French placed their main reliance upon
the photographic method, while the English and Germans also
provided for its use to a certain extent. The great advantage
of this method is, that it makes it possible to perform the
necessary measurements (upon whose accuracy every thing
depends) at leisure after the transit, without hurry, and with all
possible precautions. The field-work consists merely in obtaining
as many and as good pictures as possible. A principal
objection to the method lies in the difficulty of obtaining good
pictures, i.e., pictures free from distortion, and so distinct and
sharp as to bear high magnifying power in the microscopic
apparatus used for their measurement. The most serious difficulty,
however, is involved in the accurate determination of the
scale of the picture; that is, of the number of seconds of arc
corresponding to a linear inch upon the plate. Besides this,
we must know the exact Greenwich time at which each picture
is taken, and it is also extremely desirable that the orientation
of the picture should be accurately determined; that is, the
north and south, the east and west points of the solar image on
the finished plate. There has been a good deal of anxiety lest
the image, however accurate and sharp when first produced,
should alter, in course of time, through the contraction of the
collodion film on the glass plate; but the experiments of
// File: psp_148.png
.pn +1
Rutherfurd, Huggins, and Paschen, seem to show that this
danger is imaginary.... The Americans placed the photographic
telescope exactly in line with a meridian instrument,
and so determined, with the extremest precision, the direction
in which it was pointed. Knowing this and the time at which
any picture was taken, it becomes possible, with the help of
the plumb-line image, to determine precisely the orientation of
the picture,--an advantage possessed by the American pictures
alone, and making their value nearly twice as great as otherwise
it would have been.
"The figure below is a representation of one of the American
photographs reduced
about one-half.
V is the
image of Venus,
which, on the actual
plate, is about a
seventh of an inch
in diameter; aa' is
the image of the
plumb-line. The
centre of the reticle
is marked with a
cross."
.pm letter-end
.if h
.il fn=fig160.png w=90% alt='Examination of Sun'
.ca Fig. 160.
.if-
.if t
[Illustration: Fig. 160.]
.if-
.pm letter-start
The English photographs
proved to
be of little value,
and the results of
the measurements and calculations upon the American pictures
have not yet been published. There is a growing apprehension
that no photographic method can be relied upon.
.pm letter-end
The most recent determinations by various methods indicate
that the sun's distance is such that his parallax is about
eighty-eight seconds. This would make the linear value of
a second at the surface of the sun about four hundred and
fifty miles.
// File: psp_148p.png
.if h
.il fn=plate1.png w=90% alt='Photograph of Sun'
.ca Plate 1.
.if-
.if t
[Illustration: Plate 1.]
.if-
// File: psp_149.png
.pn +1
.sp 2
.h4 id='sun-condition'
II. PHYSICAL AND CHEMICAL CONDITION OF THE SUN.
.sp 2
.h4 id='sun-physical'
Physical Condition of the Sun.
.sp 2
143. The Sun Composed mainly of Gas.--It is now
generally believed that the sun is mainly a ball of gas, or
vapor, powerfully condensed at the centre by the weight
of the superincumbent mass, but kept from liquefying by its
exceedingly high temperature.
The gaseous interior of the sun is surrounded by a layer
of luminous clouds, which constitutes its visible surface, and
which is called its photosphere. Here and there in the
photosphere are seen dark spots, which often attain an
immense magnitude.
These clouds float in the solar atmosphere, which extends
some distance beyond them.
The luminous surface of the sun is surrounded by a rose-colored
stratum of gaseous matter, called the chromosphere.
Here and there great masses of this chromospheric matter
rise high above the general level. These masses are called
prominences.
Outside of the chromosphere is the corona, an irregular
halo of faint, pearly light, mainly composed of filaments
and streamers, which radiate from the sun to enormous distances,
often more than a million of miles.
In Fig. 161 is shown a section of the sun, according to
Professor Young.
The accompanying lithographic plate gives a general view
of the photosphere with its spots, and of the chromosphere
and its prominences.
144. The Temperature of the Sun.--Those who have
investigated the subject of the temperature of the sun have
come to very different conclusions; some placing it as high
as four million degrees Fahrenheit, and others as low as
ten thousand degrees. Professor Young thinks that Rosetti's
// File: psp_150.png
.pn +1
estimate of eighteen thousand degrees as the effective temperature
of the sun's surface is probably not far from
correct. By this is meant the temperature that a uniform
surface of lampblack of the size of the sun must have in
order to radiate as much heat as the sun does. The most
intense artificial heat does not exceed four thousand degrees
Fahrenheit.
.if h
.il fn=fig161.png w=70% alt='Section of Sun'
.ca Fig. 161.
.if-
.if t
[Illustration: Fig. 161.]
.if-
145. The Amount of Heat Radiated by the Sun.--A
unit of heat is the amount of heat required to raise a
pound of water one degree in temperature. It takes about
a hundred and forty-three units of heat to melt a pound of
ice without changing its temperature. A cubic foot of ice
weighs about fifty-seven pounds. According to Sir William
Herschel, were all the heat radiated by the sun concentrated
// File: psp_151.png
.pn +1
on a cylinder of ice forty-five miles in diameter, it would
melt it off at the rate of about a hundred and ninety thousand
miles a second.
Professor Young gives the following illustration of the
energy of solar radiation: "If we could build up a solid
column of ice from the earth to the sun, two miles and a
quarter in diameter, spanning the inconceivable abyss of
ninety-three million miles, and if then the sun should concentrate
his power upon it, it would dissolve and melt, not
in an hour, nor a minute, but in a single second. One
swing of the pendulum, and it would be water; seven more,
and it would be dissipated in vapor."
.if h
.il fn=fig162.png w=70% alt='Section of Sun'
.ca Fig. 162.
.if-
.if t
[Illustration: Fig. 162.]
.if-
This heat would be
sufficient to melt a layer
of ice nearly fifty feet
thick all around the sun
in a minute. To develop
this heat would require
the hourly consumption
of a layer of anthracite
coal, more than sixteen
feet thick, over the entire
surface of the sun; and the mechanical equivalent of this
heat is about ten thousand horse-power on every square
foot of the sun's surface.
146. The Brightness of the Sun's Surface.--The sun's
surface is a hundred and ninety thousand times as bright as
a candle-flame, a hundred and forty-six times as bright as
the calcium-light, and about three times and a half as bright
as the voltaic arc.
The sun's disk is much less bright near the margin than
near the centre, a point on the limb of the sun being only
about a fourth as bright as one near the centre of the disk.
This diminution of brightness towards the margin of the
disk is due to the increase in the absorption of the solar
// File: psp_152.png
.pn +1
atmosphere as we pass from the centre towards the margin
of the sun's disk; and this increased absorption is due to
the fact, that the rays which reach us from near the margin
have to traverse a much greater thickness of the solar
atmosphere than those which reach us from the centre of
the disk. This will be evident from Fig. 162, in which the
arrows mark the paths of rays from different parts of the
solar disk.
.sp 2
.h4 id='spectroscope'
The Spectroscope.
.sp 2
.if h
.il fn=fig163.png w=90% alt='Spectroscope'
.ca Fig. 163.
.if-
.if t
[Illustration: Fig. 163.]
.if-
147. The Spectroscope as an Astronomical Instrument.--The
spectroscope is now continually employed in the
study of the physical condition and chemical constitution
of the sun and of the other heavenly bodies. It has
become almost as indispensable to the astronomer as the
telescope.
148. The Dispersion Spectroscope.--The essential parts
of the dispersion spectroscope are shown in Fig. 163.
These are the collimator tube, the prism, and the telescope.
The collimator tube has a narrow slit at one end, through
which the light to be examined is admitted, and somewhere
within the tube a lens for condensing the light. The
light is dispersed on passing through the prism: it then
passes through the objective of the telescope, and forms
// File: psp_153.png
.pn +1
within the tube an image of the spectrum, which is examined
by means of the eye-piece. The power of the spectroscope
is increased by increasing the number of prisms,
which are arranged so
that the light shall pass
through one after another
in succession.
Such an arrangement
of prisms is shown in
Fig. 164. One end
of the collimator tube
is seen at the left, and
one end of the telescope
at the right.
Sometimes the prisms
are made long, and
the light is sent twice
through the same train
of prisms, once through
the lower, and once
through the upper, half of the prisms. This is accomplished
by placing a rectangular prism against the last
prism of the train,
as shown in Fig.
165.
.if h
.il fn=fig164.png w=80% alt='Spectroscope'
.ca Fig. 164.
.if-
.if t
[Illustration: Fig. 164.]
.if-
.if h
.il fn=fig165.png w=90% alt='Spectroscope'
.ca Fig. 165.
.if-
.if t
[Illustration: Fig. 165.]
.if-
149. The Micrometer
Scale.--Various
devices are
employed to obtain
an image of a micrometer
scale in
the tube of the
telescope beside that of the spectrum.
.if h
.il fn=fig166.png w=80% alt='Assembly'
.ca Fig. 166.
.if-
.if t
[Illustration: Fig. 166.]
.if-
One of the simplest of these methods is shown in
Fig. 166. A is the telescope, B the collimator, and C the
// File: psp_154.png
.pn +1
micrometer tube. The opening at the outer end of C contains
a piece of glass which has a micrometer scale marked
upon it. The light from the candle shines through this
glass, falls upon the surface of the prism P, and is thence
reflected into the telescope, where it forms an enlarged
image of the micrometer scale alongside the image of the
spectrum.
.if h
.il fn=fig167.png w=60% alt='Obtaining Spectra'
.ca Fig. 167.
.if-
.if t
[Illustration: Fig. 167.]
.if-
150. The Comparison of Spectra.--In order to compare
// File: psp_155.png
.pn +1
two spectra, it is desirable to be able to see them
side by side in the telescope. The images of two spectra
may be obtained side by side in the telescope tube by the
use of a little rectangular prism, which covers one-half
of the slit of the collimator tube, as shown in Fig. 167.
The light from one source is admitted directly through
the uncovered half of the slit, while the light from the
other source is sent
through the covered
portion of the slit
by reflection from
the surface of the
rectangular prism.
This arrangement and its action will be readily understood
from Fig. 167.
.if h
.il fn=fig168.png w=60% alt='Prisms'
.ca Fig. 168.
.if-
.if t
[Illustration: Fig. 168.]
.if-
151. Direct-Vision Spectroscope.--A beam of light may
be dispersed, without any ultimate deflection from its
course, by combining prisms of crown and flint glass with
equal refractive, but unequal dispersive powers. Such a
combination of prisms is called a direct-vision combination.
One of three prisms is shown in Fig. 168, and one of five
prisms in Fig. 169.
.if h
.il fn=fig169.png w=60% alt='Prisms'
.ca Fig. 169.
.if-
.if t
[Illustration: Fig. 169.]
.if-
.if h
.il fn=fig170.png w=90% alt='Spectroscope'
.ca Fig. 170.
.if-
.if t
[Illustration: Fig. 170.]
.if-
A direct-vision spectroscope (Fig. 170) is one in which
a direct-vision combination of prisms is employed. C is
the collimator tube, P the train of prisms, F the telescope,
and r the comparison prism.
.if h
.il fn=fig171.png w=60% alt='Telespectroscope'
.ca Fig. 171.
.if-
.if t
[Illustration: Fig. 171.]
.if-
152. The Telespectroscope.--The spectroscope, when
used for astronomical work, is usually combined with a
// File: psp_156.png
.pn +1
telescope. The compound instrument is called a telespectroscope.
The spectroscope is mounted at the end of the
telescope in such a way that the image formed by the
object-glass of the telescope falls upon the slit at the end
of the collimator tube. A telespectroscope of small dispersive
power is shown in Fig. 171; a being the object-glass
of the telescope, cc the tube of the telescope, and
e the comparison prism at the end of the collimator tube.
A more powerful instrument is shown in Fig. 172. A is
the telescope, C the collimator tube of the spectroscope,
// File: psp_157.png
.pn +1
P the train of prisms, and E the telescope tube. Fig. 173
shows a still more powerful spectroscope attached to the
great Newall refractor (18).
.if h
.il fn=fig172.png w=70% alt='Telespectroscope'
.ca Fig. 172.
.if-
.if t
[Illustration: Fig. 172.]
.if-
.if h
.il fn=fig173.png w=60% alt='Telespectroscope'
.ca Fig. 173.
.if-
.if t
[Illustration: Fig. 173.]
.if-
153. The Diffraction Spectroscope.--A diffraction spectroscope
is one in which the spectrum is produced by
reflection of the light from a finely ruled surface, or grating,
as it is called, instead of by dispersion in passing through a
// File: psp_158.png
.pn +1
prism. The essential parts of this instrument are shown
in Fig 174. This spectroscope may be attached to the
telescope in the same manner as the dispersion spectroscope.
When the spectroscope is thus used, the eye-piece of the
telescope is removed.
.if h
.il fn=fig174.png w=60% alt='Spectroscope'
.ca Fig. 174.
.if-
.if t
[Illustration: Fig. 174.]
.if-
.sp 2
.h4 id='sun-spectra'
Spectra.
.sp 2
154. Continuous Spectra.--Light from an incandescent
solid or liquid which has suffered no absorption in the
medium which it has traversed gives a spectrum consisting
of a continuous colored band, in which the colors, from
the red to the violet, pass gradually and imperceptibly into
one another. The spectrum is entirely free from either light
or dark lines, and is called a continuous spectrum.
155. Bright-Lined Spectra.--Light from a luminous gas
or vapor gives a spectrum composed of bright lines separated
by dark spaces, and known as a bright-lined spectrum.
It has been found that the lines in the spectrum of
a substance in the state of a gas or vapor are the most
characteristic thing about the substance, since no two vapors
give exactly the same lines: hence, when we have once
become acquainted with the bright-lined spectrum of any
substance, we can ever after recognize that substance by
the spectrum of its luminous vapor. Even when several
substances are mixed, they may all be recognized by the
bright-lined spectrum of the mixture, since the lines of
// File: psp_159.png
.pn +1
all the substances will be present in the spectrum of the
mixture. This method of identifying substances by their
spectra is called spectrum analysis.
The bright-lined spectra of several substances are given
in the frontispiece. The number of lines in the spectra of
the elements varies greatly. The spectrum of sodium is one
of the simplest, while that of iron is one of the most complex.
The latter contains over six hundred lines. Though
no two vapors give identical spectra, there are many cases
in which one or more of the spectral lines of one element
coincide in position with lines of other elements.
.pm letter-start
156. Methods of rendering Gases and Vapors Luminous.--In
order to study the spectra of vapors and gases it is necessary
to have some means of converting solids and liquids into
vapor, and also of rendering
the vapors and gases luminous.
There are four methods
of obtaining luminous
vapors and gases in common
use.
.pm letter-end
.if h
.il fn=fig175.png w=60% alt='Spectroscope'
.ca Fig. 175.
.if-
.if t
[Illustration: Fig. 175.]
.if-
(1) By means of the Bunsen
Flame.--This is a very
hot but an almost non-luminous
flame. If any
readily volatilized substance,
such as the compounds of
sodium, calcium, strontium,
etc., is introduced into this
flame on a fine platinum wire,
it is volatilized in the flame,
and its vapor is rendered
luminous, giving the flame its own peculiar color. The flame
thus colored may be examined by the spectroscope. The
arrangement of the flame is shown in Fig. 175.
.if h
.il fn=fig176.png w=60% alt='Electric Lamp'
.ca Fig. 176.
.if-
.if t
[Illustration: Fig. 176.]
.if-
(2) By means of the Voltaic Arc.--An electric lamp is
shown in Fig. 176. When this lamp is to be used for obtaining
luminous vapors, the lower carbon is made larger than the
// File: psp_160.png
.pn +1
upper one, and hollowed out at the top into a little cup. The
substance to be volatilized is placed in this cup, and the current
is allowed to pass. The heat of the voltaic arc is much more
intense than that of the Bunsen flame: hence substances that
cannot be volatilized
in the flame
are readily volatilized
in the arc,
and the vapor
formed is raised
to a very high
temperature.
(3) By means
of the Spark
from an Induction
Coil.--The
arrangement of
the coil for obtaining
luminous
vapors is shown
in Fig. 177.
.if h
.il fn=fig177.png w=60% alt='Induction Coil'
.ca Fig. 177.
.if-
.if t
[Illustration: Fig. 177.]
.if-
The terminals
of the coil between
which the
spark is to pass
are brought quite
close together.
When we wish
to vaporize any
metal, as iron,
the terminals are
made of iron.
On the passage
of the spark, a
little of the iron at the ends of the terminals is evaporated;
and the vapor is rendered luminous in the space traversed by
the spark. A condenser is usually placed in the circuit. With
the coil, the temperature may be varied at pleasure; and the
vapor may be raised even to a higher temperature than with
// File: psp_161.png
.pn +1
the electric lamp. To obtain a low temperature, the coil is
used without the condenser. By using a larger and larger
condenser, the temperature may be raised higher and higher.
By means of the induction coil we may also heat gases to
incandescence. It is only necessary to allow the spark to
pass through a space filled with the gas.
.if h
.il fn=fig178.png w=50% alt='Vacuum Tube'
.ca Fig. 178.
.if-
.if t
[Illustration: Fig. 178.]
.if-
(4) By means of a Vacuum Tube.--The form of the vacuum
tube commonly used for this purpose is shown in Fig. 178.
The gas to be examined, and which is contained in the tube,
has very slight density: but upon the passage of the discharge
from an induction
coil or a Holtz
machine, through
the tube, the gas
in the capillary
part of the tube
becomes heated to
a high temperature,
and is then
quite brilliant.
157. Reversed
Spectra.--If the
light from an incandescent
cylinder
of lime, or
from the incandescent point of an electric lamp, is allowed to pass
through luminous sodium vapor, and is then examined with
a spectroscope, the spectrum will be found to be a bright
spectrum crossed by a single dark line in the position of
the yellow line of the sodium vapor. The spectrum of
sodium vapor is reversed, its bright lines becoming dark
and its dark spaces bright. With a spectroscope of any
considerable power, the yellow line of sodium vapor is
resolved into a double line. With a spectroscope of the
same power, the dark sodium line of the reversed spectrum
is seen to be a double line.
// File: psp_162.png
.pn +1
It is found to be generally true, that the spectrum of the
light from an incandescent solid or liquid which has passed
through a luminous vapor on its way to the spectroscope
is made up of a bright ground crossed by dark lines; there
being a dark line for every bright line that the
vapor alone would give.
158. Explanation of Reversed Spectra.--It
has been found that gases absorb and quench rays
of the same degree of refrangibility as those
which they themselves emit, and no others.
When a solid is shining through a luminous
vapor, this absorbs and quenches those rays from
the solid which have the same degrees of refrangibility
as those which it is itself emitting: hence
the lines of the spectrum receive light from the
vapor alone, while the spaces between the lines
receive light from the solid. Now, solids and
liquids, when heated to incandescence, give a very
much brighter light than vapors and gases at the
same temperature: hence the lines of a reversed
spectrum, though receiving light from the vapor
or gas, appear dark by contrast.
159. Effect of Increasing the Power of the
Spectroscope upon the Brilliancy of a Spectrum.--An
increase in the power of a spectroscope
diminishes the brilliancy of a continuous spectrum,
since it makes the colored band longer, and
therefore spreads the light out over a greater
extent of surface; but, in the case of a bright-lined
spectrum, an increase of power in the spectroscope
produces scarcely any alteration in the
brilliancy of the lines, since it merely separates the lines farther
without making the lines themselves any wider. In the
case of a reversed spectrum, an increase of power in the spectroscope
dilutes the light in the spaces between the lines
without diluting that of the lines: hence lines which appear
dark in a spectroscope of slight dispersive power may appear
bright in an instrument of great dispersive power.
// File: psp_163.png
.pn +1
160. Change of the Spectrum with the Density of the Luminous
Vapor.--It has been found, that, as the density of a
luminous vapor is diminished, the lines in its spectrum become
fewer and fewer, till
they are finally reduced to
one. On the other hand, an
increase of density causes
new lines to appear in the
spectrum, and the old lines to
become thicker.
161. Change of the Spectrum
with the Temperature
of the Luminous Vapor.--It
has also been found that
the appearance of a bright-lined
spectrum changes considerably
with the temperature
of the luminous vapor.
In some cases, an increase
of temperature changes the
relative intensities of the
lines; in other cases, it causes
new lines to appear, and old
lines to disappear.
In the case of a compound
vapor, an increase
of temperature causes the
colored bands (which are
peculiar to the spectrum of
the compound) to disappear,
and to be replaced by the
spectral lines of the elements
of which the compound
is made up. The
heat appears to dissociate
the compound; that is, to resolve it into its constituent
elements. In this case, each elementary vapor would give its
own spectral lines. As the compound is not completely dissociated
at once, it is possible, of course, for one or more of
// File: psp_164.png
.pn +1
the spectral lines of the elementary vapors to co-exist in the
spectrum with the bands of the compound.
It has been found, that, in some cases, the spectra of the
elementary gases change with the temperature of the gas; and
Lockyer thinks he has discovered conclusive evidence, in the
spectra of the sun and stars, that many of the substances
regarded as elementary are really resolved into simpler substances
by the intense heat of the sun; in other words, that our
so-called elements are really compounds.
.sp 2
.h4 id='sun-chemical'
Chemical Constitution of the Sun.
.sp 2
162. The Solar Spectrum.--The solar spectrum is
crossed transversely by a great number of fine dark lines,
and hence it belongs to the class of reversed spectra.
These lines were first studied and mapped by Fraunhofer,
and from him they have been called Fraunhofer's
lines.
.if h
.il fn=fig179.png w=50% alt='Spectral Map'
.ca Fig. 179.
.if-
.if t
[Illustration: Fig. 179.]
.if-
.pm letter-start
A reduced copy of Fraunhofer's map is shown in Fig. 179.
A few of the most prominent of the dark solar lines are
designated by the letters of the alphabet. The other lines are
usually designated by the numbers at which they are found on
the scale which accompanies the map. This scale is usually
drawn at the top of the map, as will be seen in some of the
following diagrams. The two most elaborate maps of the solar
spectrum are those of Kirchhoff and Angström. The scale on
Kirchhoff's map is an arbitrary one, while that of Angström is
based upon the wave-lengths of the rays of light which would
fall upon the lines in the spectrum.
.pm letter-end
.if h
.il fn=fig180.png w=50% alt='Spectrum'
.ca Fig. 180.
.if-
.if t
[Illustration: Fig. 180.]
.if-
.pm letter-start
The appearance of the spectrum varies greatly with the
// File: psp_165.png
.pn +1
power of the spectroscope employed. Fig. 180 shows a portion
of the spectrum as it appears in a spectroscope of a single
prism: while Fig. 181 shows the b group of lines alone, as they
appear in a powerful diffraction spectroscope.
.pm letter-end
.if h
.il fn=fig181.png w=50% alt='Spectrum'
.ca Fig. 181.
.if-
.if t
[Illustration: Fig. 181.]
.if-
163. The Telluric Lines.--There are many lines of the
solar spectrum which vary considerably in intensity as the
sun passes from the horizon to the meridian, being most
intense when the sun is nearest the horizon, and when his
rays are obliged to pass through the greatest depth of the
earth's atmosphere. These lines are of atmospheric origin,
and are due to the absorption of the aqueous vapor in our
atmosphere. They are the same lines that are obtained
when a candle or other artificial light is examined with a
spectroscope through a long tube filled with steam. Since
these lines are due to the absorption of our own atmosphere,
they are called telluric lines. A map of these lines
is shown in Fig. 182.
.if h
.il fn=fig182.png w=50% alt='Spectrum'
.ca Fig. 182.
.if-
.if t
[Illustration: Fig. 182.]
.if-
164. The Solar Lines.--After deducting the telluric
lines, the remaining lines of the solar spectrum are of solar
origin. They must be due to absorption which takes place
in the sun's atmosphere. They are, in fact, the reversed
spectra of the elements which exist in the solar atmosphere
in the state of vapor: hence we conclude that the luminous
surface of the sun is surrounded with an atmosphere of
luminous vapors. The temperature of this atmosphere, at
// File: psp_166.png
.pn +1
least near the surface of the sun, must be sufficient to
enable all the elements known on the
earth to exist in it as vapors.
.if h
.il fn=fig183.png w=80% alt='Spectrum'
.ca Fig. 183.
.if-
.if t
[Illustration: Fig. 183.]
.if-
165. Chemical Constitution of the
Sun's Atmosphere.--To find whether
any element which exists on the earth
is present in the solar atmosphere, we
have merely to ascertain whether the
bright lines of its gaseous spectrum
are matched by dark lines in the
solar spectrum when the two spectra
are placed side by side. In Fig. 183,
we have in No. 1 a portion of the
red end of the solar spectra, and in
No. 2 the spectrum of sodium vapor,
both as obtained in the same spectroscope
by means of the comparison
prism. It will be seen that the
double sodium line is exactly matched
by a double dark line of the solar
spectrum: hence we conclude that
sodium vapor is present in the sun's
atmosphere. Fig. 184 shows the
matching of a great number of the
bright lines of iron vapor by dark
lines in the solar spectrum. This
matching of the iron lines establishes
the fact that iron vapor is present in
the solar atmosphere.
.if h
.il fn=fig184.png w=50% alt='Spectrum'
.ca Fig. 184.
.if-
.if t
[Illustration: Fig. 184.]
.if-
.pm letter-start
The following table (given by Professor
Young) contains a list of all the
elements which have, up to the present
time, been detected with certainty in
the sun's atmosphere. It also gives the number of bright lines
in the spectrum of each element, and the number of those
// File: psp_167.png
.pn +1
lines which have been matched by dark lines in the solar
spectrum:--
.pm letter-end
.ta l:14 r:6 r:15 l:10
| Elements.|Bright Lines.|Lines Reversed.|Observer.
1. Iron | 600 | 460 | Kirchhoff.
2. Titanium | 206 | 118 | Thalen.
3. Calcium | 89 | 75 | Kirchhoff.
4. Manganese | 75 | 57 | Angström.
5. Nickel | 51 | 33 | Kirchhoff.
6. Cobalt | 86 | 19 | Thalen.
7. Chromium | 71 | 18 | Kirchhoff.
8. Barium | 26 | 11 | Kirchhoff.
9. Sodium | 9 | 9 | Kirchhoff.
10. Magnesium | 7 | 7 | Kirchhoff.
11. Copper? | 15 | 7? | Kirchhoff.
12. Hydrogen | 5 | 5 | Angström.
13. Palladium | 29 | 5 | Lockyer.
14. Vanadium | 54 | 4 | Lockyer.
15. Molybdenum | 27 | 4 | Lockyer.
16. Strontium | 74 | 4 | Lockyer.
17. Lead | 41 | 3 | Lockyer.
18. Uranium | 21 | 3 | Lockyer.
19. Aluminium | 14 | 2 | Angström.
20. Cerium | 64 | 2 | Lockyer.
21. Cadmium | 20 | 2 | Lockyer.
22. Oxygen a | 42 | 12 ± bright | H. Draper.
Oxygen b | 4 | 4? | Schuster.
.ta-
.pm letter-start
In addition to the above elements, it is probable that several
other elements are present in the sun's atmosphere; since at
least one of their bright lines has been found to coincide with
dark lines of the solar spectrum. There are, however, a large
number of elements, no traces of which have yet been detected;
and, in the cases of the elements whose presence in the solar
atmosphere has been established, the matching of the lines is
far from complete in the majority of the cases, as will be seen
from the above table. This want of complete coincidence of
the lines is undoubtedly due to the very high temperature of
// File: psp_168.png
.pn +1
the solar atmosphere. We have already seen that the lines of
the spectrum change with the temperature; and, as the temperature
of the sun is far higher than any that we can produce
by artificial means, we might reasonably expect that it would
cause the disappearance from the spectrum of many lines
which we find to be present at our highest temperature.
Lockyer maintains that the reason why no trace of the spectral
lines of certain of our so-called elements is found in the
solar atmosphere is, that these substances are not really elementary,
and that the intense heat of the sun resolves them into
simpler constituents.
.pm letter-end
.sp 2
.h4 id='sun-surface'
Motion at the Surface of the Sun.
.sp 2
166. Change of Pitch caused by Motion of Sounding
Body.--When a sounding body is moving rapidly towards
us, the pitch of its note becomes somewhat higher than
when the body is stationary; and, when such a body is
moving rapidly from us, the pitch of its note is lowered
somewhat. We have a good illustration of this change of
pitch at a country railway station on the passage of an
express-train. The pitch of the locomotive whistle is considerably
higher when the train is approaching the station
than when it is leaving it.
167. Explanation of the Change of Pitch produced by
Motion.--The pitch of sound depends upon the rapidity
with which the pulsations of sound beat upon the drum of
// File: psp_169.png
.pn +1
the ear. The more rapidly the pulsations follow each other,
the higher is the pitch: hence
the shorter the sound-waves
(provided the sound is all the
while travelling at the same
rate), the higher the pitch of
the sound. Any thing, then,
which tends to shorten the waves
of sound tends also to raise its
pitch, and any thing which tends
to lengthen these waves tends to
lower its pitch.
When a sounding body is moving
rapidly forward, the sound-waves
are crowded together a
little, and therefore shortened;
when it is moving backward, the
sound-waves are drawn out, or
lengthened a little.
.pm letter-start
The effect of the motion of a
sounding body upon the length
of its sonorous waves will be
readily seen from the following
illustration: Suppose a number
of persons stationed at equal
intervals in a line on a long
platform capable of moving backward
and forward. Suppose the
men are four feet apart, and all
walking forward at the same
rate, and that the platform is
stationary, and that, as the men
leave the platform, they keep on
walking at the same rate: the men
will evidently be four feet apart in the line in front of the
platform, as well as on it. Suppose next, that the platform is
// File: psp_170.png
.pn +1
moving forward at the rate of one foot in the interval between
two men's leaving the platform, and that the men continue to
walk as before: it is evident that the men will then be three
feet apart in the line after they have left the platform. The
forward motion of the platform has the effect of crowding the
men together a little. Were the platform moving backward at
the same rate, the men would be five feet apart after they had
left the platform. The backward motion of the platform has
the effect of separating the men from one another.
The distance between the men in this illustration corresponds
to the length of the sound-wave, or the distance between
its two ends. Were a person to stand beside the line, and
count the men that passed him in the three cases given above,
he would find that more persons would pass him in the same
time when the platform is moving forward than when it is
stationary, and fewer persons would pass him in the same time
when the platform is moving backward than when it is stationary.
In the same way, when a sounding body is moving
rapidly forward, the sound-waves beat more rapidly upon the
ear of a person who is standing still than when the body is at
rest, and less rapidly when the sounding body is moving rapidly
backward.
Were the platform stationary, and were the person who is
counting the men to be walking along the line, either towards
or away from the platform, the effect upon the number of men
passing him in a given time would be precisely the same as it
would be were the person stationary, and the platform moving
either towards or away from him at the same rate. So the
change in the rapidity with which pulsations of sound beat upon
the ear is precisely the same whether the ear is stationary and
the sounding body moving, or the sounding body is stationary
and the ear moving.
.pm letter-end
168. Change of Refrangibility due to the Motion of a
Luminous Body.--Refrangibility in light corresponds to
pitch in sound, and depends upon the length of the luminous
waves. The shorter the luminous waves, the greater
the refrangibility of the waves. Very rapid motion of a
luminous body has the same effect upon the length of the
// File: psp_171.png
.pn +1
luminous waves that motion of a sounding body has upon
the length of the sonorous waves. When a luminous body
is moving very rapidly towards us, its luminous waves are
shortened a little, and its light becomes a little more
refrangible; when the luminous body is moving rapidly
from us, its luminous waves are lengthened a little, and its
light becomes a little less refrangible.
.if h
.il fn=fig185.png w=60% alt='Spectral Lines'
.ca Fig. 185.
.if-
.if t
[Illustration: Fig. 185.]
.if-
169. Displacement of Spectral Lines.--In examining the
spectra of the stars, we often find that certain of the dark
lines are displaced somewhat, either towards the red or the
violet end of the spectrum. As the dark lines are in the
same position as the bright lines of the absorbing vapor
would be, a displacement of
the lines towards the red end
of the spectrum indicates a
lowering of the refrangibility of
the rays, due to a motion of
the luminous vapor away from
us; and a displacement of the
lines towards the violet end of
the spectrum indicates an increase
of refrangibility, due to a
motion of the luminous vapor
towards us. From the amount of the displacement of the
lines, it is possible to calculate the velocity at which the
luminous gas is moving. In Fig. 185 is shown the displacement
of the F line in the spectrum of Sirius. This
is one of the hydrogen lines. RV is the spectrum, R
being the red, and V the violet end. The long vertical
line is the bright F line of hydrogen, and the short
dark line to the left of it is the position of the F line
in the spectrum of Sirius. It is seen that this line is displaced
somewhat towards the red end of the spectrum.
This indicates that Sirius must be moving from us; and
the amount of the displacement indicates that the star
// File: psp_172.png
.pn +1
must be moving at the rate of some twenty-five or thirty
miles a second.
.if h
.il fn=fig186.png w=60% alt='Spectral Lines'
.ca Fig. 186.
.if-
.if t
[Illustration: Fig. 186.]
.if-
170. Contortion of Lines on the Disk of the Sun.--Certain
of the dark lines seen on the centre of the sun's
disk often appear more or less distorted, as shown in
Fig. 186, which represents the contortion of the hydrogen
line as seen at various times. 1 and 2 indicate a rapid
motion of hydrogen away from us, or a down-rush at the
sun; 3 and 4 (in which the line at the centre is dark on
one side, and bent towards the red end of the spectrum,
and bright on the other side with a distortion towards the
violet end of the spectrum) indicate a down-rush of cool
hydrogen side by side with an up-rush of hot and bright
hydrogen; 5 indicates local down-rushes associated with
quiescent hydrogen.
The contorted lines, which indicate a violently agitated
state of the sun's atmosphere, appear in the midst of other
lines which indicate a quiescent state. This is owing to
the fact that the absorption which produces the dark lines
takes place at various depths in the solar atmosphere.
There may be violent commotion in the lower layers of the
sun's atmosphere, and comparative quiet in the upper layers.
In this case, the lines which are due to absorption in the
lower layers would indicate this disturbance by their contortions;
while the lines produced by absorption in the upper
layers would be free from contortion.
// File: psp_173.png
.pn +1
It often happens, too, that the contortions are confined
to one set of lines of an element, while other lines of the
same element are entirely free from contortions. This is
undoubtedly due to the fact that different layers of the
solar atmosphere differ greatly in temperature; so that the
same element would give one set of lines at one depth, and
another set at another depth: hence commotion in the
solar atmosphere at any particular depth would be indicated
by the contortion of those lines of the element only
which are produced by the temperature at that particular
depth.
A remarkable case of contortion witnessed by Professor
Young is shown in Fig. 187. Three successive appearances
of the C line are
shown. The second
view was taken three
minutes after the first,
and the third five
minutes after the second.
The contortion
in this case indicated
a velocity ranging
from two hundred to three hundred miles a second.
.if h
.il fn=fig187.png w=60% alt='Spectral Lines'
.ca Fig. 187.
.if-
.if t
[Illustration: Fig. 187.]
.if-
171. Contortion of Lines on the Sun's Limb.--When
the spectroscope is directed to the centre of the sun's
disk, the distortion of the lines indicates only vertical
motion in the sun's atmosphere; but, when the spectroscope
is directed to the limb of the sun, displacements of
the lines indicate horizontal motions in the sun's atmosphere.
When a powerful spectroscope is directed to the
margin of the sun's disk, so that the slit of the collimator
tube shall be perpendicular to the sun's limb, one or more
of the dark lines on the disk are seen to be prolonged by
a bright line, as shown in Fig. 188. But this prolongation,
instead of being straight and narrow, as shown in the figure,
// File: psp_174.png
.pn +1
is often widened and distorted in various ways, as shown in
Fig. 189. In the left-hand portion of the diagram, the line
is deflected towards the red end of the spectrum; this
indicates a violent wind on the sun's surface blowing away
from us. In the right-hand
portion of the diagram,
the line is deflected
towards the violet end of
the spectrum; this indicates
a violent wind blowing
towards us. In the
middle portion of the
figure, the line is seen to
be bent both ways; this
indicates a cyclone, on
one side of which the
wind would be blowing
from us, and on the other side towards us.
.if h
.il fn=fig188.png w=60% alt='Spectral Lines'
.ca Fig. 188.
.if-
.if t
[Illustration: Fig. 188.]
.if-
.if h
.il fn=fig189.png w=80% alt='Spectral Lines'
.ca Fig. 189.
.if-
.if t
[Illustration: Fig. 189.]
.if-
The distortions of the solar lines indicate that the wind
at the surface of the sun often blows with a velocity of
from one hundred to three hundred miles a second. The
most violent wind known on the earth has velocity of
a hundred miles an hour.
// File: psp_175.png
.pn +1
.sp 2
.h4 id='photosphere-sunspots'
III. THE PHOTOSPHERE AND SUN SPOTS.
.sp 2
.h4 id='photosphere'
The Photosphere.
.sp 2
.if h
.il fn=fig190.png w=80% alt='Photosphere'
.ca Fig. 190.
.if-
.if t
[Illustration: Fig. 190.]
.if-
172. The Granulation of the Photosphere.--When the
surface of the sun is examined with a good telescope under
favorable atmospheric conditions, it is seen to be composed
of minute grains of intense brilliancy and of irregular form,
floating in a darker medium, and arranged in streaks and
groups, as shown in Fig. 190. With a rather low power,
the general effect of the surface is much like that of rough
drawing-paper, or of curdled milk seen from a little distance.
With a high power and excellent atmospheric conditions,
the grains are seen to be irregular, rounded masses,
// File: psp_176.png
.pn +1
some hundreds of miles in diameter, sprinkled upon a less
brilliant background, and appearing somewhat like snow-flakes
sparsely scattered over a grayish cloth. Fig. 191 is
a representation of these grains according to Secchi.
.if h
.il fn=fig191.png w=80% alt='Photosphere'
.ca Fig. 191.
.if-
.if t
[Illustration: Fig. 191.]
.if-
With a very powerful telescope and the very best atmospheric
conditions, the grains themselves are resolved into
granules, or little luminous dots, not more than a hundred
miles or so in diameter, which, by their aggregation, make
up the grains, just as they, in their turn, make up the coarser
masses of the solar surface. Professor Langley estimates
that these granules constitute about one-fifth of the sun's
surface, while they
emit at least three-fourths
of its light.
173. Shape of the
Grains.--The grains
differ considerably in
shape at different
times and on different
parts of the sun's
surface. Nasmyth, in
1861, described them
as willow-leaves in
shape, several thousand miles in length, but narrow and
with pointed ends. He figured the surface of the sun as
a sort of basket-work formed by the interweaving of such
filaments. To others they have appeared to have the form
of rice-grains. On portions of the sun's disk the elementary
structure is often composed of long, narrow, blunt-ended
filaments, not so much like willow-leaves as like bits
of straw lying roughly parallel to each other,--a thatch-straw
formation, as it has been called. This is specially
common in the immediate neighborhood of the spots.
174. Nature of the Grains.--The grains are, undoubtedly,
incandescent clouds floating in the sun's atmosphere,
// File: psp_177.png
.pn +1
and composed of partially condensed metallic vapors, just
as the clouds of our atmosphere are composed of partially
condensed aqueous vapor. Rain on the sun is composed
of white-hot drops of molten iron and other metals; and
these drops are often driven with the wind with a velocity
of over a hundred miles a second.
As to the forms of the grains, Professor Young says, "If
one were to speculate as to the explanation of the grains
and thatch-straws, it might be that the grains are the upper
ends of long filaments of luminous cloud, which, over most
of the sun's surface, stand approximately vertical, but in
the neighborhood of a spot are inclined so as to lie nearly
horizontal. This is not certain, though: it may be that the
cloud-masses over the more quiet portions of the solar surface
are really, as they seem, nearly globular, while near the
spots they are drawn out into filamentary forms by atmospheric
currents."
175. Faculæ.--The faculæ are irregular streaks of greater
brightness than the general surface, looking much like the
flecks of foam on the surface of a stream below a waterfall.
They are sometimes from five to twenty thousand
miles in length, covering areas immensely larger than a
terrestrial continent.
These faculæ are elevated regions of the solar surface,
ridges and crests of luminous matter, which rise above the
general level of the sun's surface, and protrude through the
denser portions of the solar atmosphere. When one of
these passes over the edge of the sun's disk, it can be seen
to project, like a little tooth. Any elevation on the sun to
be perceptible at all must measure at least half a second
of an arc, or two hundred and twenty-five miles.
The faculæ are most numerous in the neighborhood
of the spots, and much more conspicuous near the limb
of the sun than near the centre of the disk. Fig. 192 gives
the general appearance of the faculæ, and the darkening
// File: psp_178.png
.pn +1
of the limb of the sun. Near the spots, the faculæ often
undergo very rapid change of form, while elsewhere on the
disk they change rather slowly, sometimes undergoing little
apparent alteration for several days.
.if h
.il fn=fig192.png w=80% alt='Faculae'
.ca Fig. 192.
.if-
.if t
[Illustration: Fig. 192.]
.if-
176. Why the Faculæ are most Conspicuous near the
Limb of the Sun.--The reason why the faculæ are most
conspicuous near the limb of the sun is this: The luminous
surface of the sun is covered with an atmosphere, which,
though not very thick compared with the diameter of the
sun, is still sufficient to absorb a good deal of light. Light
coming from the centre of the sun's disk penetrates this
atmosphere under the most favorable conditions, and is but
slightly reduced in amount. The edges of the disk, on the
other hand, are seen through a much greater thickness of
atmosphere; and the light is reduced by absorption some
seventy-five per cent. Suppose, now, a facula were sufficiently
elevated to penetrate quite through this atmosphere.
Its light would be undimmed by absorption on any part
of the sun's disk; but at the centre of the disk it would
be seen against a background nearly as bright as itself,
while at the margin it would be seen against one only a
// File: psp_179.png
.pn +1
quarter as bright. It is evident that the light of any facula,
owing to the elevation, would be reduced less rapidly as
we approach the edge of the disk than that of the general
surface of the sun, which lies at a lower level.
.sp 2
.h4 id='sunspots'
Sun-Spots.
.sp 2
177. General Appearance of Sun-Spots.--The general
appearance of a well-formed sun-spot is shown in Fig. 193.
The spot consists of a very dark central portion of irregular
shape, called the umbra, which is surrounded by a
less dark fringe, called the penumbra. The penumbra is
made up, for the most part, of filaments directed radially
inward.
.if h
.il fn=fig193.png w=80% alt='Sunspots'
.ca Fig. 193.
.if-
.if t
[Illustration: Fig. 193.]
.if-
There is great variety in the details of form in different
sun-spots; but they are generally nearly circular during the
middle period of their existence. During the period of
their development and of their disappearance they are much
more irregular in form.
// File: psp_180.png
.pn +1
There is nothing like a gradual shading-off of the penumbra,
either towards the umbra on the one side, or towards
the photosphere on the other. The penumbra is separated
from both the umbra and the photosphere by a sharp line
of demarcation. The umbra is much brighter on the inner
than on the outer edge, and frequently the photosphere is
excessively bright at the margin of the penumbra. The
brightness of the inner penumbra seems to be due to the
crowding together of the penumbral filaments where they
overhang the edge of the umbra.
There is a general antithesis between the irregularities of
the outer and inner edges of the penumbra. Where an
angle of the penumbral matter crowds in upon the umbra,
it is generally matched by a corresponding outward extension
into the photosphere, and vice versa.
The umbra of the spot is far from being uniformly dark.
Many of the penumbral filaments terminate in little detached
grains of luminous matter; and there are also fainter
veils of a substance less brilliant, but sometimes rose-colored,
which seem to float above the umbra. The umbra
itself is made up of masses of clouds which are really
intensely brilliant, and which appear dark only by contrast
with the intenser brightness of the solar surface. Among
these clouds are often seen one or more minute circular
spots much darker than the rest of the umbra. These
darker portions are called nuclei. They seem to be the
mouths of tubular orifices penetrating to unknown depths.
The faint veils mentioned above continually melt away, and
are replaced by others in some different position. The
bright granules at the tips of the penumbral filaments seem
to sink and dissolve, while fresh portions break off to replace
them. There is a continual indraught of luminous matter
over the whole extent of the penumbra.
At times, though very rarely, patches of intense brightness
suddenly break out, remain visible for a few minutes, and
// File: psp_181.png
.pn +1
move over the spot with velocities as great as a hundred
miles a second.
The spots change their form and size quite perceptibly
from day to day, and sometimes even from hour to
hour.
178. Duration of Sun-Spots.--The average life of a
sun-spot is two or three months: the longest on record is
that of a spot observed in 1840 and 1841, which lasted
eighteen months. There are cases, however, where the disappearance
of a spot is very soon followed by the appearance
of another at the same point; and sometimes this
alternate disappearance and re-appearance is several times
repeated. While some spots are thus long-lived, others
endure only a day or two, and sometimes only a few
hours.
179. Groups of Spots.--The spots usually appear not
singly, but in groups. A large spot is often followed by a
train of smaller ones to the east of it, many of which are
apt to be irregular in form and very imperfect in structure,
sometimes with no umbra at all, often with a penumbra only
on one side. In such cases, when any considerable change
of form or structure shows itself in the principal spot, it
seems to rush westward over the solar surface, leaving its
attendants trailing behind. When a large spot divides into
two or more, as often happens, the parts usually seem to
repel each other, and fly apart with great velocity.
180. Size of the Spots.--The spots are sometimes of
enormous size. Groups have often been observed covering
areas of more than a hundred thousand miles square, and
single spots occasionally measure from forty to fifty thousand
miles in diameter, the umbra being twenty-five or thirty
thousand miles across. A spot, however, measuring thirty
thousand miles over all, may be considered a large one.
Such a spot can easily be seen without a telescope when
the brightness of the sun's surface is reduced by clouds or
// File: psp_182.png
.pn +1
nearness to the horizon, or by the use of colored glass.
During the years 1871 and 1872 spots were visible to the
naked eye for a considerable portion of the time. The
largest spot yet recorded was observed in 1858. It had a
breadth of more than a hundred and forty-three thousand
miles, or nearly eighteen times the diameter of the earth,
and covered about a thirty-sixth of the whole surface of
the sun.
.if h
.il fn=fig194.png w=80% alt='Sunspots'
.ca Fig. 194.
.if-
.if t
[Illustration: Fig. 194.]
.if-
Fig. 194 represents a group of sun-spots observed by
Professor Langley, and drawn on the same scale as the small
circle in the upper left-hand corner, which represents the
surface of half of our globe.
.if h
.il fn=fig195.png w=80% alt='Sunspots'
.ca Fig. 195.
.if-
.if t
[Illustration: Fig. 195.]
.if-
181. The Penumbral Filaments.--Not unfrequently the
penumbral filaments are curved spirally, indicating a cyclonic
action, as shown in Fig. 195. In such cases the whole spot
usually turns slowly around, sometimes completing an entire
revolution in a few days. More frequently, however, the
spiral motion lasts but a short time; and occasionally, after
continuing for a while in one direction, the motion is
reversed. Very often in large spots we observe opposite
// File: psp_183.png
.pn +1
spiral movements in different portions of the umbra, as
shown in Figs. 196 and 197.
.if h
.il fn=fig196.png w=80% alt='Sunspots'
.ca Fig. 196.
.if-
.if t
[Illustration: Fig. 196.]
.if-
Neighboring spots show no tendency to rotate in the
same direction. The number of spots in which a decided
cyclonic motion (like that shown in Fig. 198) appears is
// File: psp_184.png
.pn +1
comparatively small, not exceeding two or three per cent
of the whole.
.if h
.il fn=fig197.png w=80% alt='Sunspots'
.ca Fig. 197.
.if-
.if t
[Illustration: Fig. 197.]
.if-
.if h
.il fn=fig198.png w=80% alt='Sunspots'
.ca Fig. 198.
.if-
.if t
[Illustration: Fig. 198.]
.if-
.if h
.il fn=plate2.png w=80% alt='Sunspots'
.ca Plate 2.
.if-
.if t
[Illustration: Plate 2.]
.if-
Plate II. represents a typical sun-spot as delineated by
Professor Langley. At the left-hand and upper portions of
this great spot the filaments present the ordinary appearance,
// File: psp_185.png
.pn +1
while at the lower edge, and upon the great overhanging
branch, they are arranged very differently. The feathery
brush below the branch, closely resembling a frost-crystal
on a window-pane, is as rare as it is curious, and has not
been satisfactorily explained.
.if h
.il fn=fig199.png w=80% alt='Sunspots'
.ca Fig. 199.
.if-
.if t
[Illustration: Fig. 199.]
.if-
182. Birth and Decay of Sun-Spots.--The formation of
a spot is sometimes gradual, requiring days or even weeks
for its full development; and sometimes a single day suffices.
Generally, for some time before its appearance, there
is an evident disturbance of the solar surface, indicated especially
by the presence of many brilliant faculæ, among which
pores, or minute black dots, are scattered. These enlarge,
and between them appear grayish patches, in which the
photospheric structure is unusually evident, as if they were
caused by a dark mass lying below a thin veil of luminous
filaments. This veil seems to grow gradually thinner, and
finally breaks open, giving us at last the complete spot with
its penumbra. Some of the pores coalesce with the principal
spot, some disappear, and others form the attendant
// File: psp_186.png
.pn +1
train before described (179). The spot when once formed
usually assumes a circular form, and remains without striking
change until it disappears. As its end approaches, the
surrounding photosphere seems to crowd in, and overwhelm
the penumbra. Bridges of light (Fig. 199), often much
brighter than the average of the solar surface, push across
the umbra; the arrangement of the penumbra filaments
becomes confused; and, as Secchi expresses it, the luminous
matter of the photosphere seems to tumble pell-mell
into the chasm, which disappears, and leaves a disturbed
surface marked with faculæ, which, in their turn, gradually
subside.
.if h
.il fn=fig200.png w=80% alt='Sunspots'
.ca Fig. 200.
.if-
.if t
[Illustration: Fig. 200.]
.if-
183. Motion of Sun-Spots.--The spots have a regular
motion across the disk of the sun
from east to west, occupying about
twelve days in the transit. A spot
generally appears first on or near
the east limb, and, after twelve or
fourteen days, disappears at the west
limb. At the end of another fourteen
days, or more, it re-appears at
the east limb, unless, in the mean
time, it has vanished from sight entirely. This motion of
the spots is indicated by the arrow in Fig. 200. The
interval between two successive appearances of the same
spot on the eastern edge of the sun is about twenty-seven
days.
.if h
.il fn=fig201.png w=80% alt='rotation'
.ca Fig. 201.
.if-
.if t
[Illustration: Fig. 201.]
.if-
184. The Rotation of the Sun.--The spots are evidently
carried around by the rotation of the sun on its axis. It is
evident, from Fig. 201, that the sun will need to make more
than a complete rotation in order to bring a spot again
upon the same part of the disk as seen from the earth.
S represents the sun, and E the earth. The arrows indicate
the direction of the sun's rotation. When the earth is at E,
a spot at a would be seen at the centre of the solar disk.
// File: psp_187.png
.pn +1
While the sun is turning on its axis, the earth moves in its
orbit from E to E': hence the sun must make a complete
rotation, and turn from a to a' in addition, in order to
bring the spot again to the centre of the disk. To carry
the spot entirely around, and
then on to a', requires about
twenty-seven days. From this
synodical period of the spot,
as it might be called, it has
been calculated that the sun
must rotate on its axis in
about twenty-five days.
.if h
.il fn=fig202.png w=80% alt='Axis'
.ca Fig. 202.
.if-
.if t
[Illustration: Fig. 202.]
.if-
185. The Inclination of the
Sun's Axis.--The paths described
by sun-spots across
the solar disk vary with the
position of the earth in its
orbit, as shown in Fig. 202.
We therefore conclude that
the sun's axis is not perpendicular to the plane of the
earth's orbit. The sun rotates on its axis from west to east,
and the axis leans about seven degrees from the perpendicular
to the earth's orbit.
186. The Proper Motion of the Spots.--When the
period of the sun's rotation is deduced from the motion of
spots in different solar latitudes, there is found to be considerable
variation in the results obtained. Thus spots near
// File: psp_188.png
.pn +1
the equator indicate that the sun rotates in about twenty-five
days; while those in latitude 20° indicate a period about
eighteen hours
longer; and those
in latitude 30° a
period of twenty-seven
days and a
half. Strictly speaking,
the sun, as a
whole, has no single
period of rotation;
but different
portions of its surface
perform their
revolutions in different
times. The
equatorial regions
not only move more rapidly in miles per hour than the
rest of the solar surface, but they complete the entire rotation
in shorter time.
.if h
.il fn=fig203.png w=70% alt='Sun-Spots'
.ca Fig. 203.
.if-
.if t
[Illustration: Fig. 203.]
.if-
There appears to
be a peculiar surface-drift
in the equatorial
regions of the sun,
the cause of which
is unknown, but which
gives the spots a
proper motion; that
is, a motion of their
own, independent of
the rotation of the
sun.
.if h
.il fn=fig204.png w=70% alt='Sun-Spots'
.ca Fig. 204.
.if-
.if t
[Illustration: Fig. 204.]
.if-
187. Distribution of the Sun-Spots.--The sun-spots are
not distributed uniformly over the sun's surface, but occur
mainly in two zones on each side of the equator, and between
// File: psp_189.png
.pn +1
the latitudes of 10° and 30°, as shown in Fig. 203.
On and near the equator itself they are comparatively rare.
There are still fewer beyond 35° of latitude, and only a
single spot has ever been recorded more than 45° from the
solar equator.
Fig. 204 shows the distribution of
the sun-spots observed by Carrington
during a period of eight years. The
irregular line on the left-hand side of
the figure indicates by its height the
comparative frequency with which the
spots occurred in different latitudes.
In Fig. 205 the same thing is indicated
by different degrees of darkness in the
shading of the belts.
.if h
.il fn=fig205.png w=50% alt='Sun-Spots'
.ca Fig. 205.
.if-
.if t
[Illustration: Fig. 205.]
.if-
188. The Periodicity of the Spots.--Careful
observations of the solar spots
indicate a period of about eleven years
in the spot-producing activity of the
sun. During two or three years the
spots increase in number and in size;
then they begin to diminish, and reach
a minimum five or six years after the
maximum. Another period of about
six years brings the return of the maximum.
The intervals are, however,
somewhat irregular.
.if h
.il fn=fig206.png w=50% alt='Sun-Spots'
.ca Fig. 206.
.if-
.if t
[Illustration: Fig. 206.]
.if-
Fig. 206 gives a graphic representation
of the periodicity of the sun-spots.
The height of the curve shows the
frequency of the sun-spots in the years
given at the bottom of the figure. It appears, from an
examination of this sun-spot curve, that the average interval
from a minimum to the next following maximum is
only about four years and a half, while that from a maximum
// File: psp_190.png
.pn +1
to the next following minimum is six years and six-tenths.
The disturbance which produces the sun-spots is
developed suddenly, but dies away
gradually.
189. Connection between Sun-Spots
and Terrestrial Magnetism.--The
magnetic needle does not
point steadily in the same direction,
but is subject to various disturbances,
some of which are regular,
and others irregular.
(1) One of the most noticeable
of the regular magnetic changes is
the so-called diurnal oscillation.
During the early part of the day
the north pole of the needle moves
toward the west in our latitude,
returning to its mean position about
ten P.M., and remaining nearly
stationary during the night. The
extent of this oscillation in the
United States is about fifteen minutes
of arc in summer, and not
quite half as much in winter; but
it differs very much in different
localities and at different times, and
the average diurnal oscillation in
any locality increases and decreases
pretty regularly during a period of
about eleven years. The maximum
and minimum of this period of
magnetic disturbance are found to
coincide with the maximum and
minimum of the sun-spot period. This is shown in Fig. 206,
in which the dotted lines indicate the variations in the intensity
of the magnetic disturbance.
(2) Occasionally so-called magnetic storms occur, during
which the compass-needle is sometimes violently disturbed,
// File: psp_191.png
.pn +1
oscillating five degrees, or even ten degrees, within an hour
or two. These storms are generally accompanied by an aurora,
and an aurora is always accompanied by magnetic disturbance.
A careful comparison of aurora observations with those of sun-spots
shows an almost perfect parallelism between the curves
of auroral and sun-spot frequency.
(3) A number of observations render it very probable that
every intense disturbance of the solar surface is propagated
to our terrestrial magnetism with the speed of light.
.if h
.il fn=fig207.png w=90% alt='Solar Lines'
.ca Fig. 207.
.if-
.if t
[Illustration: Fig. 207.]
.if-
Fig. 207 shows certain of the solar lines as they were
observed by Professor Young on Aug. 3, 1872. The contortions
of the F line indicated an intense disturbance in the
atmosphere of the sun. There were three especially notable
paroxysms in this distortion, occurring at a quarter of nine,
half-past ten, and ten minutes of twelve, A.M.
.if h
.il fn=fig208.png w=90% alt='Solar Lines'
.ca Fig. 208.
.if-
.if t
[Illustration: Fig. 208.]
.if-
Fig. 208 shows the curve of magnetic disturbance as traced
at Greenwich on the same day. It will be seen from the curve
that it was a day of general magnetic disturbance. At the
// File: psp_192.png
.pn +1
times of the three paroxysms, which are given at the bottom
of the figure, it will be observed that there is a peculiar shivering
of the magnetic curve.
190. The Spots are Depressions in the Photosphere.--This
fact was first clearly brought out by Dr. Wilson of
Glasgow, in 1769, from observations upon the penumbra of
a spot in November of that year. He found, that when
the spot appeared at the eastern limb, or edge of the sun,
just moving into sight, the penumbra was well marked on
the side of the spot nearest to the edge of the disk; while
on the other edge of the spot, towards the centre of the
sun, there was no penumbra
visible at all, and the umbra
itself was almost hidden, as if
behind a bank. When the spot
had moved a day's journey
toward the centre of the disk,
the whole of the umbra came
into sight, and the penumbra on
the inner edge of the spot
began to be visible as a narrow
line. After the spot was well
advanced upon the disk, the penumbra was of the same
width all around the spot. When the spot approached the
sun's western limb, the same phenomena were repeated, but
in the inverse order. The penumbra on the inner edge of
the spot narrowed much faster than that on the outer, disappeared
entirely, and finally seemed to hide from sight
much of the umbra nearly a whole day before the spot
passed from view around the limb. This is precisely what
would occur (as Fig. 209 clearly shows) if the spot were a
saucer-shaped depression in the solar surface, the bottom
of the saucer corresponding to the umbra, and the sloping
sides to the penumbra.
.if h
.il fn=fig209.png w=70% alt='Sun-Spots'
.ca Fig. 209.
.if-
.if t
[Illustration: Fig. 209.]
.if-
// File: psp_193.png
.pn +1
.if h
.il fn=fig210.png w=90% alt='Sun-Spot Spectrum'
.ca Fig. 210.
.if-
.if t
[Illustration: Fig. 210.]
.if-
191. Sun-Spot Spectrum.--When the image of a sun-spot
is thrown upon the slit of the spectroscope, the spectrum is
seen to be crossed longitudinally by a continuous dark band,
showing an increased general absorption in the region of the
sun-spot. Many of the spectral lines are greatly thickened, as
shown in Fig. 210. This thickening of the lines shows that
the absorption is taking place at a greater depth. New lines
and shadings often appear, which indicate, that, in the cooler
nucleus of the spot, certain compound vapors exist, which are
dissociated elsewhere on the sun's surface. These lines and
shadings are shown in Fig. 211.
.if h
.il fn=fig211.png w=90% alt='Sun-Spot Spectrum'
.ca Fig. 211.
.if-
.if t
[Illustration: Fig. 211.]
.if-
It often happens that certain of the spectral lines are reversed
in the spectrum of the spot, a thin bright line appearing
over the centre of a thick dark one, as shown in Fig. 212.
These reversals are due to very bright vapors floating over the
spot.
.if h
.il fn=fig212.png w=70% alt='Sun-Spot Spectrum'
.ca Fig. 212.
.if-
.if t
[Illustration: Fig. 212.]
.if-
// File: psp_194.png
.pn +1
At times, also, the spectrum of a spot indicates violent
motion in the overlying gases by distortion and displacement
of the lines. This phenomenon occurs oftener at points near
the outer edge of the penumbra
than over the centre of
the spot; but occasionally the
whole neighborhood is violently
agitated. In such cases,
lines in the spectrum side by
side are often affected in entirely
different ways, one being
greatly displaced while its
neighbor is not disturbed in
the least, showing that the
vapors which produce the lines are at different levels in the
solar atmosphere, and moving independently of each other.
.if h
.il fn=fig213.png w=70% alt='Sun Clouds'
.ca Fig. 213.
.if-
.if t
[Illustration: Fig. 213.]
.if-
192. The Cause and Nature of Sun-Spots.--According to
Professor Young,
the arrangement
and relations of
the photospheric
clouds in the
neighborhood of
a spot are such as
are represented in
Fig. 213. "Over
the sun's surface
generally, these
clouds probably
have the form of
vertical columns,
as at aa. Just
outside the spot,
the level of the
photosphere is
the most part, overtopped by eruptions of hydrogen and
usually raised into faculæ, as at bb. These faculæ are, for
metallic vapors, as indicated by the shaded clouds.... While
the great clouds of hydrogen are found everywhere upon the
// File: psp_195.png
.pn +1
sun, these spiky, vivid outbursts of metallic vapors seldom
occur except just in the neighborhood of a spot, and then only
during its season of rapid change. In the penumbra of the
spot the photospheric filaments become more or less nearly
horizontal, as at pp; in the umbra at u it is quite uncertain
what the true state of affairs may be. We have conjecturally
represented the filaments there as vertical also, but depressed
and carried down by a descending current. Of course, the
cavity is filled by the gases which overlie the photosphere; and
it is easy to see, that, looked at from above, such a cavity
and arrangement of the luminous filaments would present the
appearances actually observed."
Professor Young also suggests that the spots may be depressions
in the photosphere caused "by the diminution of upward
pressure from below, in consequence of eruptions in the neighborhood;
the spots thus being, so to speak, sinks in the
photosphere. Undoubtedly the photosphere is not a strictly
continuous shell or crust; but it is heavy as compared with the
uncondensed vapors in which it lies, just as a rain-cloud in our
terrestrial atmosphere is heavier than the air; and it is probably
continuous enough to have its upper level affected by any
diminution of pressure below. The gaseous mass below the
photosphere supports its weight and the weight of the products
of condensation, which must always be descending in an inconceivable
rain and snow of molten and crystallized material.
To all intents and purposes, though nothing but a layer of
clouds, the photosphere thus forms a constricting shell, and
the gases beneath are imprisoned and compressed. Moreover,
at a high temperature the viscosity of gases is vastly increased,
so that quite probably the matter of the solar nucleus resembles
pitch or tar in its consistency more than what we usually
think of as a gas. Consequently, any sudden diminution of
pressure would propagate itself slowly from the point where
it occurred. Putting these things together, it would seem, that,
whenever a free outlet is obtained through the photosphere at
any point, thus decreasing the inward pressure, the result would
be the sinking of a portion of the photosphere somewhere in
the immediate neighborhood, to restore the equilibrium; and, if
the eruption were kept up for any length of time, the depression
// File: psp_196.png
.pn +1
in the photosphere would continue till the eruption ceased.
This depression, filled with the overlying gases, would constitute
a spot. Moreover, the line of fracture (if we may call it so) at
the edges of the sink would be a region of weakness in the
photosphere, so that we should expect a series of eruptions
all around the spot. For a time the disturbance, therefore,
would grow, and the spot would enlarge and deepen, until, in
spite of the viscosity of the internal gases, the equilibrium of
pressure was gradually restored beneath. So far as we know
the spectroscopic and visual phenomena, none of them contradict
this hypothesis. There is nothing in it, however, to
account for the distribution of the spots in solar latitudes, nor
for their periodicity."
.sp 2
.h4 id='prominences'
IV. THE CHROMOSPHERE AND PROMINENCES.
.sp 2
193. The Sun's Outer Atmosphere.--What we see of
the sun under ordinary circumstances is but a fraction of
his total bulk. While by far the greater portion of the
solar mass is included within the photosphere, the larger
portion of his volume lies without, and constitutes a gaseous
envelope whose diameter is at least double, and its bulk
therefore sevenfold, that of the central globe.
This outer envelope, though mainly gaseous, is not spherical,
but has an exceedingly irregular and variable outline.
It seems to be made up, not of regular strata of different
density, like our atmosphere, but rather of flames, beams,
and streamers, as transient and unstable as those of the
aurora borealis. It is divided into two portions by a
boundary as definite, though not so regular, as that which
separates them both from the photosphere. The outer and
far more extensive portion, which in texture and rarity seems
to resemble the tails of comets, is known as the coronal
atmosphere, since to it is chiefly due the corona, or glory,
which surrounds the darkened sun during an eclipse.
194. The Chromosphere.--At the base of the coronal
atmosphere, and in contact with the photosphere, is what
// File: psp_197.png
.pn +1
resembles a sheet of scarlet fire. It appears as if countless
jets of heated gas were issuing through vents over the
whole surface, clothing it with flame, which heaves and
tosses like the blaze of a conflagration. This is the chromosphere,
or color-sphere. It owes its vivid redness to the
predominance of hydrogen in the flames. The average
depth of the chromosphere is not far from ten or twelve
seconds, or five thousand or six thousand miles.
195. The Prominences.--Here and there masses of this
hydrogen, mixed with other substances, rise far above the
general level into the coronal regions, where they float like
clouds, or are torn
to pieces by conflicting
currents. These
cloud-masses are
known as solar prominences,
or protuberances.
196. Magnitude and
Distribution of the
Prominences.--The
prominences differ
greatly in magnitude.
Of the 2,767 observed
by Secchi, 1,964 attained an altitude of eighteen thousand
miles; 751, or nearly a fourth of the whole, reached a
height of twenty-eight thousand miles; several exceeded
eighty-four thousand miles. In rare instances they reach
elevations as great as a hundred thousand miles. A few
have been seen which exceeded a hundred and fifty thousand
miles; and Secchi has recorded one of three hundred
thousand miles.
.if h
.il fn=fig214.png w=60% alt='Prominences'
.ca Fig. 214.
.if-
.if t
[Illustration: Fig. 214.]
.if-
The irregular lines on the right-hand side of Fig. 214
show the proportion of the prominences observed by Secchi,
that were seen in different parts of the sun's surface. The
// File: psp_198.png
.pn +1
outer line shows the distribution of the smaller prominences,
and the inner dotted line that of the larger prominences.
By comparing these lines with those
on the opposite side of the circle,
which show the distribution of the
spots, it will be seen, that, while the
spots are confined mainly to two
belts, the prominences are seen in all
latitudes.
197. The Spectrum of the Chromosphere.--The
spectrum of the
chromosphere is comparatively simple.
There are eleven lines only which are
always present; and six of these are
lines of hydrogen, and the others,
with a single exception, are of unknown
elements. There are sixteen
other lines which make their appearance
very frequently. Among these
latter are lines of sodium, magnesium,
and iron.
Where some special disturbance is
going on, the spectrum at the base of
the chromosphere is very complicated,
consisting of hundreds of bright lines.
"The majority of the lines, however,
are seen only occasionally, for a few
minutes at a time, when the gases
and vapors, which generally lie low
(mainly in the interstices of the
clouds which constitute the photosphere),
and below its upper surface,
are elevated for the time being by some eruptive action.
For the most part, the lines which appear only at such
times are simply reversals of the more prominent dark lines
// File: psp_199.png
.pn +1
of the ordinary solar spectrum. But the selection of the
lines seems most capricious: one is taken, and another left,
though belonging to the same element, of equal intensity,
and close beside the first." Some of the main lines of the
chromosphere and prominences are shown in Fig. 215.
.if h
.il fn=fig215.png w=50% alt='Solar Lines'
.ca Fig. 215.
.if-
.if t
[Illustration: Fig. 215.]
.if-
198. Method of Studying the Chromosphere and Prominences.--Until
recently, the solar atmosphere could be seen
only during a total eclipse of the sun; but now the spectroscope
enables us to study the chromosphere and the prominences
with nearly the same facility as the spots and faculæ.
The protuberances are ordinarily invisible, for the same
reason that the stars cannot be seen in the daytime; they are
hidden by the intense light reflected from our own atmosphere.
If we could only get rid of this aerial illumination, without at
the same time weakening the light of
the prominences, the latter would become
visible. This the spectroscope
enables us to accomplish. Since the
air-light is reflected sunshine, it of
course presents the same spectrum as
sunlight,--a continuous band of color
crossed by dark lines. Now, this sort
of spectrum is weakened by increase of dispersive power (159),
because the light is spread out into a longer ribbon, and made
to cover a greater area. On the other hand, the spectrum of
the prominences, being composed of bright lines, undergoes
no such diminution by increased dispersion.
.if h
.il fn=fig216.png w=60% alt='Spectroscope'
.ca Fig. 216.
.if-
.if t
[Illustration: Fig. 216.]
.if-
When the spectroscope is used as a means of examining the
prominences, the slit is more or less widened. The telescope
is directed so that the image of that portion of the solar limb
which is to be examined shall be tangent to the opened slit,
as in Fig. 216, which represents the slit-plate of the spectroscope
of its actual size, with the image of the sun in the
proper position for observation.
.if h
.il fn=fig217.png w=60% alt='Prominence'
.ca Fig. 217.
.if-
.if t
[Illustration: Fig. 217.]
.if-
If, now, a prominence exists at this part of the solar limb,
and if the spectroscope itself is so adjusted that the C line
falls in the centre of the field of view, then one will see something
like Fig. 217. "The red portion of the spectrum will
// File: psp_200.png
.pn +1
stretch athwart the field of view like a scarlet ribbon with a
darkish band across it; and in that band will appear the prominences,
like scarlet clouds, so like our own terrestrial clouds,
indeed, in form and texture, that the resemblance is quite
startling. One might almost think he was looking out through
a partly-opened door upon a sunset sky, except that there is
no variety or contrast of color; all the cloudlets are of the
same pure scarlet hue. Along the edge of the opening is seen
the chromosphere, more brilliant than the clouds which rise
from it or float above it, and, for the most part, made up of
minute tongues and filaments."
199. Quiescent Prominences.--The prominences differ
as widely in form
and structure as in
magnitude. The two
principal classes are
the quiescent, cloud-formed,
or hydrogenous,
and the eruptive,
or metallic.
.if h
.il fn=plate3.png w=70% alt='Prominence'
.ca Plate 3.
.if-
.if t
[Illustration: Plate 3.]
.if-
The quiescent
prominences resemble
almost exactly
our terrestrial clouds, and differ among themselves in the
same manner. They are often of enormous dimensions,
especially in horizontal extent, and are comparatively permanent,
often undergoing little change for hours and days.
Near the poles they sometimes remain during a whole solar
revolution of twenty-seven days. Sometimes they appear
to lie upon the limb of the sun, like a bank of clouds in
the terrestrial horizon, probably because they are so far
from the edge that only their upper portions are in sight.
When fully seen, they are usually connected to the chromosphere
by slender columns, generally smallest at the base,
and often apparently made up of separate filaments closely
// File: psp_201.png
.pn +1
intertwined, and expanding upward. Sometimes the whole
under surface is fringed with pendent filaments. Sometimes
they float entirely
free from the chromosphere;
and in
most cases the
larger clouds are
attended by detached
cloudlets.
Various forms of
quiescent prominences
are shown
in Plate III.
Other forms are
given in Figs. 218
and 219.
.if h
.il fn=fig218.png w=60% alt='Prominences'
.ca Fig. 218.
.if-
.if t
[Illustration: Fig. 218.]
.if-
Their spectrum
is usually very simple, consisting of the four lines of hydrogen
and the orange
D^3: hence the appellation
hydrogenous.
Occasionally
the sodium and
magnesium lines
also appear, even
near the tops of
the clouds.
.if h
.il fn=fig219.png w=60% alt='Prominences'
.ca Fig. 219.
.if-
.if t
[Illustration: Fig. 219.]
.if-
200. Eruptive
Prominences.--The
eruptive prominences
ordinarily
consist of brilliant
spikes or jets, which
change very rapidly in form and brightness. As a rule, their
altitude is not more than twenty thousand or thirty thousand
// File: psp_202.png
.pn +1
miles; but occasionally they rise far higher than even the
largest of the quiescent protuberances. Their spectrum is
very complicated, especially near their base, and often filled
with bright lines. The most conspicuous lines are those of
sodium, magnesium, barium, iron, and titanium: hence
Secchi calls them metallic prominences.
.if h
.il fn=fig220.png w=70% alt='Prominences'
.ca Fig. 220.
.if-
.if t
[Illustration: Fig. 220.]
.if-
They usually appear in the immediate vicinity of a spot,
never very near the solar poles. They change with such
rapidity, that the motion can almost be seen with the eye.
Sometimes, in the course of fifteen or twenty minutes, a
mass of these flames, fifty thousand miles high, will undergo
a total transformation; and in some instances their complete
development or disappearance takes no longer time.
Sometimes they consist of pointed rays, diverging in all
directions, as represented in Fig. 220. "Sometimes they
look like flames, sometimes like sheaves of grain, sometimes
like whirling water-spouts capped with a great cloud;
occasionally they present most exactly the appearance of
jets of liquid fire, rising and falling in graceful parabolas;
frequently they carry on their edges spirals like the volutes
// File: psp_203.png
.pn +1
of an Ionic column; and continually they detach filaments,
which rise to a great elevation, gradually expanding
and growing fainter as they ascend, until the eye loses
them."
.if h
.il fn=fig221.png w=70% alt='Prominences'
.ca Fig. 221.
.if-
.if t
[Illustration: Fig. 221.]
.if-
201. Change of
Form in Prominences.--Fig.
221 represents
a prominence as
seen by Professor
Young, Sept. 7, 1871.
It was an immense
quiescent cloud, a
hundred thousand miles long and fifty-four thousand miles
high. At a there was a brilliant lump, somewhat in the
form of a thunder-head. On returning to the spectroscope
less than half an hour
afterwards, he found that the
cloud had been literally blown
into shreds by some inconceivable
uprush from beneath.
The prominence then presented
the form shown in
Fig. 222. The débris of the
cloud had already attained a
height of a hundred thousand
miles. While he was
watching them for the next
ten minutes, they rose, with
a motion almost perceptible
to the eye, till the uppermost
reached an altitude of
two hundred thousand miles. As the filaments rose, they
gradually faded away like a dissolving cloud.
.if h
.il fn=fig222.png w=70% alt='Prominences'
.ca Fig. 222.
.if-
.if t
[Illustration: Fig. 222.]
.if-
Meanwhile the little thunder-head had grown and developed
into what appeared to be a mass of rolling and ever-changing
// File: psp_204.png
.pn +1
flame. Figs. 223 and 224 give the appearance
of this portion of the prominence at intervals of fifteen
minutes. Other similar eruptions have been observed.
.if h
.il fn=fig223.png w=70% alt='Prominences'
.ca Fig. 223.
.if-
.if t
[Illustration: Fig. 223.]
.if-
.if h
.il fn=fig224.png w=70% alt='Prominences'
.ca Fig. 224.
.if-
.if t
[Illustration: Fig. 224.]
.if-
.sp 2
.h4 id='corona'
V. THE CORONA.
.sp 2
202. General Appearance of the Corona.--At the
time of a total eclipse of the sun, if the sky is clear, the
moon appears as a huge black ball, the illumination at
the edge of the disk being just sufficient to bring out
its rotundity. "From behind it," to borrow Professor
Young's vivid description, "stream out on all sides radiant
filaments, beams, and sheets of pearly light, which reach
to a distance sometimes of several degrees from the solar
surface, forming an irregular stellate halo, with the black
globe of the moon in its apparent centre. The portion
nearest the sun is of dazzling brightness, but still less brilliant
than the prominences which blaze through it like
carbuncles. Generally this inner corona has a pretty uniform
height, forming a ring three or four minutes of arc
in width, separated by a somewhat definite outline from
the outer corona, which reaches to a much greater distance,
and is far more irregular in form. Usually there are
several rifts, as they have been called, like narrow beams of
darkness, extending from the very edge of the sun to the
outer night, and much resembling the cloud-shadows which
radiate from the sun before a thunder-shower; but the
edges of these rifts are frequently curved, showing them
// File: psp_205.png
.pn +1
to be something else than real shadows. Sometimes there
are narrow bright streamers, as long as the rifts, or longer.
These are often inclined, occasionally are even nearly
tangential to the solar surface, and frequently are curved.
On the whole, the corona is usually less extensive and
brilliant over the solar poles, and there is a recognizable
tendency to accumulations above the middle latitudes, or
spot-zones; so that, speaking roughly, the corona shows a
disposition to assume the form of a quadrilateral or four-rayed
star, though in almost every individual case this form
is greatly modified by abnormal streamers at some point or
other."
.if h
.il fn=fig225.png w=70% alt='Corona'
.ca Fig. 225.
.if-
.if t
[Illustration: Fig. 225.]
.if-
203. The Corona as seen at Recent Eclipses.--The
// File: psp_206.png
.pn +1
corona can be seen only at the time of a total eclipse of
the sun, and then for only a few minutes. Its form varies
considerably from one eclipse to another, and apparently
also during the same eclipse. At least, different observers
at different stations depict the same corona under very
different forms. Fig. 225 represents the corona of 1857 as
observed by Liais. In this view the petal-like forms, which
have been noticed in the corona at other times, are especially
prominent.
.if h
.il fn=fig226.png w=70% alt='Corona'
.ca Fig. 226.
.if-
.if t
[Illustration: Fig. 226.]
.if-
Fig. 226 shows the corona of 1860 as it was observed
by Temple.
.if h
.il fn=fig227.png w=70% alt='Corona'
.ca Fig. 227.
.if-
.if t
[Illustration: Fig. 227.]
.if-
Fig. 227 shows the corona of 1867. This is interesting
as being a corona at the time of sun-spot minimum.
// File: psp_207.png
.pn +1
.if h
.il fn=fig228.png w=70% alt='Corona'
.ca Fig. 228.
.if-
.if t
[Illustration: Fig. 228.]
.if-
Fig. 228 represents the corona of 1868. This is a larger
and more irregular corona than usual.
.if h
.il fn=fig229.png w=70% alt='Corona'
.ca Fig. 229.
.if-
.if t
[Illustration: Fig. 229.]
.if-
The corona of 1869 is shown in Fig. 229.
.if h
.il fn=fig230.png w=70% alt='Corona'
.ca Fig. 230.
.if-
.if t
[Illustration: Fig. 230.]
.if-
Fig. 230 is a view of the corona of 1871 as seen by
Capt. Tupman.
.if h
.il fn=fig231.png w=70% alt='Corona'
.ca Fig. 231.
.if-
.if t
[Illustration: Fig. 231.]
.if-
Fig. 231 shows the same corona as seen by Foenander.
.if h
.il fn=fig232.png w=70% alt='Corona'
.ca Fig. 232.
.if-
.if t
[Illustration: Fig. 232.]
.if-
Fig. 232 shows the same corona as photographed by
Davis.
.if h
.il fn=fig233.png w=70% alt='Corona'
.ca Fig. 233.
.if-
.if t
[Illustration: Fig. 233.]
.if-
Fig. 233 shows the corona of 1878 made up from several
views as combined by Professor Young.
204. The Spectrum of the Corona.--The chief line in the
spectrum of the corona is the one usually designated as 1474,
and now known as the coronal line. It is seen as a dark line
// File: psp_208.png
.pn +1
on the disk of the sun; and a spectroscope of great dispersive
power shows this dark line to be closely double, the lower
component being one of the iron lines, and the upper the
coronal line. This dark line is shown at x, Fig. 234.
.if h
.il fn=fig234.png w=70% alt='Spectral Lines'
.ca Fig. 234.
.if-
.if t
[Illustration: Fig. 234.]
.if-
Besides this bright line, the hydrogen lines appear faintly
in the spectrum of the corona. The 1474 line has been
sometimes traced with the spectroscope to an elevation of
nearly twenty minutes above the moon's limb, and the hydrogen
lines nearly as far; and the lines were just as strong
in the middle of a dark rift as anywhere else.
The substance which produces the 1474 line is unknown
as yet. It seems to be something with a vapor-density far
below that of hydrogen, which is the lightest substance of
which we have any knowledge. It can hardly be an "allotropic"
// File: psp_209.png
.pn +1
form of any terrestrial element, as some scientists have
suggested; for in the most violent disturbances in prominences
and near sun-spots, when the lines of hydrogen, magnesium,
and other metals, are contorted and shattered by the rush of
the contending elements, this line alone remains fine, sharp,
and straight, a little brightened, but not otherwise affected.
For the present it remains, like a few other lines in the spectrum,
an unexplained mystery.
Besides bright lines, the corona shows also a faint continuous
spectrum, in which have been observed a few of the more
prominent dark lines of the solar spectrum.
This shows, that, while the corona may be in the main
composed of glowing gas (as indicated by the bright lines
of its spectrum), it also contains considerable matter in such
// File: psp_210.png
.pn +1
a state as to reflect the sunlight, probably in the form of dust
or fog.
.sp 2
.h3 id='eclipses'
V. ECLIPSES.
.sp 2
.if h
.il fn=fig235.png w=60% alt='Eclipses'
.ca Fig. 235.
.if-
.if t
[Illustration: Fig. 235.]
.if-
205. The Shadows of the Earth and Moon.--The
shadows cast by the earth and moon are shown in Fig. 235.
Each shadow is seen to be made up of a dark portion
called the umbra, and of a lighter portion called the
penumbra. The light of the sun is completely excluded
from the umbra, but only partially from the penumbra.
The umbra is in the form of a cone, with its apex away
from the sun; though in the case of the earth's shadow
it tapers very slowly. The penumbra surrounds the umbra,
// File: psp_211.png
.pn +1
and increases in size as we recede from the sun. The axis
of the earth's shadow lies in the plane of the ecliptic, which
in the figure is the surface of the page. As the moon's
orbit is inclined five degrees to the plane of the ecliptic,
the axis of the moon's shadow will sometimes lie above,
and sometimes below, the ecliptic. It will lie on the ecliptic
only when the moon is at one of her nodes.
206. When there will be an Eclipse of the Moon.--The
moon is eclipsed whenever it passes into the umbra
of the earth's shadow. It will be seen from the figure that
the moon can pass into the shadow of the earth only when
she is in opposition, or at full. Owing to the inclination
of the moon's orbit to the ecliptic, the moon will pass
// File: psp_212.png
.pn +1
either above or below the earth's shadow when she is at
full, unless she happens to be near her node at this time:
hence there is not an eclipse of the moon every month.
When the moon simply passes into the penumbra of the
earth's shadow, the light of the moon is somewhat dimmed,
but not sufficiently to attract attention, or to be denominated
an eclipse.
.if h
.il fn=fig236.png w=60% alt='Eclipses'
.ca Fig. 236.
.if-
.if t
[Illustration: Fig. 236.]
.if-
207. The Lunar Ecliptic Limits.--In Fig. 236 the line
AB represents the plane of the ecliptic, and the line CD the
moon's orbit. The large black circles on the line AB represent
sections of the umbra of the earth's shadow, and the
smaller circles on CD represent the moon at full. It will be
seen, that, if the moon is full at E, she will just graze the
// File: psp_213.png
.pn +1
umbra of the earth's shadow. In this case she will suffer no
eclipse. Were the moon full at any point nearer her node, as
at F, she would pass into the umbra of the earth's shadow, and
would be partially eclipsed. Were the moon full at G, she
would pass through the centre of the earth's shadow, and be
totally eclipsed.
It will be seen from the figure that full moon must occur
when the moon is within a certain distance from her node, in
order that there may be a lunar eclipse; and this space is
called the lunar ecliptic limits.
The farther the earth is from the sun, the less rapidly does
its shadow taper, and therefore the greater its diameter at the
distance of the moon; and, the nearer the moon to the earth,
the greater the diameter of the earth's shadow at the distance
of the moon. Of course, the greater the diameter of the
// File: psp_214.png
.pn +1
earth's shadow, the greater the ecliptic limits: hence the lunar
ecliptic limits vary somewhat from time to time, according to
the distance from the earth to the sun and from the earth
to the moon. The limits within which an eclipse is inevitable
under all circumstances are called the minor ecliptic limits;
and those within which an eclipse is possible under some circumstances,
the major ecliptic limits.
.if h
.il fn=fig237.png w=60% alt='Eclipses'
.ca Fig. 237.
.if-
.if t
[Illustration: Fig. 237.]
.if-
208. Lunar Eclipses.--Fig. 237 shows the path of the
moon through the earth's shadow in the case of a partial
eclipse. The magnitude of such an eclipse depends upon
the nearness of the moon to her nodes. The magnitude of
an eclipse is usually denoted in digits, a digit being one-twelfth
of the diameter of the moon.
.if h
.il fn=fig238.png w=70% alt='Eclipses'
.ca Fig. 238.
.if-
.if t
[Illustration: Fig. 238.]
.if-
Fig. 238 shows the path of the moon through the earth's
shadow in the
case of a total
eclipse. It will
be seen from
the figure that
it is not necessary
for the moon
to pass through
the centre of the
earth's shadow in order to have a total eclipse. When
the moon passes through the centre of the earth's shadow,
the eclipse is both total and central.
At the time of a total eclipse, the moon is not entirely
invisible, but shines with a faint copper-colored light. This
light is refracted into the shadow by the earth's atmosphere,
and its amount varies with the quantity of clouds and vapor
in that portion of the atmosphere which the sunlight must
graze in order to reach the moon.
The duration of an eclipse varies between very wide
limits, being, of course, greatest when the eclipse is central.
A total eclipse of the moon may last nearly two hours, or,
// File: psp_215.png
.pn +1
including the partial portions of the eclipse, three or four
hours.
Every eclipse of the moon, whether total or partial, is
// File: psp_216.png
.pn +1
visible at the same time to the whole hemisphere of the
earth which is turned towards the moon; and the eclipse
will have exactly the same magnitude at every point of
observation.
209. When there will be an Eclipse of the Sun.--There
will be an eclipse of the sun whenever any portion
of the moon's shadow is thrown on the earth. It will be
seen from Fig. 235 that this can occur only when the moon
is in conjunction, or at new. It does not occur every
month, because, owing to the inclination of the moon's orbit
to the ecliptic, the moon's shadow is usually thrown either
above or below the earth at the time of new moon. There
// File: psp_217.png
.pn +1
can be an eclipse of the sun only when new moon occurs
at or near one of the nodes of her orbit.
210. Solar Ecliptic Limits.--The distances from the moon's
node within which a new moon would throw some portion of
its shadow on the earth so as to produce an eclipse of the
sun are called the solar ecliptic limits. As in the case of
the moon, there are major and minor ecliptic limits; the former
being the limits within which an eclipse of the sun is possible
under some circumstances, and the latter those under which
an eclipse is inevitable under all circumstances.
The limits within which a solar eclipse may occur are
greater than those within which a lunar eclipse may occur.
This will be evident from an examination of Fig. 235. Were
the moon in that figure just outside of the lines AB and CD,
it will be seen that the penumbra of her shadow would just
graze the earth: hence the moon must be somewhere within
the space bounded by these lines in order to cause an eclipse
of the sun. Now, these lines mark the prolongation to the
sun of the cone of the umbra of the earth's shadow: hence,
in order to produce an eclipse of the sun, new moon must
occur somewhere within this prolongation of the umbra of the
earth's shadow. Now, it is evident that the diameter of this
// File: psp_218.png
.pn +1
cone is greater on the side of the earth toward the sun than
on the opposite side: hence the solar ecliptic limits are greater
than the lunar ecliptic limits.
211. Solar Eclipses.--An observer in the umbra of the
moon's shadow would see a total eclipse of the sun, while
one in the penumbra would see only a partial eclipse. The
magnitude of this partial eclipse would depend upon the
distance of the observer from the umbra of the moon's
shadow.
.if h
.il fn=fig239.png w=90% alt='Eclipses'
.ca Fig. 239.
.if-
.if t
[Illustration: Fig. 239.]
.if-
.if h
.il fn=fig240.png w=90% alt='Eclipses'
.ca Fig. 240.
.if-
.if t
[Illustration: Fig. 240.]
.if-
The umbra of the moon's shadow is just about long
enough to reach the earth. Sometimes the point of this
shadow falls short of the earth's surface, as shown in
Fig. 239, and sometimes it falls upon the earth, as shown
in Fig. 240, according to the varying distance of the sun
and moon from the earth. The diameter of the umbra at
the surface of the earth is seldom more than a hundred
// File: psp_219.png
.pn +1
miles: hence the belt of a total eclipse is, on the average,
not more than a hundred miles wide; and a total eclipse
seldom lasts more than five or six minutes, and sometimes
only a few seconds. Owing, however, to the rotation of
the earth, the umbra of the moon's shadow may pass over
a long reach of the earth's surface. Fig. 241 shows the
track of the umbra of the moon's shadow over the earth
in the total eclipse of 1860.
.if h
.il fn=fig241.png w=90% alt='Eclipses'
.ca Fig. 241.
.if-
.if t
[Illustration: Fig. 241.]
.if-
.if h
.il fn=fig242.png w=90% alt='Eclipses'
.ca Fig. 242.
.if-
.if t
[Illustration: Fig. 242.]
.if-
Fig. 242 shows the track of the total eclipse of 1871
across India and the adjacent seas.
.if h
.il fn=fig243.png w=70% alt='Eclipses'
.ca Fig. 243.
.if-
.if t
[Illustration: Fig. 243.]
.if-
.if h
.il fn=fig244.png w=70% alt='Eclipses'
.ca Fig. 244.
.if-
.if t
[Illustration: Fig. 244.]
.if-
In a partial eclipse of the sun, more or less of one side
of the sun's disk is usually concealed, as shown in Fig. 243.
Occasionally, however, the centre of the sun's disk is covered,
leaving a bright ring around the margin, as shown in
Fig. 244. Such an eclipse is called an annular eclipse.
// File: psp_220.png
.pn +1
An eclipse can be annular only when the cone of the
moon's shadow is too short to reach the earth, and then
only to observers who are in the central portion of the
penumbra.
212. Comparative Frequency of Solar and Lunar
Eclipses.--There are more eclipses of the sun in the year
// File: psp_221.png
.pn +1
than of the moon; and yet, at any one place, eclipses of
the moon are more frequent than those of the sun.
There are more lunar than solar eclipses, because, as we
have seen, the limits within which a solar eclipse may occur
are greater than those within which a lunar eclipse may occur.
There are more eclipses of the moon visible at any one place
than of the sun; because, as we have seen, an eclipse of the
moon, whenever it does occur, is visible to a whole hemisphere
at a time, while an eclipse of the sun is visible to only a portion
of a hemisphere, and a total eclipse to only a very small
portion of a hemisphere. A total eclipse of the sun is, therefore,
a very rare occurrence at any one place.
The greatest number of eclipses that can occur in a year is
seven, and the least number, two. In the former case, five
may be of the sun and two of the moon, or four of the sun
and three of the moon. In the latter case, both must be of the
sun.
.sp 2
.h3 id='three-groups'
VI. THE THREE GROUPS OF PLANETS.
.sp 2
.h4 id='group-chars'
I. GENERAL CHARACTERISTICS OF THE GROUPS.
.sp 2
213. The Inner Group.--The inner group of planets
is composed of Mercury, Venus, the Earth, and Mars;
that is, of all the planets which lie between the asteroids
// File: psp_222.png
.pn +1
and the sun. The planets of this group are comparatively
small and dense. So far as known, they rotate on their
axes in about twenty-four hours, and they are either entirely
without moons, or are attended by comparatively few.
The comparative sizes and eccentricities of the orbits of
this group are shown in Fig. 245. The dots round the orbits
show the position of the planets at intervals of ten days.
.if h
.il fn=fig245.png w=90% alt='Planetary Orbits'
.ca Fig. 245.
.if-
.if t
[Illustration: Fig. 245.]
.if-
214. The Outer Group.--The outer group of planets
is composed of Jupiter, Saturn, Uranus, and Neptune.
These planets are all very large and of slight density. So
far as known, they rotate on their axes in about ten hours,
// File: psp_223.png
.pn +1
and are accompanied with complicated systems of moons.
Fig. 246, which represents the comparative sizes of the
planets, shows at a glance the immense difference between
those of the inner and outer group. Fig. 247 shows the
comparative sizes and eccentricities of the orbits of the
outer planets. The dots round the orbits show the position
of the planets at intervals
of a thousand days.
.if h
.il fn=fig246.png w=70% alt='Planets'
.ca Fig. 246.
.if-
.if t
[Illustration: Fig. 246.]
.if-
.if h
.il fn=fig247.png w=90% alt='Planetary Orbits'
.ca Fig. 247.
.if-
.if t
[Illustration: Fig. 247.]
.if-
215. The Asteroids.--Between
the inner and
outer groups of planets
there is a great number
of very small planets
known as the minor planets,
or asteroids. Over
two hundred planets belonging
to this group
have already been discovered.
Their orbits are
shown by the dotted lines
in Fig. 247. The sizes
of the four largest of
these planets, compared
with the earth, are shown
in Fig. 248.
.if h
.il fn=fig248.png w=70% alt='Planet Sizes'
.ca Fig. 248.
.if-
.if t
[Illustration: Fig. 248.]
.if-
The asteroids of this
group are distinguished from the other planets, not only by
their small size, but by the great eccentricities and inclinations
of their orbits. If we except Mercury, none of the
larger planets has an eccentricity amounting to one-tenth
the diameter of its orbit (43), nor is any orbit inclined more
than two or three degrees to the ecliptic; but the inclinations
of many of the minor planets exceed ten degrees, and
the eccentricities frequently amount to an eighth of the
orbital diameter. The orbit of Pallas is inclined thirty-four
// File: psp_224.png
.pn +1
degrees to the ecliptic, while there are some planets of
this group whose orbits nearly coincide with the plane of
the ecliptic.
.if h
.il fn=fig249.png w=90% alt='Planetary Orbits'
.ca Fig. 249.
.if-
.if t
[Illustration: Fig. 249.]
.if-
Fig. 249 shows one of the most and one of the least
eccentric of the orbits of this group as compared with that
of the earth.
.if h
.il fn=fig250.png w=90% alt='Asteroid Orbits'
.ca Fig. 250.
.if-
.if t
[Illustration: Fig. 250.]
.if-
The intricate complexity of the orbits of the asteroids is
shown in Fig. 250.
// File: psp_225.png
.pn +1
.sp 2
.h4 id='inner-group'
II. THE INNER GROUP OF PLANETS.
.sp 2
.h4 id='mercury'
Mercury.
.sp 2
216. The Orbit of Mercury.--The orbit of Mercury is
more eccentric than that of any of the larger planets, and
it has also a greater
inclination to the ecliptic.
Its eccentricity (43)
is a little over a fifth,
and its inclination to
the ecliptic somewhat
over seven degrees. The
mean distance of Mercury
from the sun is
about thirty-five million miles; but, owing to the great
eccentricity of its orbit, its distance from the sun varies
from about forty-three
million miles
at aphelion to
about twenty-eight
million at perihelion.
.if h
.il fn=fig251.png w=70% alt='Mercury'
.ca Fig. 251.
.if-
.if t
[Illustration: Fig. 251.]
.if-
217. Distance of
Mercury from the
Earth.--It is evident,
from Fig.
251, that an inferior
planet, like
Mercury, is the
whole diameter of
its orbit nearer the
earth at inferior conjunction than at superior conjunction:
hence Mercury's distance from the earth varies considerably.
Owing to the great eccentricity of its orbit, its distance
// File: psp_226.png
.pn +1
from the earth at inferior conjunction also varies considerably.
Mercury is nearest to the earth when its inferior
conjunction occurs at its own aphelion and at the earth's
perihelion.
.if h
.il fn=fig252.png w=70% alt='Mercury'
.ca Fig. 252.
.if-
.if t
[Illustration: Fig. 252.]
.if-
218. Apparent Size of Mercury.--Since Mercury's distance
from the earth is variable, the apparent size of the
planet is also variable. Fig. 252 shows its apparent size at
its extreme and mean distances from the earth. Its apparent
diameter varies from five seconds to twelve seconds.
.if h
.il fn=fig253.png w=70% alt='Mercury'
.ca Fig. 253.
.if-
.if t
[Illustration: Fig. 253.]
.if-
219. Volume and Density of Mercury.--The real
diameter of Mercury is about three thousand miles. Its
size, compared with that of the earth, is shown in Fig.
253. The earth is about sixteen times as large as Mercury;
// File: psp_227.png
.pn +1
but Mercury is about one-fifth more dense than the
earth.
220. Greatest Elongation of Mercury.--Mercury, being
an inferior planet (or one within the orbit of the earth),
appears to oscillate
to and fro across
the sun. Its greatest
apparent distance
from the sun, or its
greatest elongation,
varies considerably.
The farther Mercury
is from the sun, and
the nearer the earth
is to Mercury, the
greater is its angular
distance from the sun
at the time of its
greatest elongation. Under the most favorable circumstances,
the greatest elongation amounts to about twenty-eight
degrees, and under the least favorable to only sixteen
or seventeen degrees.
221. Sidereal and Synodical Periods of Mercury.--Mercury
accomplishes
a complete revolution
around the sun
in about eighty-eight
days; but it
takes it a hundred
and sixteen days to
pass from its greatest elongation east to the same elongation
again. The orbital motion of this planet is at the
rate of nearly thirty miles a second.
In Fig. 251, P''' represents elongation east of the sun,
and P' elongation west. It will be seen that it is much
// File: psp_228.png
.pn +1
farther from P' around to P''' than from P''' on to P'.
Mercury is only about forty-eight days in passing from
greatest elongation east to greatest elongation west, while
it is about sixty-eight days in passing back again.
222. Visibility of Mercury.--Mercury is too close to
the sun for favorable observation. It is never seen long
after sunset, or long before sunrise, and never far from the
horizon. When visible at all, it must be sought for low
down in the west shortly after sunset, or low in the east
shortly before sunrise, according
as the planet is at its east
or west elongation. It is often
visible to the naked eye in our
latitude; but the illumination
of the twilight sky, and the
excess of vapor in our atmosphere
near the horizon, combine
to make the telescopic
study of the planet difficult
and unsatisfactory.
.if h
.il fn=fig254.png w=70% alt='Mercury'
.ca Fig. 254.
.if-
.if t
[Illustration: Fig. 254.]
.if-
223. The Atmosphere and Surface of Mercury.--Mercury
seems to be surrounded by a dense atmosphere. One
proof of the existence of such an atmosphere is furnished
at the time of the planet's transit across the disk of the
sun, which occasionally happens. The planet is then seen
// File: psp_229.png
.pn +1
surrounded by a border, as shown in Fig. 254. A bright
spot has also been observed on the dark disk of the planet
during a transit, as shown in Fig. 255. The border around
the planet seems to
be due to the action
of the planet's atmosphere;
but it is difficult
to account for
the bright spot.
.if h
.il fn=fig255.png w=70% alt='Mercury'
.ca Fig. 255.
.if-
.if t
[Illustration: Fig. 255.]
.if-
.if h
.il fn=fig256.png w=70% alt='Mercury'
.ca Fig. 256.
.if-
.if t
[Illustration: Fig. 256.]
.if-
Schröter, a celebrated
German astronomer,
at about the
beginning of the present
century, thought
that he detected spots
and shadings on the
disk of the planet,
which indicated both
the presence of an atmosphere and of elevations. The
shading along the terminator, which seemed to indicate
the presence of a twilight, and therefore of an atmosphere,
are shown in Fig. 256. It also shows the blunted
aspect of one of the cusps, which Schröter noticed at times,
and which he attributed to the shadow of a mountain,
estimated to be ten or twelve miles high. Fig. 257 shows
// File: psp_230.png
.pn +1
this mountain near the upper cusp, as Schröter believed he
saw it in the year 1800. By watching certain marks upon
the disk of Mercury, Schröter came to the conclusion that
the planet rotates on its axis in about twenty-four hours.
Modern observers, with more powerful telescopes, have
failed to verify Schröter's observations as to the indications
of an atmosphere and of elevations. Nothing is known
with certainty about the rotation of the planet.
.if h
.il fn=fig257.png w=70% alt='Mercury'
.ca Fig. 257.
.if-
.if t
[Illustration: Fig. 257.]
.if-
The border around Mercury, and the bright spot on its
disk at the time of the transit of the planet across the sun,
have been seen since Schröter's time, and the existence of
these phenomena is now well established; but astronomers
are far from being agreed as to
their cause.
224. Intra-Mercurial Planets.--It
has for some time been
thought probable that there is a
group of small planets between
Mercury and the sun; and at various
times the discovery of such
bodies has been announced. In
1859 a French observer believed
that he had detected an intra-Mercurial
planet, to which the name of Vulcan was given,
and for which careful search has since been made, but without
success. During the total eclipse of 1878 Professor
Watson observed two objects near the sun, which he thought
to be planets; but this is still matter of controversy.
.sp 2
.h4 id='venus'
Venus.
.sp 2
225. The Orbit of Venus.--The orbit of Venus has
but slight eccentricity, differing less from a circle than that
of any other large planet. It is inclined to the ecliptic somewhat
more than three degrees. The mean distance of the
planet from the sun is about sixty-seven million miles.
// File: psp_231.png
.pn +1
226. Distance of Venus from the Earth.--The distance
of Venus from the earth varies within much wider
limits than that of Mercury. When Venus is at inferior
conjunction, her distance from the earth is ninety-two
million miles minus sixty-seven million miles, or twenty-five
million miles; and when at superior conjunction it is
ninety-two million miles plus sixty-seven million miles, or
a hundred and fifty-nine million miles. Venus is considerably
more than six times as far off at superior conjunction
as at inferior conjunction.
.if h
.il fn=fig258.png w=70% alt='Venus'
.ca Fig. 258.
.if-
.if t
[Illustration: Fig. 258.]
.if-
227. Apparent Size of Venus.--Owing to the great
variation in the distance of Venus from the earth, her
apparent diameter varies from about ten seconds to about
sixty-six seconds. Fig. 258 shows the apparent size of
Venus at her extreme and mean distances from the earth.
228. Volume and Density of Venus.--The real size of
Venus is about the same as that of the earth, her diameter
being only about three hundred miles less. The comparative
sizes of the two planets are shown in Fig. 259. The
density of Venus is a little less than that of the earth.
.if h
.il fn=fig259.png w=70% alt='Venus'
.ca Fig. 259.
.if-
.if t
[Illustration: Fig. 259.]
.if-
229. The Greatest Elongation of Venus.--Venus, like
Mercury, appears to oscillate to and fro across the sun.
The angular value of the greatest elongation of Venus
varies but slightly, its greatest value being about forty-seven
degrees.
// File: psp_232.png
.pn +1
230. Sidereal and Synodical Periods of Venus.--The
sidereal period of Venus, or that of a complete revolution
around the sun, is about two hundred and twenty-five days;
her orbital motion being at the rate of nearly twenty-two
miles a second. Her synodical period, or the time it takes
her to pass around from her greatest eastern elongation
to the same elongation again, is about five hundred and
eighty-four days, or eighteen months. Venus is a hundred
and forty-six days, or nearly five months, in passing
from her greatest elongation east through inferior conjunction
to her greatest elongation west.
231. Venus as a Morning and an Evening Star.--For
a period of about nine months, while Venus is passing from
superior conjunction to her greatest eastern elongation, she
will be east of the sun, and will therefore set after the
sun. During this period she is the evening star, the Hesperus
of the ancients. While passing from inferior conjunction
to superior conjunction, Venus is west of the sun, and
therefore rises before the sun. During this period of nine
months she is the morning star, the Phosphorus, or Lucifer,
of the ancients.
232. Brilliancy of Venus.--Next to the sun and moon,
Venus is at times the most brilliant object in the heavens,
being bright enough to be seen in daylight, and to cast
// File: psp_233.png
.pn +1
a distinct shadow at night. Her brightness, however, varies
considerably, owing to her phases and to her varying distance
from the earth. She does not appear brightest when
at full, for she is then farthest from the earth, at superior
conjunction; nor does she appear brightest when nearest the
earth, at inferior conjunction, for her phase is then a thin
crescent (see Fig. 258). She is most conspicuous while
passing from her greatest eastern elongation to her greatest
western elongation. After she has passed her eastern
elongation, she becomes brighter and brighter till she is
within about forty degrees of the sun. Her phase at
this point in her orbit is
shown in Fig. 260. Her
brilliancy then begins to
wane, until she comes too
near the sun to be visible.
When she re-appears
on the west of the sun,
she again becomes more
brilliant; and her brilliancy
increases till she is about
forty degrees from the sun,
when she is again at her
brightest. Venus passes
from her greatest brilliancy as an evening star to her greatest
brilliancy as a morning star in about seventy-three days.
She has the same phase, and is at the same distance from
the earth, in both cases of maximum brilliancy. Of course,
the brilliancy of Venus when at the maximum varies somewhat
from time to time, owing to the eccentricities of the
orbits of the earth and of Venus, which cause her distance
from the earth, at her phase of greatest brilliancy, to vary.
She is most brilliant when the phase of her greatest brilliancy
occurs when she is at her aphelion and the earth at
its perihelion.
.if h
.il fn=fig260.png w=70% alt='Venus'
.ca Fig. 260.
.if-
.if t
[Illustration: Fig. 260.]
.if-
// File: psp_234.png
.pn +1
233. The Atmosphere and Surface of Venus.--Schröter
believed that he saw shadings and markings on Venus similar
to those on Mercury, indicating the presence of an
atmosphere and of elevations on the surface of the planet.
Fig. 261 represents the surface of Venus as it appeared
to this astronomer. By watching certain markings on the
disk of Venus, Schröter came to the conclusion that Venus
rotates on her axis in about twenty-four hours.
.if h
.il fn=fig261.png w=70% alt='Venus'
.ca Fig. 261.
.if-
.if t
[Illustration: Fig. 261.]
.if-
It is now generally conceded that Venus has a dense
atmosphere; but Schröter's observations
of the spots on her disk
have not been verified by modern
astronomers, and we really know
nothing certainly of her rotation.
234. Transits of Venus.--When
Venus happens to be near one of
the nodes of her orbit when she is
in inferior conjunction, she makes
a transit across the sun's disk.
These transits occur in pairs, separated by an interval of
over a hundred years. The two transits of each pair are
separated by an interval of eight years, the dates of the most
recent being 1874 and 1882. Venus, like Mercury, appears
surrounded with a border on passing across the sun's disk,
as shown in Fig. 262.
.if h
.il fn=fig262.png w=70% alt='Venus'
.ca Fig. 262.
.if-
.if t
[Illustration: Fig. 262.]
.if-
// File: psp_235.png
.pn +1
.sp 2
.h4 id='mars'
Mars.
.sp 2
235. The Orbit of Mars.--The orbit of Mars is more
eccentric than that of any of the larger planets, except
Mercury; its eccentricity being about one-eleventh. The
inclination of the orbit to the ecliptic is somewhat under
two degrees. The mean distance of Mars from the sun
is about a hundred and forty million miles; but, owing to
the eccentricity of his orbit, the distance varies from a hundred
and fifty-three million miles to a hundred and twenty-seven
million miles.
.if h
.il fn=fig263.png w=70% alt='Mars'
.ca Fig. 263.
.if-
.if t
[Illustration: Fig. 263.]
.if-
236. Distance of
Mars from the
Earth.--It will be
seen, from Fig. 263,
that a superior
planet (or one outside
the orbit of
the earth), like
Mars, is nearer the
earth, by the whole
diameter of the
earth's orbit, when
in opposition than
when in conjunction. The mean distance of Mars from
the earth, at the time of opposition, is a hundred and forty
million miles minus ninety-two million miles, or forty-eight
million miles. Owing to the eccentricity of the orbit of
the earth and of Mars, the distance of this planet when
in opposition varies considerably. When the earth is in
aphelion, and Mars in perihelion, at the time of opposition,
the distance of the planet from the earth is only about
thirty-three million miles. On the other hand, when the
earth is in perihelion, and Mars in aphelion, at the time of
opposition, the distance of the planet is over sixty-two
million miles.
// File: psp_236.png
.pn +1
The mean distance of Mars from the earth when in
conjunction is a hundred and forty million miles plus
ninety-two million miles, or two hundred and thirty-two
million miles. It will therefore be seen that Mars is nearly
five times as far off at conjunction as at opposition.
.if h
.il fn=fig264.png w=70% alt='Mars'
.ca Fig. 264.
.if-
.if t
[Illustration: Fig. 264.]
.if-
237. The Apparent Size of Mars.--Owing to the varying
distance of Mars from the earth, the apparent size of
the planet varies almost as much as that of Venus. Fig.
264 shows the apparent size of Mars at its extreme and
mean distances from the earth. The apparent diameter
varies from about four seconds to about thirty seconds.
.if h
.il fn=fig265.png w=70% alt='Mars'
.ca Fig. 265.
.if-
.if t
[Illustration: Fig. 265.]
.if-
238. The Volume and Density of Mars.--Among the
larger planets Mars is next in size to Mercury. Its real
diameter is somewhat more than four thousand miles, and
its bulk is about one-seventh of that of the earth. Its size,
compared with that of the earth, is shown in Fig. 265.
.if h
.il fn=plate4.png w=90% alt='Mars'
.ca Plate 4.
.if-
.if t
[Illustration: Plate 4.]
.if-
// File: psp_237.png
.pn +1
The density of Mars is only about three-fourths of that
of the earth.
239. Sidereal and Synodical Periods of Mars.--The
sidereal period of Mars, or the time in which he makes a
complete revolution around the sun, is about six hundred
and eighty-seven days, or nearly twenty-three months; but
he is about seven hundred and eighty days in passing from
opposition to opposition again, or in performing a synodical
revolution. Mars moves in his orbit at the rate of about
fifteen miles a second.
240. Brilliancy of Mars.--When near his opposition,
Mars is easily recognized with the naked eye by his fiery-red
light. He is much more brilliant at some oppositions than
at others, for reasons already explained (236), but always
shines brighter than an ordinary star of the first magnitude.
241. Telescopic Appearance of Mars.--When viewed
with a good telescope (see Plate IV.), Mars is seen to be
covered with dusky, dull-red patches, which are supposed
to be continents, like those of our own globe. Other portions,
of a greenish hue, are believed to be tracts of water.
The ruddy color, which overpowers the green, and makes
the whole planet seem red to the naked eye, was believed
by Sir J. Herschel to be due to an ochrey tinge in the
general soil, like that of the red sandstone districts on the
earth. In a telescope, Mars appears less red, and the higher
the power the less the intensity of the color. The disk,
when well seen, is mapped out in a way which gives at once
the impression of land and water. The bright part is red inclining
to orange, sometimes dotted with brown and greenish
points. The darker spaces, which vary greatly in depth of
tone, are of a dull gray-green, having the aspect of a fluid
which absorbs the solar rays. The proportion of land to
water on the earth appears to be reversed on Mars. On the
earth every continent is an island; on Mars all seas are
lakes. Long, narrow straits are more common than on the
// File: psp_238.png
.pn +1
earth; and wide expanses of water, like our Atlantic Ocean,
are rare. (See Fig. 266.)
.if h
.il fn=fig266.png w=70% alt='Mars'
.ca Fig. 266.
.if-
.if t
[Illustration: Fig. 266.]
.if-
.if h
.il fn=fig267.png w=70% alt='Mars'
.ca Fig. 267.
.if-
.if t
[Illustration: Fig. 267.]
.if-
Fig. 267 represents a chart of the surface of Mars, which
has been constructed from careful telescopic observation.
The outlines, as seen in the telescope, are, however, much
// File: psp_239.png
.pn +1
less distinct than they are represented here; and it is by
no means certain that the light and dark portions are bodies
of land and water.
In the vicinity of the poles brilliant white spots may be
noticed, which are considered by many astronomers to be
masses of snow. This conjecture is favored by the fact
that they appear to diminish under the sun's influence at
the beginning of the Martial summer, and to increase again
on the approach of winter.
242. Rotation of Mars.--On watching Mars with a
telescope, the spots on the disk are found to move (as
shown in Fig. 268) in a manner which indicates that the
planet rotates in about twenty-four hours on an axis inclined
about twenty-eight degrees from a perpendicular
to the plane of its orbit. The inclination of the axis is
shown in Fig. 269. It is evident from the figure that the
variation in the length of day and night, and the change
of seasons, are about the same on Mars as on the earth.
The changes will, of course, be somewhat greater, and the
seasons will be about twice as long.
.if h
.il fn=fig268.png w=70% alt='Mars'
.ca Fig. 268.
.if-
.if t
[Illustration: Fig. 268.]
.if-
.if h
.il fn=fig269.png w=70% alt='Mars'
.ca Fig. 269.
.if-
.if t
[Illustration: Fig. 269.]
.if-
.if h
.il fn=fig270.png w=70% alt='Mars'
.ca Fig. 270.
.if-
.if t
[Illustration: Fig. 270.]
.if-
243. The Satellites of Mars.--In 1877 Professor Hall
of the Washington Observatory discovered that Mars is
accompanied by two small moons, whose orbits are shown
in Fig. 270. The inner satellite has been named Phobos,
and the outer one Deimos. It is estimated that the diameter
of the outer moon is from five to ten miles, and that
of the inner one from ten to forty miles.
// File: psp_240.png
.pn +1
Phobos is remarkable for its nearness to the planet and
the rapidity of its revolution, which is performed in seven
hours thirty-eight minutes. Its distance from the centre of
the planet is about six thousand miles, and from the surface
less than four thousand. Astronomers on Mars, with telescopes
and eyes like ours, could readily find out whether
this satellite is inhabited, the distance being less than one-sixtieth
of that of our moon.
It will be seen that Phobos makes about three revolutions
// File: psp_241.png
.pn +1
to one rotation of the planet. It will, of course, rise in the
west; though the sun, the stars, and the other satellite rise
in the east. Deimos makes a complete revolution in about
thirty hours.
.sp 2
.h4 id='asteroids'
III. THE ASTEROIDS.
.sp 2
244. Bode's Law of Planetary Distances.--There is a
very remarkable law connecting the distances of the planets
from the sun, which is generally known by the name of
Bode's Law. Attention was drawn to it in 1778 by the
astronomer Bode, but he was not really its author.
To express this law we write the following series of numbers:--
.pm verse-start
0, 3, 6, 12, 24, 48, 96;
.pm verse-end
each number, with the exception of the first, being double
the one which precedes it. If we add 4 to each of these
numbers, the series becomes--
.pm verse-start
4, 7, 10, 16, 28, 52, 100;
.pm verse-end
which series was known to Kepler. These numbers, with
the exception of 28, are sensibly proportional to the distances
of the principal planets from the sun, the actual
distances being as follows:--
.ta l:8 l:7 l:6 l:5 l:4 l:8 l:8
Mercury.| Venus.| Earth.| Mars.| ----| Jupiter.| Saturn.
||||||
3·9| 7·2| 10| 15·2| | 52·9| 95·4
.ta-
245. The First Discovery of the Asteroids.--The great
gap between Mars and Jupiter led astronomers, from the
time of Kepler, to suspect the existence of an unknown
planet in this region; but no such planet was discovered
till the beginning of the present century. Ceres was discovered
Jan. 1, 1801, Pallas in 1802, Juno in 1804, and
Vesta in 1807. Then followed a long interval of thirty-eight
years before Astræa, the fifth of these minor planets,
was discovered in 1845.
246. Olbers's Hypothesis.--After the discovery of Pallas,
// File: psp_242.png
.pn +1
Olbers suggested his celebrated hypothesis, that the two
bodies might be fragments of a single planet which had
been shattered by some explosion. If such were the case,
the orbits of all the fragments would at first intersect each
other at the point where the explosion occurred. He therefore
thought it likely that other fragments would be found,
especially if a search were kept up near the intersection of
the orbits of Ceres and Pallas.
.pm letter-start
Professor Newcomb makes the following observations concerning
this hypothesis:--
"The question whether these bodies could ever have formed
a single one has now become one of cosmogony rather than of
astronomy. If a planet were shattered, the orbit of each fragment
would at first pass through the point at which the explosion
occurred, however widely they might be separated through
the rest of their course; but, owing to the secular changes
produced by the attractions of the other planets, this coincidence
would not continue. The orbits would slowly move
away, and after the lapse of a few thousand years no trace
of a common intersection would be seen. It is therefore
curious that Olbers and his contemporaries should have expected
to find such a region of intersection, as it implied that
the explosion had occurred within a few thousand years. The
fact that the required conditions were not fulfilled was no argument
against the hypothesis, because the explosion might have
occurred millions of years ago; and in the mean time the perihelion
and node of each orbit would have made many entire
revolutions, so that the orbits would have been completely
mixed up.... A different explanation of the group is given
by the nebular hypothesis; so that Olbers's hypothesis is no
longer considered by astronomers."
.pm letter-end
247. Later Discoveries of Asteroids.--Since 1845 over
two hundred asteroids have been discovered. All these are
so small, that it requires a very good telescope to see them;
and even in very powerful telescopes they appear as mere
points of light, which can be distinguished from the stars
only by their motions.
// File: psp_243.png
.pn +1
To facilitate the discovery of these bodies, very accurate
maps have been constructed, including all the stars down to
the thirteenth magnitude in the neighborhood of the ecliptic.
A reduced copy of one of these maps is shown in Fig. 271.
.if h
.il fn=fig271.png w=70% alt='Asteroids'
.ca Fig. 271.
.if-
.if t
[Illustration: Fig. 271.]
.if-
Furnished with a map of this kind, and with a telescope
powerful enough to show all the stars marked on it, the
observer who is searching for these small planets will place
in the field of view of his telescope six spider-lines at right
angles to each other, and at equal distances apart, in such
a manner that several small squares will be formed, embracing
just as much of the heavens as do those shown in the
map. He will then direct his telescope to the region of the
sky he wishes to examine, represented by the map, so as to
be able to compare successively each square with the corresponding
// File: psp_244.png
.pn +1
portion of the sky. Fig. 272 shows at the right
hand the squares in the telescopic field of view, and at the
left hand the corresponding squares of the map.
.if h
.il fn=fig272.png w=70% alt='Asteroids'
.ca Fig. 272.
.if-
.if t
[Illustration: Fig. 272.]
.if-
He can then assure himself if the numbers and positions of
the stars mapped, and of the stars observed, are identical. If
he observes in the field of view a luminous point which is not
marked in the map, it is evident that either the new body is a
star of variable brightness which was not visible at the time
the map was made, or it is a planet, or perhaps a comet. If
the new body remains fixed at the same point, it is the former;
but, if it changes its position with regard to the neighboring
stars, it is the latter. The motion is generally so sensible, that
in the course of one evening the change of position may be
detected; and it can soon be determined, by the direction and
rate of the motion, whether the body is a planet or a comet.
.sp 2
.h4 id='outer-group'
IV. OUTER GROUP OF PLANETS.
.sp 2
.h4 id='jupiter'
Jupiter.
.sp 2
248. Orbit of Jupiter.--The orbit of Jupiter is inclined
only a little over one degree to the ecliptic; and its eccentricity
is only about half of that of Mars, being less than
one-twentieth. The mean distance of Jupiter from the sun
is about four hundred and eighty million miles; but, owing
to the eccentricity of his orbit, his actual distance from the
sun ranges from four hundred and fifty-seven to five hundred
and three million miles.
// File: psp_245.png
.pn +1
249. Distance of Jupiter from the Earth.--When
Jupiter is in opposition, his mean distance from the earth
is four hundred and eighty million miles minus ninety-two
million miles, or three hundred and eighty-eight million
miles, and, when he is in conjunction, four hundred and
eighty million miles plus ninety-two million miles, or five
hundred and seventy-two million miles. It will be seen
that he is less than twice as far off in conjunction as in
opposition, and that the ratio of his greatest to his least
distance is very much less than in the case of Venus and
Mars. This is owing to his very much greater distance from
the sun. Owing to the eccentricities of the orbits of the
earth and of Jupiter, the greatest and least distances of
Jupiter from the earth vary somewhat from year to year.
.if h
.il fn=fig273.png w=70% alt='Jupiter'
.ca Fig. 273.
.if-
.if t
[Illustration: Fig. 273.]
.if-
250. The Brightness and Apparent Size of Jupiter.--The
apparent diameter of Jupiter varies from about fifty
seconds to about thirty seconds. His apparent size at his
extreme and mean distances from the earth is shown in
Fig. 273.
Jupiter shines with a brilliant white light, which exceeds
that of every other planet except Venus. The planet is,
of course, brightest when near opposition.
251. The Volume and Density of Jupiter.--Jupiter is
the "giant planet" of our system, his mass largely exceeding
that of all the other planets combined. His mean
// File: psp_246.png
.pn +1
diameter is about eighty-five thousand miles; but the equatorial
exceeds the polar diameter by five thousand miles.
In volume he exceeds our earth about thirteen hundred
times, but in mass only about two hundred and thirteen
times. His specific gravity is, therefore, far less than that
of the earth, and even less than that of water. The comparative
size of Jupiter and the earth is shown in Fig. 274.
.if h
.il fn=fig274.png w=70% alt='Jupiter'
.ca Fig. 274.
.if-
.if t
[Illustration: Fig. 274.]
.if-
252. The Sidereal and Synodical Periods of Jupiter.--It
takes Jupiter nearly twelve years to make a sidereal revolution,
or a complete revolution around the sun, his orbital
motion being at the rate of about eight miles a second.
His synodical period, or the time of his passage from opposition
to opposition again, is three hundred and ninety-eight
days.
253. The Telescopic Aspect of Jupiter.--There are no
really permanent markings on the disk of Jupiter; but his
surface presents a very diversified appearance. The earlier
telescopic observers descried dark belts across it, one north
of the equator, and the other south of it. With the increase
of telescopic power, it was seen that these bands
// File: psp_247.png
.pn +1
were of a more complex structure than had been supposed,
and consisted of stratified, cloud-like appearances, varying
greatly in form and number. These change so rapidly, that
the face of the planet rarely presents the same appearance
on two successive nights. They are most strongly marked
at some distance on each side of the planet's equator, and
thus appear as two belts under a low magnifying power.
Both the outlines of the belts, and the color of portions
of the planet, are subject to considerable changes. The
equatorial regions, and the spaces between the belts generally,
are often of a rosy tinge. This color is sometimes
strongly marked, while at other times hardly a trace of it
can be seen. A general telescopic view of Jupiter is given
in Plate V.
.if h
.il fn=plate5.png w=90% alt='Jupiter'
.ca Plate 5.
.if-
.if t
[Illustration: Plate 5.]
.if-
254. The Physical Constitution of Jupiter.--From the
changeability of the belts, and of nearly all the visible
features of Jupiter, it is clear that what we see on that
planet is not the solid nucleus, but cloud-like formations,
which cover the entire surface to a great depth. The planet
appears to be covered with a deep and dense atmosphere,
filled with thick masses of clouds and vapor. Until recently
this cloud-laden atmosphere was supposed to be somewhat
like that of our globe; but at present the physical constitution
of Jupiter is believed to resemble that of the sun rather
than that of the earth. Like the sun, he is brighter in the
centre than near the edges, as is shown in the transits of
the satellites over his disk. When the satellite first enters
on the disk, it commonly seems like a bright spot on a
dark background; but, as it approaches the centre, it
appears like a dark spot on the bright surface of the
planet. The centre is probably two or three times brighter
than the edges. This may be, as in the case of the sun,
because the light near the edge passes through a greater
depth of atmosphere, and is diminished by absorption.
It has also been suspected that Jupiter shines partly by
// File: psp_248.png
.pn +1
his own light, and not wholly by reflected sunlight. The
planet cannot, however, emit any great amount of light;
for, if it did, the satellites would shine by this light when
they are in the shadow of the planet, whereas they totally
disappear. It is possible that the brighter portions
of the surface are from time to time slightly self-luminous.
.if h
.il fn=fig275.png w=70% alt='Jupiter'
.ca Fig. 275.
.if-
.if t
[Illustration: Fig. 275.]
.if-
Again: the interior of Jupiter seems to be the seat of an
activity so enormous that it can be ascribed only to intense
heat. Rapid movements are always occurring on his surface,
often changing its aspect in a few hours. It is therefore
probable that Jupiter is not yet covered by a solid
crust, and that the fiery interior, whether liquid or gaseous,
is surrounded by the dense vapors which cease to be luminous
on rising into the higher and cooler regions of the
atmosphere. Figs. 275 and 276 show the disk of Jupiter
as it appeared in December, 1881.
.if h
.il fn=fig276.png w=70% alt='Jupiter'
.ca Fig. 276.
.if-
.if t
[Illustration: Fig. 276.]
.if-
255. Rotation of Jupiter.--Spots are sometimes visible
// File: psp_249.png
.pn +1
which are much more permanent than the ordinary markings
on the belts. The most remarkable of these is "the
great red spot," which was first observed in July, 1878,
and is still to be seen in February, 1882. It is shown
just above the centre of the disk in Fig. 275. By watching
these spots from day to day, the time of Jupiter's axial
rotation has been found to be about nine hours and fifty
minutes.
// File: psp_250.png
.pn +1
The axis of Jupiter deviates but slightly from a perpendicular
to the plane of its orbit, as is shown in Fig. 277.
.if h
.il fn=fig277.png w=70% alt='Jupiter'
.ca Fig. 277.
.if-
.if t
[Illustration: Fig. 277.]
.if-
.sp 2
.h4 id='jupiter-satellites'
THE SATELLITES OF JUPITER.
.sp 2
.if h
.il fn=fig278.png w=70% alt='Jupiter'
.ca Fig. 278.
.if-
.if t
[Illustration: Fig. 278.]
.if-
256. Jupiter's Four Moons.--Jupiter is accompanied
by four moons, as shown in Fig. 278. The diameters of
these moons range from about twenty-two hundred to thirty-seven
hundred miles. The second from the planet is the
smallest, and the third the largest. The smallest is about
the size of our moon; the largest considerably exceeds
Mercury, and almost rivals Mars, in bulk. The sizes of
these moons, compared with those of the earth and its
moon, are shown in Fig. 279.
.if h
.il fn=fig279.png w=70% alt='Jupiter'
.ca Fig. 279.
.if-
.if t
[Illustration: Fig. 279.]
.if-
The names of these satellites, in the order of their distance
from the planet, are Io, Europa, Ganymede, and Callisto.
// File: psp_251.png
.pn +1
Their times of revolution range from about a day
and three-fourths up to about sixteen days and a half.
Their orbits are shown in Fig. 280.
.if h
.il fn=fig280.png w=70% alt='Jupiter'
.ca Fig. 280.
.if-
.if t
[Illustration: Fig. 280.]
.if-
257. The Variability of Jupiter's Satellites.--Remarkable
variations in the light of these moons have led to the
supposition that violent changes are taking place on their
surfaces. It was formerly believed, that, like our moon,
they always present the same face to the planet, and that
the changes in their brilliancy are due to differences in the
luminosity of parts of their surface which are successively
turned towards us during a revolution; but careful measurements
of their light show that this hypothesis does not
account for the changes, which are sometimes very sudden.
The satellites are too distant for examination of their surfaces
with the telescope: hence it is impossible to give any
certain explanation of these phenomena.
// File: psp_252.png
.pn +1
.if h
.il fn=fig281.png w=70% alt='Jupiter and Satellites'
.ca Fig. 281.
.if-
.if t
[Illustration: Fig. 281.]
.if-
258. Eclipses of Jupiter's Satellites.--Jupiter, like the
earth, casts a shadow away from the sun, as shown in
Fig. 281; and, whenever one of his moons passes into this
shadow, it becomes eclipsed. On the other hand, whenever
// File: psp_253.png
.pn +1
one of the moons throws its shadow on Jupiter, the sun is
eclipsed to that part of the planet which lies within the
shadow.
To the inhabitants of Jupiter (if there are any, and if
they can see through the clouds) these eclipses must be
very familiar affairs; for in consequence of the small inclinations
of the orbits of the satellites to the planet's equator,
and the small inclination of the latter to the plane of
Jupiter's orbit, all the satellites, except the most distant one,
// File: psp_254.png
.pn +1
are eclipsed in every revolution. A spectator on Jupiter
might therefore witness during the planetary year forty-five
hundred eclipses of the moons, and about the same number
of the sun.
.if h
.il fn=fig282.png w=70% alt='Jupiter and Satellites'
.ca Fig. 282.
.if-
.if t
[Illustration: Fig. 282.]
.if-
259. Transits of Jupiter's Satellites.--Whenever one
of Jupiter's moons passes in front of the planet, it is said
to make a transit across his disk. When a moon is making
a transit, it presents its bright hemisphere towards the earth,
as will be seen from Fig. 282: hence it is usually seen as a
bright spot on the planet's disk; though sometimes, on the
brighter central portions of the disk, it appears dark.
.if h
.il fn=fig283.png w=80% alt='Jupiter'
.ca Fig. 283.
.if-
.if t
[Illustration: Fig. 283.]
.if-
It will be seen from Fig. 282 that the shadow of a moon
does not fall upon the part of the planet's disk that is
covered by the moon: hence we may observe the transit
of both the moon and its shadow. The shadow appears
as a small black spot, which will precede or follow the
moon according to the position of the earth in its orbit.
Fig. 283 shows two moons of Jupiter in transit.
260. Occultations of Jupiter's Satellites.--The eclipse
of a moon of Jupiter must be carefully distinguished from
the occultation of a moon by the planet. In the case of
an eclipse, the moon ceases to be visible, because the mass
// File: psp_255.png
.pn +1
of Jupiter is interposed between the sun and the moon,
which ceases to be luminous, because the sun's light is cut
off; but, in the case of an occupation, the moon gets into
such a position that the body of Jupiter is interposed between
it and the earth, thus rendering the moon invisible
to us. The third satellite, m'' (Fig. 282), is invisible from
the earth E, having become occulted when it passed behind
the planet's disk; but
it will not be eclipsed
until it passes into the
shadow of Jupiter.
261. Jupiter without
Satellites.--It occasionally
happens that every
one of Jupiter's satellites
will disappear at
the same time, either
by being eclipsed or
occulted, or by being in
transit. In this event,
Jupiter will appear without
satellites. This occurred
on the 21st of
August, 1867. The position
of Jupiter's satellites at this time is shown in
Fig. 284.
.if h
.il fn=fig284.png w=80% alt='Jupiter'
.ca Fig. 284.
.if-
.if t
[Illustration: Fig. 284.]
.if-
.sp 2
.h4 id='saturn'
Saturn.
.sp 4
.h4 id='saturn-planet'
THE PLANET AND HIS MOONS.
.sp 2
262. The Orbit of Saturn.--The orbit of Saturn is
rather more eccentric than that of Jupiter, its eccentricity
being somewhat more than one-twentieth. Its inclination
to the ecliptic is about two degrees and a half. The mean
distance of Saturn from the sun is about eight hundred and
eighty million miles. It is about a hundred million miles
nearer the sun at perihelion than at aphelion.
// File: psp_256.png
.pn +1
263. Distance of Saturn from the Earth.--The mean
distance of Saturn from the earth at opposition is eight hundred
and eighty million miles minus ninety-two million
miles, or seven hundred and eighty-eight million; and at
conjunction, eight hundred and eighty million miles plus
ninety-two million, or nine hundred and seventy-two million.
Owing to the eccentricity of the orbit of Saturn, his distance
from the earth at opposition and at conjunction varies
by about a hundred million miles at different times; but he
is so immensely far away, that this is only a small fraction
of his mean distance.
264. Apparent Size and Brightness of Saturn.--The
apparent diameter of Saturn varies from about twenty seconds
to about fourteen seconds. His apparent size at his
extreme and mean distances from the earth is shown in
Fig. 285.
.if h
.il fn=fig285.png w=80% alt='Saturn'
.ca Fig. 285.
.if-
.if t
[Illustration: Fig. 285.]
.if-
The planet generally shines with the brilliancy of a moderate
first-magnitude star, and with a dingy, reddish light,
as if seen through a smoky atmosphere.
265. Volume and Density of Saturn.--The real diameter
of Saturn is about seventy thousand miles, and its
volume over seven hundred times that of the earth. The
comparative size of the earth and Saturn is shown in Fig.
286. This planet is a little more than half as dense as
Jupiter.
.if h
.il fn=fig286.png w=80% alt='Saturn'
.ca Fig. 286.
.if-
.if t
[Illustration: Fig. 286.]
.if-
266. The Sidereal and Synodical Periods of Saturn.--Saturn
makes a complete revolution round the sun in a
period of about twenty-nine years and a half, moving in
his orbit at the rate of about six miles a second. The
// File: psp_257.png
.pn +1
planet passes from opposition to opposition again in a
period of three hundred and seventy-eight days, or thirteen
days over a year.
267. Physical Constitution of Saturn.--The physical
constitution of Saturn seems to resemble that of Jupiter;
but, being twice as far away, the planet cannot be so well
studied. The farther an object is from the sun, the less it
is illuminated; and, the farther it is from the earth, the
smaller it appears: hence there is a double difficulty in
examining the more distant planets. Under favorable circumstances,
the surface of Saturn is seen to be diversified
with very faint markings; and, with high telescopic powers,
two or more very faint streaks, or belts, may be discerned
parallel to its equator. These belts, like those of Jupiter,
change their aspect from time to time; but they are so faint
that the changes cannot be easily followed. It is only on
rare occasions that the time of rotation can be determined
from a study of the markings.
// File: psp_258.png
.pn +1
268. Rotation of Saturn.--On the evening of Dec. 7,
1876, Professor Hall, who had been observing the satellites
of Saturn with the great Washington telescope (18), saw a
brilliant white spot near the equator of the planet. It
seemed as if an immense eruption of incandescent matter
had suddenly burst up from the interior. The spot gradually
spread itself out into a long light streak, of which the
brightest point was near the western end. It remained visible
until January, when it became faint and ill-defined, and
the planet was lost in the rays of the sun.
From all the observations on this spot, Professor Hall
found the period of Saturn to be ten hours fourteen minutes,
reckoning by the brightest part of the streak. Had the
middle of the streak been taken, the time would have been
less, because the bright matter seemed to be carried along
in the direction of the planet's rotation. If this motion
was due to a wind, the velocity of the current must have
been between fifty and a hundred miles an hour. The axis
of Saturn is inclined twenty-seven degrees from the perpendicular
to its orbit.
// File: psp_259.png
.pn +1
.if h
.il fn=fig287.png w=70% alt='Saturn'
.ca Fig. 287.
.if-
.if t
[Illustration: Fig. 287.]
.if-
269. The Satellites of Saturn.--Saturn is accompanied
by eight moons. Seven of these are shown in Fig. 287.
The names of these satellites, in the order of their distances
from the planet, are given in the accompanying table:--
.ta r:7 l:9 r:9 r:10 r:12 l:11
| Number.| Name. |Distance from Planet|Sidereal Period.||Discoverer.
1 |Mimas | 120,800| 0 22 37 | 0.94|Herschel
2 |Enceladus| 155,000| 1 8 53 | 1.37|Herschel
3 |Tethys | 191,900| 1 21 18 | 1.88|Cassini
4 |Dione | 245,800| 2 17 41 | 2.73|Cassini
5 |Rhea | 343,400| 4 12 25 | 4.51|Cassini
6 |Titan | 796,100| 15 22 41 |15.94|Huyghens
7 |Hyperion | 963,300| 21 7 7 |21.29|Bond
8 |Japetus | 2,313,800| 79 7 53 |79.33|Cassini
.ta-
The apparent brightness or visibility of these satellites
follows the order of their discovery. The smallest telescope
will show Titan, and one of very moderate size will show
Japetus in the western part of its orbit. An instrument of
four or five inches aperture will show Rhea, and perhaps
Tethys and Dione; while seven or eight inches are required
for Enceladus, even at its greatest elongation from the planet.
Mimas can rarely be seen except at its greatest elongation, and
then only with an aperture of twelve inches or more. Hyperion
can be detected only with the most powerful telescopes,
on account of its faintness and the difficulty of distinguishing
it from minute stars.
Japetus, the outermost satellite, is remarkable for the fact,
that while, in one part of its orbit, it is the brightest of the
satellites except Titan, in the opposite part it is almost as
// File: psp_260.png
.pn +1
// File: psp_261.png
.pn +1
faint as Hyperion, and can be seen only in large telescopes.
When west of the planet, it is bright; when east of it, faint.
This peculiarity has been accounted for by supposing that the
satellite, like our moon, always presents the same face to the
planet, and that one side of it is white and the other intensely
black; but it is doubtful whether any known substance is so
black as one side of the satellite must be to account for such
extraordinary changes of brilliancy.
.if h
.il fn=fig288.png w=70% alt='Titan'
.ca Fig. 288.
.if-
.if t
[Illustration: Fig. 288.]
.if-
Titan, the largest of these satellites, is about the size
of the largest satellite of Jupiter. The relative sizes of
the satellites are shown in Fig. 288, and their orbits in
Fig. 289.
.if h
.il fn=fig289.png w=70% alt='Saturn'
.ca Fig. 289.
.if-
.if t
[Illustration: Fig. 289.]
.if-
.if h
.il fn=fig290.png w=70% alt='Saturn'
.ca Fig. 290.
.if-
.if t
[Illustration: Fig. 290.]
.if-
Fig. 290 shows the transit of one of the satellites, and
of its shadow, across the disk of the planet.
.sp 2
.h4 id='saturn-rings'
THE RINGS OF SATURN.
.sp 2
270. General Appearance of the Rings.--Saturn is surrounded
by a thin flat ring lying in the plane of its equator.
This ring is probably less than a hundred miles thick. The
part of it nearest Saturn reflects little sunlight to us; so that
it has a dusky appearance, and is not easily seen, although
it is not quite so dark as the sky seen between it and the
planet. The outer edge of this dusky portion of the ring
is at a distance from Saturn of between two and three
times the earth's diameter. Outside of this dusky part of
// File: psp_262.png
.pn +1
// File: psp_263.png
.pn +1
the ring is a much brighter portion, and outside of this
another, which is somewhat fainter, but still so much brighter
than the dusky part as to be easily seen. The width of the
brighter parts of the ring is over three times the earth's
diameter. To distinguish the parts, the outer one is called
ring A, the middle one ring B, and the dusky one ring C.
Between A and B is an apparently open space, nearly two
thousand miles wide, which looks like a black line on the
ring. Other divisions in the ring have been noticed at
times; but this is the only one always seen with good telescopes
at times when either side of the ring is in view from
the earth. The general telescopic appearance of the ring
is shown in Fig. 291.
.if h
.il fn=fig291.png w=70% alt='Saturn'
.ca Fig. 291.
.if-
.if t
[Illustration: Fig. 291.]
.if-
.if h
.il fn=fig292.png w=70% alt='Saturn'
.ca Fig. 292.
.if-
.if t
[Illustration: Fig. 292.]
.if-
Fig. 292 shows the divisions of the rings as they were
seen by Bond.
271. Phases of Saturn's Ring.--The ring is inclined
to the plane of the planet's orbit by an angle of twenty-seven
degrees. The general aspect from the earth is nearly
the same as from the sun. As the planet revolves around
the sun, the axis and plane of the ring keep the same
direction in space, just as the axis of the earth and the
plane of the equator do.
When the planet is in one part of its orbit, we see the
// File: psp_264.png
.pn +1
upper or northern side of the ring at an inclination of
twenty-seven degrees, the greatest angle at which the ring
can ever be seen. This phase of the ring is shown in
Fig. 293.
.if h
.il fn=fig293.png w=70% alt='Saturn'
.ca Fig. 293.
.if-
.if t
[Illustration: Fig. 293.]
.if-
When the planet has moved through a quarter of a
revolution, the edge of the ring is turned towards the sun
and the earth; and, owing to its extreme thinness, it is
visible only in the most powerful telescopes as a fine line
of light, stretching out on each side of the planet. This
phase of the ring is shown in Fig. 294.
.if h
.il fn=fig294.png w=70% alt='Saturn'
.ca Fig. 294.
.if-
.if t
[Illustration: Fig. 294.]
.if-
All the satellites, except Japetus, revolve very nearly in
the plane of the ring: consequently, when the edge of the
ring is turned towards the earth, the satellites seem to swing
// File: psp_265.png
.pn +1
from one side of the planet to the other in a straight line,
running along the thin edge of the ring like beads on a
string. This phase affords the best opportunity of seeing
the inner satellites, Mimas and Enceladus, which at other
times are obscured by the brilliancy of the ring.
.if h
.il fn=fig295.png w=70% alt='Saturn'
.ca Fig. 295.
.if-
.if t
[Illustration: Fig. 295.]
.if-
Fig. 295 shows a phase of the ring intermediate between
the last two.
When the planet has moved ninety degrees farther, we
// File: psp_266.png
.pn +1
again see the ring at an angle of twenty-seven degrees; but
now it is the lower or southern side which is visible. When
it has moved ninety degrees farther, the edge of the ring
is again turned towards the earth and sun.
.if h
.il fn=fig296.png w=70% alt='Saturn'
.ca Fig. 296.
.if-
.if t
[Illustration: Fig. 295.]
.if-
The successive phases of Saturn's ring during a complete
revolution are shown in Fig. 296.
It will be seen that there are two opposite points of
Saturn's orbit in which the rings are turned edgewise to
us, and two points half-way between the former in which
the ring is seen at its maximum inclination of about twenty-seven
degrees. Since the planet performs a revolution in
twenty-nine years and a half, these phases occur at average
intervals of about seven years and four months.
// File: psp_267.png
.pn +1
.if h
.il fn=fig297.png w=70% alt='Saturn'
.ca Fig. 297.
.if-
.if t
[Illustration: Fig. 297.]
.if-
.if h
.il fn=fig298.png w=70% alt='Saturn'
.ca Fig. 298.
.if-
.if t
[Illustration: Fig. 298.]
.if-
272. Disappearance of Saturn's Ring.--It will be seen
from Fig. 297 that the plane of the ring may not be turned
towards the sun and the earth at exactly the same time, and
also that the earth may sometimes come on one side of the
plane of the ring while the sun is shining on the other. In
the figure, E, E', E'', and E''' is the orbit of the earth.
When Saturn is at S', or opposite, at F, the plane of the ring
will pass through the sun, and then only the edge of the ring
will be illumined. Were Saturn at S, and the earth at E', the
plane of the ring would pass through the earth. This would
also be the case were the earth at E''', and Saturn at S''.
Were Saturn at S or at S'', and the earth farther to the left or
to the right, the sun would be shining on one side of the ring
while we should be looking on the other. In all these cases
the ring will disappear entirely in a telescope of ordinary
power. With very powerful telescopes the ring will appear, in
the first two cases, as a thin line of light (Fig. 298). It will
be seen that all these cases of disappearance must take place
when Saturn is in the parts of his orbit intercepted between
the parallel lines AC and BD. These lines are tangent to
the earth's orbit, which they enclose, and are parallel to the
plane of Saturn's ring. As Saturn passes away from these
two lines on either side, the rings appear more and more open.
When the dark side of the ring is in view, it appears as a
// File: psp_268.png
.pn +1
black line crossing the planet; and on such occasions the sunlight
reflected from the outer and inner edges of the rings A
and B enables us to see traces of the ring on each side of
Saturn, at least in places where two such reflections come
nearly together. Fig. 299 illustrates this reflection from the
edges at the divisions of the rings.
.if h
.il fn=fig299.png w=70% alt='Saturn'
.ca Fig. 299.
.if-
.if t
[Illustration: Fig. 299.]
.if-
273. Changes in Saturn's Ring.--The question whether
changes are going on in the rings of Saturn is still unsettled.
Some observers have believed that they saw additional divisions
in the rings from time to time; but these may have been
errors of vision, due partly to the shading which is known to
exist on portions of the ring.
Professor Newcomb says, "As seen with the great Washington
equatorial in the autumn of 1874, there was no great or
sudden contrast between the inner or dark edge of the bright
ring and the outer edge of the dusky ring. There was some
suspicion that the one shaded into the other by insensible
gradations. No one could for a moment suppose, as some
observers have, that there was a separation between these two
rings. All these considerations give rise to the question
whether the dusky ring may not be growing at the expense
of the inner bright ring."
Struve, in 1851, advanced the startling theory that the inner
edge of the ring was gradually approaching the planet, the
// File: psp_269.png
.pn +1
whole ring spreading inwards, and making the central opening
smaller. The theory was based upon the descriptions and
drawings of the rings by the astronomers of the seventeenth
century, especially Huyghens, and the measures made by later
astronomers up to 1851. This supposed change in the dimension
of the ring is shown in Fig. 300.
.if h
.il fn=fig300.png w=70% alt='Saturn'
.ca Fig. 300.
.if-
.if t
[Illustration: Fig. 300.]
.if-
274. Constitution of Saturn's Ring.--The theory now generally
held by astronomers is, that the ring is composed of a
cloud of satellites too small to be separately seen in the telescope,
and too close together to admit of visible intervals
between them. The ring looks solid, because its parts are
too small and too numerous to be seen singly. They are like
the minute drops of water that make up clouds and fogs,
which to our eyes seem like solid masses. In the dusky ring
the particles may be so scattered that we can see through
the cloud, the duskiness being due to the blending of light and
darkness. Some believe, however, that the duskiness is caused
by the darker color of the particles rather than by their being
farther apart.
.sp 2
.h4 id='uranus'
Uranus.
.sp 2
275. Orbit and Dimensions of Uranus.--Uranus, the
smallest of the outer group of planets, has a diameter of
nearly thirty-two thousand miles. It is a little less dense
than Jupiter, and its mean distance from the sun is about
seventeen hundred and seventy millions of miles. Its orbit
has about the same eccentricity as that of Jupiter, and is
inclined less than a degree to the ecliptic. Uranus makes
// File: psp_270.png
.pn +1
a revolution around the sun in eighty-four years, moving at
the rate of a little over four miles a second. It is visible
to the naked eye as a star of the sixth magnitude.
As seen in a large telescope, the planet has a decidedly
sea-green color; but no markings have with certainty been
detected on its disk, so that
nothing is really known with
regard to its rotation. Fig.
301 shows the comparative
size of Uranus and the
earth.
.if h
.il fn=fig301.png w=70% alt='Uranus'
.ca Fig. 301.
.if-
.if t
[Illustration: Fig. 301.]
.if-
276. Discovery of Uranus.--This
planet was discovered
by Sir William Herschel in March, 1781. He was engaged
at the time in examining the small stars of the constellation
Gemini, or the Twins. He noticed that this object which
had attracted his attention had an appreciable disk, and
therefore could not be a star. He also perceived by its
motion that it could not be
a nebula; he therefore concluded
that it was a comet,
and announced his discovery
as such. On attempting to
compute its orbit, it was
soon found that its motions
could be accounted for only
on the supposition that it
was moving in a circular
orbit at about twice the distance
of Saturn from the
sun. It was therefore recognized as a new planet, whose
discovery nearly doubled the dimensions of the solar system
as it was then known.
277. The Name of the Planet.--Herschel, out of compliment
to his patron, George III., proposed to call the new
// File: psp_271.png
.pn +1
planet Georgium Sidus (the Georgian Star); but this name
found little favor. The name of Herschel was proposed, and
continued in use in England for a time, but did not meet with
general approval. Various other names were suggested, and
finally that of Uranus was adopted.
.if h
.il fn=fig302.png w=70% alt='Uranus'
.ca Fig. 302.
.if-
.if t
[Illustration: Fig. 302.]
.if-
278. The Satellites of Uranus.--Uranus is accompanied
by four satellites, whose orbits are shown in Fig. 302. These
satellites are remarkable for the great inclination of their
orbits to the plane of the planet's orbit, amounting to about
eighty degrees, and for their retrograde motion; that is,
they move from east to west, instead of from west to east,
as in the case of all the planets and of all the satellites
previously discovered.
.sp 2
.h4 id='neptune'
Neptune.
.sp 2
279. Orbit and Dimensions of Neptune.--So far as
known, Neptune is the most remote member of the solar
system, its mean distance from the sun being twenty-seven
hundred and seventy-five million miles. This distance is
considerably less than twice that of Uranus. Neptune
revolves around the sun in a period of a little less than
a hundred and sixty-five years. Its orbit has but slight
eccentricity, and is inclined less than two degrees to the
ecliptic. This planet is considerably larger than Uranus,
its diameter being nearly thirty-five thousand miles. It is
somewhat less dense than Uranus. Neptune is invisible to
the naked eye, and no telescope has revealed any markings
on its disk: hence nothing is certainly known as to its
rotation. Fig. 303 shows the comparative size of Neptune
and the earth.
.if h
.il fn=fig303.png w=70% alt='Neptune and Earth'
.ca Fig. 303.
.if-
.if t
[Illustration: Fig. 303.]
.if-
280. The Discovery of Neptune.--The discovery of
Neptune was made in 1846, and is justly regarded as one
of the grandest triumphs of astronomy.
Soon after Uranus was discovered, certain irregularities in
its motion were observed, which could not be explained. It
// File: psp_272.png
.pn +1
is well known that the planets are all the while disturbing
each other's motions, so that none of them describe perfect
ellipses. These mutual disturbances are called perturbations.
In the case of Uranus it was found, that, after
making due allowance for the action of all the known
planets, there were still certain perturbations in its course
which had not been accounted for. This led astronomers
to the suspicion that these might be caused by an unknown
planet. Leverrier in France, and Adams in England, independently
of each other, set themselves the difficult problem
of computing the position and magnitude of a planet which
would produce these perturbations. Both, by a most laborious
computation, showed that the
perturbations were such as would be
produced by a planet revolving about
the sun at about twice the distance
of Uranus, and having a mass somewhat
greater than that of this planet;
and both pointed out the same part
of the heavens as that in which the
planet ought to be found at that time. Almost immediately
after they had announced the conclusion to which
they had arrived, the planet was found with the telescope.
The astronomer who was searching for the planet
at the suggestion of Leverrier was the first to recognize
it: hence Leverrier has obtained the chief credit of the
discovery.
The observed planet is proved to be nearer than the one
predicted by Leverrier and Adams, and therefore of smaller
magnitude.
281. The Observed Planet not the Predicted One.--Professor
Peirce always maintained that the planet found by observation
was not the one whose existence had been predicted by
Leverrier and Adams, though its action would completely explain
all the irregularities in the motion of Uranus. His last
// File: psp_273.png
.pn +1
statement on this point is as follows: "My position is, that
there were two possible planets, either of which might have
caused the observed irregular
motions of Uranus. Each
planet excluded the other; so
that, if one was, the other was
not. They coincided in direction
from the earth at certain
epochs, once in six hundred
and fifty years. It was at one
of these epochs that the prediction
was made, and at no
other time for six centuries
could the prediction of the
one planet have revealed the
other. The observed planet was not the predicted one."
282. Bode's Law Disproved.--The following table gives
the distances of the planets according to Bode's law, their
actual distances, and the error of the law in each case:--
.ta l:8 r:14 r:10 r:7
| Planet. |Numbers of Bode.|Actual Distances.|Errors.
| | |
Mercury | 0 + 4 = 4 | 3.9 | 0.1
Venus | 3 + 4 = 7 | 7.2 | 0.2
Earth | 6 + 4 = 10 | 10.0 | 0.0
Mars | 12 + 4 = 16 | 15.2 | 0.8
Minor planets| 24 + 4 = 28 | 20 to 35 |
Jupiter | 48 + 4 = 52 | 52.0 | 0.0
Saturn | 96 + 4 = 100 | 95.4 | 4.6
Uranus | 192 + 4 = 196 | 191.9 | 4.1
Neptune | 384 + 4 = 388 | 300.6 | 87.4
.ta-
It will be seen, that, before the discovery of Neptune, the
agreement was so close as to indicate that this was an actual
law of the distances; but the discovery of this planet completely
disproved its existence.
// File: psp_274.png
.pn +1
.if h
.il fn=fig304.png w=70% alt='Neptune'
.ca Fig. 304.
.if-
.if t
[Illustration: Fig. 304.]
.if-
283. The Satellite of Neptune.--Neptune is accompanied
by at least one moon, whose orbit is shown in Fig.
304. The orbit of this satellite is inclined about thirty
degrees to the plane of the ecliptic, and the motion of the
satellite is retrograde, or from east to west.
.sp 2
.h3 id='comets-meteors'
VII. COMETS AND METEORS.
.sp 2
.h4 id='comets'
I. COMETS.
.sp 2
.h4 id='comets-general'
General Phenomena of Comets.
284. General Appearance of a Bright Comet.--Comets
bright enough to be seen with the naked eye are composed
of three parts, which run into each other by insensible
gradations. These
are the nucleus, the
coma, and the tail.
The nucleus is the
bright centre of the
comet, and appears
to the eye as a
star or planet.
The coma is a
nebulous mass surrounding
the nucleus
on all sides.
Close to the nucleus
it is almost as bright
as the nucleus itself;
but it gradually
shades off in every direction. The nucleus and coma combined
appear like a star shining through a small patch of
fog; and these two together form what is called the head
of the comet.
The tail is a continuation of the coma, and consists of a
// File: psp_275.png
.pn +1
stream of milky light, growing wider and fainter as it
recedes from the head, till the eye is unable to trace it.
.if h
.il fn=fig305.png w=70% alt='Comet'
.ca Fig. 305.
.if-
.if t
[Illustration: Fig. 305.]
.if-
The general appearance of one of the smaller of the
brilliant comets is shown in Fig. 305.
.if h
.il fn=fig306.png w=70% alt='Comet'
.ca Fig. 306.
.if-
.if t
[Illustration: Fig. 306.]
.if-
.if h
.il fn=fig307.png w=70% alt='Comet'
.ca Fig. 307.
.if-
.if t
[Illustration: Fig. 307.]
.if-
285. General Appearance of a Telescopic Comet.--The
great majority of comets are too faint to be visible with the
naked eye, and are called
telescopic comets. In these
comets there seems to be
a development of coma
at the expense of nucleus
and tail. In some cases
the telescope fails to reveal
any nucleus at all in
one of these comets; at
other times the nucleus is
so faint and ill-defined as
to be barely distinguishable.
Fig. 306 shows a
telescopic comet without
any nucleus at all, and
another with a slight condensation at the centre. In these
comets it is generally impossible to distinguish the coma
from the tail, the latter being either entirely invisible, as in
// File: psp_276.png
.pn +1
Fig. 306, or else only an elongation of the coma, as shown
in Fig. 307. Many comets appear simply as patches of
foggy light of more or less irregular form.
.if h
.il fn=fig308.png w=70% alt='Comet'
.ca Fig. 308.
.if-
.if t
[Illustration: Fig. 308.]
.if-
// File: psp_277.png
.pn +1
286. The Development of Telescopic Comets on their
Approach to the Sun.--As a rule, all comets look nearly
alike when they first come within the reach of the telescope.
They appear at first as little foggy patches, without
any tail, and often without
any visible nucleus. As
they approach the sun their
peculiarities are rapidly developed.
Fig. 308 shows
such a comet as first seen,
and the gradual development
of its nucleus, head, and
tail, as it approaches the
sun.
.if h
.il fn=fig309.png w=70% alt='Comet'
.ca Fig. 309.
.if-
.if t
[Illustration: Fig. 309.]
.if-
.if h
.il fn=fig310.png w=70% alt='Comet'
.ca Fig. 310.
.if-
.if t
[Illustration: Fig. 310.]
.if-
.if h
.il fn=fig311.png w=70% alt='Comet'
.ca Fig. 311.
.if-
.if t
[Illustration: Fig. 311.]
.if-
If the comet is only a
small one, the tail developed
is small; but these small appendages have a great variety
of form in different comets. Fig. 309 shows the singular
form into which Encke's comet was developed in 1871.
Figs. 310 and 311 show
other peculiar developments
of telescopic comets.
287. Development of Brilliant
Comets on their Approach
to the Sun.--Brilliant
comets, as well as
telescopic comets, appear
nearly alike when they come
into the view of the telescope;
and it is only on
their approach to the sun
that their distinctive features are developed. Not only do
these comets, when they first come into view, resemble
each other, but they also bear a close resemblance to telescopic
comets.
// File: psp_278.png
.pn +1
As the comet approaches the sun, bright vaporous jets,
two or three in number, are emitted from the nucleus on
the side of the sun and in the direction of the sun. These
jets, though directed towards the sun, are soon more or less
carried backward, as if repelled by the sun. Fig. 312
shows a succession of views of these jets as they were
developed in the case of Halley's comet in 1835.
.if h
.il fn=fig312.png w=70% alt='Comet'
.ca Fig. 312.
.if-
.if t
[Illustration: Fig. 312.]
.if-
The jets in this case seemed to have an oscillatory motion.
At 1 and 2 they seemed to be attracted towards the sun,
and in 3 to be repelled by him. In 4 and 5 they seemed
to be again attracted, and in 6 to be repelled, but in a
reverse direction to that in 3. In 7 they appeared to be
again attracted. Bessel likened this oscillation of the jets
to the vibration of a magnetic needle when presented to
the pole of a magnet.
In the case of larger comets these luminous jets are surrounded
// File: psp_279.png
.pn +1
by one or more envelops, which are thrown off in
succession as the comet approaches the sun. The formation
of these envelops was a conspicuous feature of Donati's
comet of 1858. A rough view of the jets and the surrounding
envelops is given in Fig. 313. Fig. 314 gives a view
of the envelops without the jets.
.if h
.il fn=fig313.png w=70% alt='Comet'
.ca Fig. 313.
.if-
.if t
[Illustration: Fig. 313.]
.if-
.if h
.il fn=fig314.png w=70% alt='Comet'
.ca Fig. 314.
.if-
.if t
[Illustration: Fig. 314.]
.if-
288. The Tails of Comets.--The tails of brilliant comets
are rapidly formed as the comet approaches the sun, their
increase in length often
being at the rate of several
million miles a day. These
appendages seem to be
formed entirely out of the
matter which is emitted
from the nucleus in the
luminous jets which are
at first directed towards
the sun. The tails of
comets are, however, always
directed away from the sun,
as shown in Fig. 315.
.if h
.il fn=fig315.png w=50% alt='Comet'
.ca Fig. 315.
.if-
.if t
[Illustration: Fig. 315.]
.if-
It will be seen that the
comet, as it approaches the
sun, travels head foremost;
but as it leaves the sun it
goes tail foremost.
The apparent length of the tail of a comet depends
partly upon its real length, partly upon the distance of the
comet, and partly upon the direction of the axis of the tail
with reference to the line of vision. The longer the tail,
the nearer the comet; and the more nearly at right angles
to the line of vision is the axis of the tail, the greater is
the apparent length of the tail. In the majority of cases
the tails of comets measure only a few degrees; but, in the
case of many comets recorded in history, the tail has extended
half way across the heavens.
// File: psp_280.png
.pn +1
The tail of a comet, when seen at all, is usually several
million miles in length; and in some instances the tail is
long enough to reach across the orbit of the earth, or twice
as far as from the earth to the sun.
The tails of comets are apparently hollow, and are sometimes
a million of miles in diameter. So great, however,
is the tenuity of the matter in them, that the faintest stars
are seen through it without any apparent obscuration. See
Fig. 316, which is a view of the great comet of 1264.
.if h
.il fn=fig316.png w=50% alt='Comet'
.ca Fig. 316.
.if-
.if t
[Illustration: Fig. 316.]
.if-
.if h
.il fn=fig317.png w=70% alt='Comet'
.ca Fig. 317.
.if-
.if t
[Illustration: Fig. 317.]
.if-
.if h
.il fn=fig318.png w=70% alt='Comet'
.ca Fig. 318.
.if-
.if t
[Illustration: Fig. 318.]
.if-
.if h
.il fn=fig319.png w=70% alt='Comet'
.ca Fig. 319.
.if-
.if t
[Illustration: Fig. 319.]
.if-
.if h
.il fn=fig320.png w=50% alt='Comet'
.ca Fig. 320.
.if-
.if t
[Illustration: Fig. 320.]
.if-
The tails of comets are sometimes straight, as in Fig.
316, but usually more or less curved, as in Fig. 317, which
is a view of Donati's comet as it appeared at one time.
The tail of a comet is occasionally divided into a number
of streamers, as in Figs. 318 and 319. Fig. 318 is a
view of the great comet of 1744, and Fig. 319 of the
// File: psp_281.png
.pn +1
great comet of 1861. No. 1, in Fig. 320, is a view of the
comet of 1577; No. 2, of the comet of 1680; and No. 3,
of the comet of 1769.
.if h
.il fn=fig321.png w=50% alt='Comet'
.ca Fig. 321.
.if-
.if t
[Illustration: Fig. 321.]
.if-
Fig. 321 shows some of the
forms which the imagination
of a superstitious age saw depicted
in comets, when these
heavenly visitants were thought
to be the forerunners of wars,
pestilence, famine, and other
dire calamities.
289. Visibility of Comets.--Even
the brightest comets are
visible only a short time near
their perihelion passage. When
near the sun, they sometimes
become very brilliant, and on
rare occasions have been visible
even at mid-day. It is seldom
that a comet can be seen, even
with a powerful telescope, during
its perihelion passage, unless
its perihelion is either inside
of the earth's orbit, or
but little outside of it.
.sp 2
.h4 id='comets-motion'
Motion and Origin of Comets.
.sp 2
290. Recognition of a Telescopic Comet.--It is impossible
to distinguish telescopic comets by their appearance
from another class of heavenly bodies known as nebulæ.
Such comets can be recognized only by their motion.
// File: psp_282.png
.pn +1
Thus, in Fig. 322, the upper and lower bodies look exactly
alike; but the upper one is found to remain stationary,
while the lower one moves across the field of view. The
upper one is thus shown to be a nebula, and the lower
one a comet.
.if h
.il fn=fig322.png w=70% alt='Comet'
.ca Fig. 322.
.if-
.if t
[Illustration: Fig. 322.]
.if-
291. Orbits of Comets.--All comets are found to move
in very eccentric ellipses, in parabolas, or in hyperbolas.
Since an ellipse is a
closed curve (48), all comets
that move in ellipses,
no matter how eccentric,
are permanent members
of the solar system, and
will return to the sun at
intervals of greater or less
length, according to the
size of the ellipses and
the rate of the comet's
motion.
Parabolas and hyperbolas
being open curves (48),
comets that move in either
of these orbits are only
temporary members of our
solar system. After passing
the sun, they move
off into space, never to return, unless deflected hither by
the action of some heavenly body which they pass in their
journey.
.if h
.il fn=fig323.png w=70% alt='Comet'
.ca Fig. 323.
.if-
.if t
[Illustration: Fig. 323.]
.if-
.pm letter-start
Since a comet is visible only while it is near the sun, it is
impossible to tell, by the form of the portion of the orbit
which it describes during the period of its visibility, whether
it is a part of a very elongated ellipse, a parabola, or a hyperbola.
Thus in Fig. 323 are shown two orbits, one of which
is a very elongated ellipse, and the other a parabola. The
// File: psp_283.png
.pn +1
part ab, in each case, is the portion of the orbit described
by the comet during its visibility. While describing the dotted
portions of the orbit, the comet is invisible. Now it is impossible
// File: psp_284.png
.pn +1
to distinguish the form of the visible portion in the
two orbits. The same would be true were one of the orbits
a hyperbola.
Whether a comet will describe an ellipse, a parabola, or a
hyperbola, can be determined only by its velocity, taken in connection
with its distance from the sun. Were a comet ninety-two
and a half million miles from the sun, moving away from
the sun at the rate of twenty-six miles a second, it would have
just the velocity necessary to describe a parabola. Were it
moving with a greater velocity, it would necessarily describe
a hyperbola, and, with a less velocity, an ellipse. So, at any
distance from the sun, there is a certain velocity which would
cause a comet to describe a parabola; while a greater velocity
would cause it to describe a hyperbola, and a less velocity to
describe an ellipse. If the comet is moving in an ellipse, the
less its velocity, the less the eccentricity of its orbit: hence, in
order to determine the form of the orbit of any comet, it is
only necessary to ascertain its distance from the sun, and its
velocity at any given time.
// File: psp_285.png
.pn +1
Comets move in every direction in their orbits, and these
orbits have every conceivable inclination to the ecliptic.
.pm letter-end
// File: psp_286.png
.pn +1
292. Periodic Comets.--There are quite a number of
// File: psp_287.png
.pn +1
comets which are known to be periodic, returning to the
sun at regular intervals in elliptic orbits. Some of these
have been observed at several
returns, so that their
period has been determined
with great certainty. In the
case of others the periodicity
is inferred from the
fact that the velocity fell
so far short of the parabolic
limit that the comet must
move in an ellipse. The
number of known periodic
comets is increasing every
year, three having been added to the list in 1881.
The velocity of most comets is so near the parabolic limit
that it is not possible to decide, from observations, whether it
falls short of it, or exceeds it. In the case of a few comets
the observations indicate
a minute excess of
velocity; but this cannot
be confidently asserted.
It is not, therefore, absolutely
certain that any
known comet revolves
in a hyperbolic orbit;
and thus it is possible
that all comets belong
to our system, and will
ultimately return to it.
It is, however, certain,
that, in the majority of
cases, the return will be
delayed for many centuries,
and perhaps for many thousand years.
293. Origin of Comets.--It is now generally believed that
the original home of the comets is in the stellar spaces outside
// File: psp_288.png
.pn +1
of our solar system, and that they are drawn towards the sun,
one by one, in the long lapse of ages. Were the sun unaccompanied
by planets, or were the planets immovable, a comet
thus drawn in would whirl around the sun in a parabolic orbit,
and leave it again never to return, unless its path were again
deflected by its approach to some star. But, when a comet is
moving in a parabola, the slightest retardation would change
its orbit to an ellipse, and the slightest acceleration into a
hyperbola. Owing to the motion of the several planets in
their orbits, the velocity of a comet would be changed on
passing each of them. Whether its velocity would be accelerated
or retarded, would depend upon the way in which it passed.
Were the comet accelerated by the action of the planets, on
its passage through our system, more than it was retarded by
them, it would leave the system with a more than parabolic
orbit, and would therefore move in a hyperbola. Were it, on
the contrary, retarded more than accelerated by the action of
the planets, its velocity would be reduced, so that the comet
would move in a more or less elongated ellipse, and thus
become a permanent member of the solar system.
In the majority of cases the retardation would be so slight
that it could not be detected by the most delicate observation,
and the comet would return to the sun only after the expiration
of tens or hundreds of thousands of years; but, were the
comet to pass very near one of the larger planets, the retardation
might be sufficient to cause the comet to revolve in an
elliptical orbit of quite a short period. The orbit of a comet
thus captured by a planet would have its aphelion point near
the orbit of the planet which captured it. Now, it happens
that each of the larger planets has a family of comets whose
aphelia are about its own distance from the sun. It is therefore
probable that these comets have been captured by the
action of these planets. As might be expected from the gigantic
size of Jupiter, the Jovian family of comets is the largest.
The orbits of several of the comets of this group are shown
in Fig. 324.
.if h
.il fn=fig324.png w=70% alt='Comet'
.ca Fig. 324.
.if-
.if t
[Illustration: Fig. 324.]
.if-
294. Number of Comets.--The number of comets recorded
as visible to the naked eye since the birth of Christ
// File: psp_289.png
.pn +1
is about five hundred, while about two hundred telescopic
comets have been observed since the invention of the telescope.
The total number of comets observed since the
Christian era is therefore about seven hundred. It is certain,
however, that only an insignificant fraction of all existing
comets have ever been observed. Since they can be
seen only when near their perihelion, and since it is probable
that the period of most of those which have been observed
is reckoned by thousands of years (if, indeed, they ever
return at all), our observations must be continued for many
thousand years before we have seen all which come within
range of our telescopes. Besides, as already stated (289),
a comet can seldom be seen unless its perihelion is either
// File: psp_290.png
.pn +1
inside the orbit of the earth, or but little outside of it; and
it is probable that the perihelia of the great majority of
comets are beyond this limit of visibility.
.sp 2
.h4 id='comets-remarkable'
Remarkable Comets.
.sp 2
295. The Comet of 1680.--The great comet of 1680, shown
in Fig. 320, is one of the most celebrated on record. It was
by his study of its motions that Newton proved the orbit of
a comet to be one of the conic sections, and therefore that
these bodies move under the influence of gravity. This comet
descended almost in a direct line to the sun, passing nearer to
that luminary than any comet before known. Newton estimated,
that, at its perihelion point, it was exposed to a temperature
two thousand times that of red-hot iron. During its
perihelion passage it was exceedingly brilliant. Halley suspected
that this comet had a period of five hundred and
seventy-five years, and that its first recorded appearance was
in 43 B.C., its third in 1106, and its fourth in 1680. If this is
its real period, it will return in 2255. The comet of 43 B.C.
made its appearance just after the assassination of Julius
Cæsar. The Romans called it the Julian Star, and regarded
it as a celestial chariot sent to convey the soul of Cæsar to the
skies. It was seen two or three hours before sunset, and continued
visible for eight successive days. The great comet of
1106 was described as an object of terrific splendor, and was
visible in close proximity to the sun. The comet of 1680 has
become celebrated, not only on account of its great brilliance,
and on account of Newton's investigation of its orbit, but also
on account of the speculation of the theologian Whiston in
regard to it. He accepted five hundred and seventy-five years
as its period, and calculated that one of its earlier apparitions
must have occurred at the date of the flood, which he supposed
to have been caused by its near approach to the earth; and he
imagined that the earth is doomed to be destroyed by fire on
some future encounter with this comet.
.if h
.il fn=fig325.png w=70% alt='Comet'
.ca Fig. 325.
.if-
.if t
[Illustration: Fig. 325.]
.if-
296. The Comet of 1811.--The great comet of 1811, a view
of which is given in Fig. 325, is, perhaps, the most remarkable
comet on record. It was visible for nearly seventeen months,
// File: psp_291.png
.pn +1
and was very brilliant, although at its perihelion passage it
was over a hundred million miles from the sun. Its tail was
a hundred and twenty million miles in length, and several
million miles through. It has been calculated that its aphelion
point is about two hundred times as far from the sun as its
perihelion point, or some seven times the distance of Neptune
from the sun. Its period is estimated at about three thousand
years. It was an object of superstitious terror, especially in
the East. The Russians regarded it as presaging Napoleon's
great and fatal war with Russia.
.if h
.il fn=fig326.png w=70% alt='Comet'
.ca Fig. 326.
.if-
.if t
[Illustration: Fig. 326.]
.if-
.if h
.il fn=fig327.png w=90% alt='Comet'
.ca Fig. 327.
.if-
.if t
[Illustration: Fig. 327.]
.if-
297. Halley's Comet.--Halley's comet has become one of
the most celebrated of modern times. It is the first comet
whose return was both predicted and observed. It made its
appearance in 1682. Halley computed its orbit, and compared
it with those of previous comets, whose orbits he also computed
from recorded observations. He found that it coincided
so exactly with that of the comet observed by Kepler in 1607,
that there could be no doubt of the identity of the two orbits.
So close were they together, that, were they both drawn in the
// File: psp_292.png
.pn +1
heavens, the naked eye would almost see them joined into one
line. There could therefore be no doubt that the comet of
1682 was the same that had appeared in 1607, and that it moved
in an elliptic orbit, with a period of about seventy-five years.
He found that this
comet had previously
appeared in
1531 and in 1456;
and he predicted
that it would return
about 1758. Its
actual return was
retarded somewhat
by the action of
the planets on it in
its passage through
the solar system.
It, however, appeared
again in
1759, and a third time in 1835. Its next appearance will be
about 1911. The orbit of this comet is shown in Fig. 326.
Fig. 327 shows the comet as it appeared to the naked eye, and
in a telescope of moderate power, in 1835. This comet appears
to be growing less brilliant. In 1456 it appeared as a comet
of great splendor; and coming as it did in a very superstitious
age, soon after the fall of Constantinople, and during the threatened
// File: psp_293.png
.pn +1
invasion of Europe by the Turks, it caused great alarm.
Fig. 328 shows the changes undergone by the nucleus of this
comet during its perihelion passage in 1835.
.if h
.il fn=fig328.png w=70% alt='Comet'
.ca Fig. 328.
.if-
.if t
[Illustration: Fig. 328.]
.if-
.if h
.il fn=fig329.png w=70% alt='Comet'
.ca Fig. 329.
.if-
.if t
[Illustration: Fig. 329.]
.if-
.if h
.il fn=fig330.png w=70% alt='Comet'
.ca Fig. 330.
.if-
.if t
[Illustration: Fig. 330.]
.if-
298. Encke's Comet.--This telescopic comet, two views of
which are given in Figs. 329 and 330, appeared in 1818. Encke
computed its orbit, and found it to lie wholly within the orbit
of Jupiter (Fig. 324), and the period to be about three years
and a third. By comparing the intervals between the successive
returns of this comet, it has been ascertained that its
orbit is continually growing smaller and smaller. To account
for the retardation of this comet, Olbers announced his celebrated
hypothesis, that the celestial spaces are filled with a
subtile resisting medium. This hypothesis was adopted by
Encke, and has been accepted by certain other astronomers;
but it has by no means gained universal assent.
299. Biela's Comet.--This comet appeared in 1826, and
was found to have a period of about six years and two thirds.
On its return in 1845, it met with a singular, and as yet unexplained,
// File: psp_294.png
.pn +1
accident, which has rendered the otherwise rather
insignificant comet famous. In November and December of
that year it was observed as usual, without any thing remarkable
about it; but, in
January of the following
year, it was
found to have been
divided into two distinct
parts, so as to
appear as two comets
instead of one. The
two parts were at
first of very unequal
brightness; but, during
the following
month, the smaller
of the two increased
in brilliancy until it equalled its companion; it then grew
fainter till it entirely disappeared, a month before its companion.
The two parts were about two hundred thousand miles apart.
Fig. 331 shows these
two parts as they
appeared on the 19th
of February, and Fig.
332 as they appeared
on the 21st of February.
On its return
in 1852, the comets
were found still to
be double; but the
two components were
now about a million
and a half miles
apart. They are
shown in Fig. 333
as they appeared at
this time. Sometimes one of the parts appeared the brighter,
and sometimes the other; so that it was impossible to decide
which was really the principal comet. The two portions passed
// File: psp_295.png
.pn +1
out of view in September, and have not been seen since;
although in 1872 the position of the comet would have been
especially favorable for observation. The comet appears to
have become completely broken up.
.if h
.il fn=fig331.png w=70% alt='Comet'
.ca Fig. 331.
.if-
.if t
[Illustration: Fig. 331.]
.if-
.if h
.il fn=fig332.png w=70% alt='Comet'
.ca Fig. 332.
.if-
.if t
[Illustration: Fig. 332.]
.if-
.if h
.il fn=fig333.png w=70% alt='Comet'
.ca Fig. 333.
.if-
.if t
[Illustration: Fig. 333.]
.if-
.if h
.il fn=fig334.png w=50% alt='Comet'
.ca Fig. 334.
.if-
.if t
[Illustration: Fig. 334.]
.if-
300. The Comet of 1843.--The great comet of 1843, a view
of which is given in Fig. 334, was favorably situated for observation
only in southern latitudes. It was exceedingly brilliant,
and was easily seen in full daylight, in close proximity to the
sun. The apparent length of its tail was sixty-five degrees,
and its real length a hundred and fifty million miles, or nearly
// File: psp_296.png
.pn +1
twice the distance from the earth to the sun. This comet is
especially remarkable on account of its near approach to the
sun. At the time of its perihelion passage the distance of
the comet from the photosphere of the sun was less than
one-fourteenth of the diameter of the sun. This distance was
only one-half that of the comet of 1680 when at its perihelion.
When at perihelion, this comet was plunging through the sun's
outer atmosphere at the rate of one million, two hundred and
eighty thousand miles an hour. It passed half way round the
sun in the space of two hours, and its tail was whirled round
through a hundred and eighty degrees in that brief time. As
the tail extended almost double the earth's distance from the
sun, the end of the tail must have traversed in two hours a
space nearly equal to the circumference of the earth's orbit,--a
distance which the earth, moving at the rate of about twenty
miles a second, is a whole year in passing. It is almost impossible
to suppose that the matter forming this tail remained the
same throughout this tremendous sweep.
301. Donati's Comet.--The great comet of 1858, known as
Donati's comet, was one of the most magnificent of modern
times. When at its brightest it was only about fifty million
miles from the earth. Its tail was then more than fifty million
miles long. Had the comet at this time been directly
between the earth and sun, the earth must have passed through
// File: psp_297.png
.pn +1
its tail; but this did not occur. The orbit of this comet was
found to be decidedly elliptic, with a period of about two thousand
years. This comet is especially celebrated on account
of the careful telescopic observations of its nucleus and coma
at the time of its perihelion passage. Attention has already
been called (287) to the changes it
underwent at that time. Its tail was
curved, and of a curious feather-like
form, as shown in Fig. 335. At times
it developed lateral streamers, as shown
in Fig. 336. Fig. 337 shows the head
of the comet as it was seen by Bond
of the Harvard Observatory, whose
delineations of this comet have been
justly celebrated.
.if h
.il fn=fig335.png w=70% alt='Comet'
.ca Fig. 335.
.if-
.if t
[Illustration: Fig. 335.]
.if-
.if h
.il fn=fig336.png w=70% alt='Comet'
.ca Fig. 336.
.if-
.if t
[Illustration: Fig. 336.]
.if-
.if h
.il fn=fig337.png w=70% alt='Comet'
.ca Fig. 337.
.if-
.if t
[Illustration: Fig. 337.]
.if-
302. The Comet of 1861.--The great
comet of 1861 is remarkable for its
great brilliancy, for its peculiar fan-shaped
tail, and for the probable passage
of the earth through its tail. Sir
John Herschel declared that it far exceeded
in brilliancy any comet he had
ever seen, not excepting those of 1811
and 1858. Secchi found its tail to be
a hundred and eighteen degrees in
length, the largest but one on record.
Fig. 338 shows this comet as it appeared
at one time. Fig. 339 shows the position
of the earth at E, in the tail of
this comet, on the 30th of June, 1861.
Fig. 340 shows the probable passage of
the earth through the tail of the comet
on that date. As the tail of a comet
doubtless consists of something much
less dense than our atmosphere, it is not surprising that no
noticeable effect was produced upon us by the encounter, if it
occurred.
.if h
.il fn=fig338.png w=70% alt='Comet'
.ca Fig. 338.
.if-
.if t
[Illustration: Fig. 338.]
.if-
.if h
.il fn=fig339.png w=70% alt='Comet'
.ca Fig. 339.
.if-
.if t
[Illustration: Fig. 339.]
.if-
.if h
.il fn=fig340.png w=70% alt='Comet'
.ca Fig. 340.
.if-
.if t
[Illustration: Fig. 340.]
.if-
303. Coggia's Comet.--This comet, which appeared in 1874,
looked very large, because it came very near the earth. It was
// File: psp_298.png
.pn +1
not at all brilliant. Its nucleus was carefully studied, and was
// File: psp_299.png
.pn +1
found to develop a series of envelops similar to those of
Donati's comet. Figs. 341 and 342 are two views of the head
of this comet. Fig. 343 shows the system of envelops that
were developed during its perihelion passage.
.if h
.il fn=fig341.png w=70% alt='Comet'
.ca Fig. 341.
.if-
.if t
[Illustration: Fig. 341.]
.if-
.if h
.il fn=fig342.png w=70% alt='Comet'
.ca Fig. 342.
.if-
.if t
[Illustration: Fig. 342.]
.if-
.if h
.il fn=fig343.png w=70% alt='Comet'
.ca Fig. 343.
.if-
.if t
[Illustration: Fig. 343.]
.if-
// File: psp_300.png
.pn +1
304. The Comet of June, 1881.--This comet, though far
from being one of the largest of modern times, was still very
brilliant. It will ever be memorable as the first brilliant comet
which has admitted of careful examination with the spectroscope.
.sp 2
.h4 id='comets-connection'
Connection between Meteors and Comets.
.sp 2
305. Shooting-Stars.--On watching the heavens any
clear night, we frequently see an appearance as of a star
// File: psp_301.png
.pn +1
shooting rapidly through a short space in the sky, and then
suddenly disappearing. Three or four such shooting-stars
may, on the average, be observed in the course of an hour.
They are usually seen only a second or two; but they sometimes
move slowly, and are visible much longer. These
stars begin to be visible at an average height of about
seventy-five miles, and they disappear at an average height
of about fifty miles. They are occasionally seen as high
as a hundred and fifty miles, and continue to be visible
till within thirty miles of the earth. Their visible paths
vary from ten to a hundred miles in length, though they
are occasionally two hundred or three hundred miles long.
Their average velocity, relatively to the earth's surface, varies
from ten to forty-five miles a second.
The average number of shooting-stars visible to the
naked eye at any one place is estimated at about a thousand
an hour; and the average number large enough to
be visible to the naked eye, that traverse the atmosphere
daily, is estimated at over eight millions. The number of
telescopic shooting-stars would of course be much greater.
Occasionally, shooting-stars leave behind them a trail of
// File: psp_302.png
.pn +1
light which lasts for several seconds. These trails are sometimes
straight, as shown in Fig. 344, and sometimes curved,
as in Figs. 345 and 346. They often disappear like trails
of smoke, as shown in Fig. 347.
.if h
.il fn=fig344.png w=70% alt='Meteors'
.ca Fig. 344.
.if-
.if t
[Illustration: Fig. 344.]
.if-
.if h
.il fn=fig345.png w=70% alt='Meteors'
.ca Fig. 345.
.if-
.if t
[Illustration: Fig. 345.]
.if-
.if h
.il fn=fig346.png w=70% alt='Meteors'
.ca Fig. 346.
.if-
.if t
[Illustration: Fig. 346.]
.if-
.if h
.il fn=fig347.png w=70% alt='Meteors'
.ca Fig. 347.
.if-
.if t
[Illustration: Fig. 347.]
.if-
Shooting-stars are seen to move in all directions through
the heavens. Their apparent paths are, however, generally
inclined downward, though sometimes upward; and after
midnight they come in the greatest numbers from that
quarter of the heavens toward which the earth is moving
in its journey around the sun.
// File: psp_303.png
.pn +1
306. Meteors.--Occasionally these bodies are brilliant
enough to illuminate the whole heavens. They are then
called meteors, although this term is equally applicable
to ordinary shooting-stars. Such a meteor is shown in
Fig. 348.
.if h
.il fn=fig348.png w=70% alt='Meteors'
.ca Fig. 348.
.if-
.if t
[Illustration: Fig. 348.]
.if-
Sometimes these brilliant meteors are seen to explode,
as shown in Fig. 349; and the explosion is accompanied
with a loud detonation, like the discharge of cannon.
.if h
.il fn=fig349.png w=70% alt='Meteors'
.ca Fig. 349.
.if-
.if t
[Illustration: Fig. 349.]
.if-
Ordinary shooting-stars are not accompanied by any
// File: psp_304.png
.pn +1
audible sound, though they are sometimes seen to break
in pieces. Meteors which explode with an audible sound
are called detonating meteors.
307. Aerolites.--There is no certain evidence that any
deposit from ordinary shooting-stars ever reaches the surface
of the earth; though a peculiar dust has been found
in certain localities, which has been supposed to be of
meteoric origin, and which has been called meteoric dust.
// File: psp_305.png
.pn +1
But solid bodies occasionally descend to the earth from
beyond our atmosphere. These generally penetrate a foot
or more into the earth, and, if picked up soon after their
fall, are found to be warm, and sometimes even hot. These
bodies are called aerolites. When they have a stony appearance,
and contain but little iron, they are called meteoric
stones; when they have a metallic appearance, and are
composed largely of iron, they are called meteoric iron.
There are eighteen well-authenticated cases in which aerolites
have fallen in the United States during the last sixty
years, and their aggregate weight is twelve hundred and fifty
pounds. The entire number of known aerolites the date of
whose fall is well determined is two hundred and sixty-one.
There are also on record seventy-four cases of which the date
// File: psp_306.png
.pn +1
is more or less uncertain. There have also been found eighty-six
masses, which, from their peculiar composition, are believed
to be aerolites, though their fall was not seen. The weight
of these masses varies from a few pounds to several tons.
The entire number of aerolites of which we have any knowledge
is therefore about four hundred and twenty.
Aerolites are composed of the same elementary substances
as occur in terrestrial minerals, not a single new element
having been found in their analysis. Of the sixty or more
// File: psp_307.png
.pn +1
elements now recognized by chemists, about twenty have been
found in aerolites.
While aerolites contain no new elements, their appearance
is quite peculiar; and the compounds found in them are so
peculiar as to enable us by chemical analysis to distinguish
an aerolite from any terrestrial substance.
Iron ores are very abundant in nature, but iron in the
metallic state is exceedingly rare. Now, aerolites invariably
contain metallic iron, sometimes from ninety to ninety-six per
cent. This iron is malleable, and may be readily worked into
cutting instruments. It always contains eight or ten per cent
of nickel, together with small quantities of cobalt, copper, tin,
and chromium. This composition has never been found in
any terrestrial mineral. Aerolites also contain, usually in
small amount, a compound of iron, nickel, and phosphorus,
which has never been found elsewhere.
// File: psp_308.png
.pn +1
Meteorites often present the appearance of having been
fused on the surface to a slight depth, and meteoric iron is
found to have a peculiar crystalline structure. The external
appearance of a piece of meteoric iron found near Lockport,
N.Y., is shown in Fig. 350. Fig. 351 shows the peculiar
internal structure of meteoric iron.
.if h
.il fn=fig350.png w=70% alt='Meteors'
.ca Fig. 350.
.if-
.if t
[Illustration: Fig. 350.]
.if-
.if h
.il fn=fig351.png w=70% alt='Meteor'
.ca Fig. 351.
.if-
.if t
[Illustration: Fig. 351.]
.if-
308. Meteoroids.--Astronomers now universally hold
that shooting-stars, meteors, and aerolites are all minute
bodies, revolving, like the comets, about the sun. They
are moving in every possible direction through the celestial
spaces. They may not average more than one in a million
of cubic miles, and yet their total number exceeds all calculation.
Of the nature of the minuter bodies of this class
nothing is certainly known. The earth is continually encountering
them in its journey around the sun. They are
burned by passing through the upper regions of our atmosphere,
and the shooting-star is simply the light of that
burning. These bodies, which are invisible till they plunge
into the earth's atmosphere, are called meteoroids.
309. Origin of the Light of Meteors.--When one of
// File: psp_309.png
.pn +1
these meteoroids enters our atmosphere, the resistance of
the air arrests its motion to some extent, and so converts
a portion of its energy of motion into that of heat. The
heat thus developed is sufficient to raise the meteoroid and
the air around it to incandescence, and in most cases
either to cause the meteoroid to burn up, or to dissipate it
as vapor. The luminous vapor thus formed constitutes the
luminous train which occasionally accompanies a meteor,
and often disappears as a puff of smoke. When a meteoroid
is large enough and refractory enough to resist the
heat to which it is exposed, its motion is sufficiently arrested,
on entering the lower layers of our atmosphere, to cause
it to fall to the earth. We then have an aerolite. A
brilliant meteor differs from a shooting-star simply in magnitude.
310. The Intensity of the Heat to which a Meteoroid is
Exposed.--It has been ascertained by experiment that a body
moving through the atmosphere at the rate of a hundred and
twenty-five feet a second raises the temperature of the air
immediately in front of it one degree, and that the temperature
increases as the square of the velocity of the moving body;
that is to say, that, with a velocity of two hundred and fifty
feet, the temperature in front of the body would be raised
four degrees; with a velocity of five hundred feet, sixteen
degrees; and so on. To find, therefore, the temperature to
which a meteoroid would be exposed in passing through our
atmosphere, we have merely to divide its velocity in feet per
second by a hundred and twenty-five, and square the quotient.
With a velocity of forty-four miles a second in our atmosphere,
a meteoroid would therefore be exposed to a temperature of
between three and four million degrees. The air acts upon
the body as if it were raised to this intense heat. At such a
temperature small masses of the most refractory or incombustible
substances known to us would flash into vapor with
the evolution of intense light and heat.
If one of these meteoric bodies is large enough to pass
// File: psp_310.png
.pn +1
through the atmosphere and reach the earth, without being
volatilized by the heat, we have an aerolite. As it is only a
few seconds in making the passage, the heat has not time to
penetrate far into its interior, but is expended in melting and
vaporizing the outer portions. The resistance of the denser
strata of the atmosphere to the motion of the aerolite sometimes
becomes so enormous that the body is suddenly rent to
pieces with a loud detonation. It seems like an explosion produced
by some disruptive action within the mass; but there
can be little doubt that it is due to the velocity--perhaps ten,
twenty, or thirty miles a second--with which the body strikes
the air.
If, on the other hand, the meteoroid is so small as to be
burned up or volatilized in the upper regions of the atmosphere,
we have a common shooting-star, or a meteor of greater
or less brilliancy.
.if h
.il fn=fig352.png w=70% alt='Meteor'
.ca Fig. 352.
.if-
.if t
[Illustration: Fig. 352.]
.if-
311. Meteoric Showers.--On ordinary nights only four
or five shooting-stars are seen in an hour, and these move
in every direction. Their orbits lie in all possible positions,
and are seemingly scattered at random. Such meteors are
called sporadic meteors. On occasional nights, shooting-stars
are more numerous, and all move in a common direction.
Such a display is called a meteoric shower. These
showers differ greatly in brilliancy; but during any one
shower the meteors all appear to radiate from some one
point in the heavens. If we mark on a celestial globe the
apparent paths of the meteors which fall during a shower,
or if we trace them back on the celestial sphere, we shall
find that they all meet in the same point, as shown in Fig.
352. This point is called the radiant point. It always
appears in the same position, wherever the observer is situated,
and does not partake of the diurnal motion of the
earth. As the stars move towards the west, the radiant
point moves with them. The point in question is purely
an effect of perspective, being the "vanishing point" of
the parallel lines in which the meteors are actually moving.
// File: psp_311.png
.pn +1
These lines are seen, not in their real direction in space,
but as projected on the celestial sphere. If we look upwards,
and watch snow falling through a calm atmosphere,
the flakes which fall directly towards us do not seem to
move at all, while the surrounding flakes seem to diverge
from them on all sides. So, in a meteoric shower, a
meteor coming directly towards the observer does not seem
to move at all, and marks the point from which all the
others seem to radiate.
312. The August Meteors.--A meteoric shower of no
great brilliancy occurs annually about the 10th of August.
The radiant point of this shower is in the constellation Perseus,
and hence these meteors are often called the Perseids.
The orbit of these meteoroids has been pretty accurately
determined, and is shown in Fig. 353.
.if h
.il fn=fig353.png w=50% alt='Meteors'
.ca Fig. 353.
.if-
.if t
[Illustration: Fig. 353.]
.if-
// File: psp_312.png
.pn +1
It will be seen that the perihelion point of this orbit is
at about the distance of the earth from the sun; so that
the earth encounters the meteors
once a year, and this takes
place in the month of August.
The orbit is a very eccentric
ellipse, reaching far beyond
Neptune. As the meteoric display
is about equally brilliant
every year, it seems probable
that the meteoroids form a
stream quite uniformly distributed
throughout the whole
orbit. It probably takes one
of the meteoroids about a hundred
and twenty-four years to
pass around this orbit.
.if h
.il fn=fig354.png w=50% alt='Meteor Orbits'
.ca Fig. 354.
.if-
.if t
[Illustration: Fig. 354.]
.if-
313. The November Meteors.--A
somewhat brilliant meteoric
shower also occurs annually,
about the 13th of November.
The radiant point of
these meteors is in the constellation
Leo, and hence they
are often called the Leonids.
Their orbit has been determined
with great accuracy, and
is shown in Fig. 354. While
the November meteors are not
usually very numerous or bright,
a remarkably brilliant display
of them has been seen once in
about thirty-three or thirty-four years: hence we infer, that,
while there are some meteoroids scattered throughout the
whole extent of the orbit, the great majority are massed in
// File: psp_313.png
.pn +1
a group which traverses the orbit in a little over thirty-three
years. A conjectural form of this condensed group is
shown in Fig. 355. The group is so large that it takes it
two or three years to pass the perihelion point: hence there
may be a brilliant meteoric display two or three years in
succession.
.if h
.il fn=fig355.png w=50% alt='Meteors'
.ca Fig. 355.
.if-
.if t
[Illustration: Fig. 355.]
.if-
The last brilliant display of these meteors was in the
years 1866 and 1867.
The display was visible
in this country only a
short time before sunrise,
and therefore did
not attract general attention.
The display of
1833 was remarkably
brilliant in this country,
and caused great consternation
among the
ignorant and superstitious.
.if h
.il fn=fig356.png w=50% alt='Meteors and Comets'
.ca Fig. 356.
.if-
.if t
[Illustration: Fig. 356.]
.if-
314. Connection between
Meteors and Comets.--It
has been found that a
comet which appeared in
1866, and which is designated
as 1866, I., has
exactly the same orbit
and period as the November
meteors, and that another comet, known as the 1862,
III., has the same orbit as the August meteors. It has also
been ascertained that a third comet, 1861, I., has the same
orbit as a stream of meteors which the earth encounters in
April. Furthermore, it was found, in 1872, that there was a
small stream of meteors following in the train of the lost
comet of Biela. These various orbits of comets and meteoric
streams are shown in Fig. 356. The coincidence of the orbits
// File: psp_314.png
.pn +1
of comets and of meteoric streams indicates that these two
classes of bodies are very closely related. They undoubtedly
have a common origin. The fact that there is a stream of
meteors in the train of Biela's comet has led to the supposition
that comets may become gradually disintegrated into
meteoroids.
.sp 2
.h4 id='comets-physical'
Physical and Chemical Constitution of Comets.
.sp 2
315. Physical Constitution of Telescopic Comets.--We have
no certain knowledge of the physical constitution of telescopic
comets. They are usually tens
of thousands of miles in diameter,
and yet of such tenuity that
the smallest stars can readily be
seen through them. It would
seem that they must shine in
part by reflected light; yet the
spectroscope shows that their
spectrum is composed of bright
bands, which would indicate that
they are composed, in part at
least, of incandescent gases. It
is, however, difficult to conceive
how these gases become sufficiently
heated to be luminous;
and at the same time such
gases would reflect no sunlight.
It seems probable that these
comets are really made up of a
combination of small, solid particles
in the form of minute
meteoroids, and of gases which
are, perhaps, rendered luminous
by electric discharges of slight
intensity.
316. Physical Constitution of Large Comets.--In the case
of large comets the nucleus is either a dense mass of solid
matter several hundred miles in diameter, or a dense group
of meteoroids. Professor Peirce estimated that the density
// File: psp_315.png
.pn +1
of the nucleus is at least equal to that of iron. As such a
comet approaches the sun, the nucleus is, to a slight extent,
vaporized, and out of this vapor is formed the coma and the
tail.
That some evaporating process is going on from the nucleus
of the comet is proved by the movements of the tail. It is
evident that the tail cannot be an appendage carried along with
the comet, as it seems to be. It is impossible that there
should be any cohesion in matter of such tenuity that the
smallest stars could be seen through a million of miles of it,
and which is, moreover, continually changing its form. Then,
again, as a comet is passing its perihelion, the tail appears to
be whirled from one side of the sun to another with a rapidity
which would tear it to pieces if the movement were real. The
tail seems to be, not something attached to the comet, and
carried along with it, but a stream of vapor issuing from it,
like smoke from a chimney. The matter of which it is composed
is continually streaming outwards, and continually being
replaced by fresh vapor from the nucleus.
The vapor, as it emanates from the nucleus, is repelled by
the sun with a force often two or three times as great as the
ordinary solar attraction. The most probable explanation of
this phenomenon is, that it is a case of electrical repulsion, the
sun and the particles of the cometary mist being similarly
// File: psp_316.png
.pn +1
electrified. With reference to this electrical theory of the
formation of comets' tails, Professor Peirce makes the following
observation: "In its approach to the sun, the surface of
the nucleus is rapidly heated: it is melted and vaporized, and
subjected to frequent explosions. The vapor rises in its atmosphere
with a well-defined upper surface, which is known to
observers as an envelop.... The electrification of the cometary
mist is analogous to that of our own thunder-clouds. Any
portion of the coma which has received the opposite kind of
electricity to the sun and to the repelled tail will be attracted.
This gives a simple explanation of the negative tails which have
been sometimes seen directed towards the sun. In cases of
violent explosion, the whole nucleus might be broken to pieces,
and the coma dashed
around so as to give
varieties of tail, and even
multiple tails. There
seems, indeed, to be no
observed phenomenon of
the tail or the coma
which is not consistent
with a reasonable modification
of the theory."
Professor Peirce regarded
comets simply as the
largest of the meteoroids.
They appear to shine partly by reflected sunlight, and partly
by their own proper light, which seems to be that of vapor
rendered luminous by an electric discharge of slight intensity.
.if h
.il fn=fig357.png w=70% alt='Meteors and Comets'
.ca Fig. 357.
.if-
.if t
[Illustration: Fig. 357.]
.if-
317. Collision of a Comet and the Earth.--It sometimes
happens that the orbit of a comet intersects that of the earth,
as is shown in Fig. 357, which shows a portion of the orbit
of Biela's comet, with the positions of the comet and of the
earth in 1832. Of course, were a comet and the earth both to
reach the intersection of their orbits at the same time, a collision
of the two bodies would be inevitable. With reference
to the probable effect of such a collision, Professor Newcomb
remarks,--
"The question is frequently asked, What would be the
// File: psp_317.png
.pn +1
effect if a comet should strike the earth? This would depend
upon what sort of a comet it was, and what part of the comet
came in contact with our planet. The latter might pass through
the tail of the largest comet without the slightest effect being
produced; the tail being so thin and airy that a million miles
thickness of it looks only like gauze in the sunlight. It is not
at all unlikely that such a thing may have happened without
ever being noticed. A passage through a telescopic comet
would be accompanied by a brilliant meteoric shower, probably
a far more brilliant one than has ever been recorded. No
more serious danger would be encountered than that arising
from a possible fall of meteorites; but a collision between
the nucleus of a large comet and the earth might be a serious
matter. If, as Professor Peirce supposes, the nucleus is a solid
body of metallic density, many miles in diameter, the effect
where the comet struck would be terrific beyond conception.
At the first contact in the upper regions of the atmosphere, the
whole heavens would be illuminated with a resplendence
beyond that of a thousand suns, the sky radiating a light which
would blind every eye that beheld it, and a heat which would
melt the hardest rocks. A few seconds of this, while the
huge body was passing through the atmosphere, and the collision
at the earth's surface would in an instant reduce everything
there existing to fiery vapor, and bury it miles deep in
the solid earth. Happily, the chances of such a calamity are
so minute that they need not cause the slightest uneasiness.
There is hardly a possible form of death which is not a thousand
times more probable than this. So small is the earth in
// File: psp_318.png
.pn +1
comparison with the celestial spaces, that if one should shut
his eyes, and fire a gun at random in the air, the chance of
bringing down a bird would be better than that of a comet of
any kind striking the earth."
.if h
.il fn=fig358.png w=70% alt='Spectrum'
.ca Fig. 358.
.if-
.if t
[Illustration: Fig. 358.]
.if-
.if h
.il fn=fig359.png w=70% alt='Spectrum'
.ca Fig. 359.
.if-
.if t
[Illustration: Fig. 359.]
.if-
318. The Chemical Constitution of Comets.--Fig. 358 shows
the bands of the spectrum of a telescopic comet of 1873, as
seen by two different observers. Fig. 359 shows the spectrum
of the coma and tail of the comet of 1874; and the spectrum
of the bright comet of 1881 showed the same three bands for
the coma and tail. Now, these three bands are those of certain
hydrocarbon vapors: hence it would seem that the coma
and tails of comets are composed chiefly of such vapors (315).
.sp 2
.h4 id='zodiacal'
II. THE ZODIACAL LIGHT.
.sp 2
319. The General Appearance of the Zodiacal Light.--The
phenomenon known as the zodiacal light consists of a
very faint luminosity, which may be seen rising from the
western horizon after twilight on any clear winter or spring
evening, also from the eastern horizon just before daybreak
in the summer or autumn. It extends out on each side
of the sun, and lies nearly in the plane of the ecliptic. It
grows fainter the farther it is from the sun, and can generally
be traced to about ninety degrees from that luminary,
// File: psp_319.png
.pn +1
// File: psp_320.png
.pn +1
when it gradually fades away. In a very clear, tropical
atmosphere, it has been traced all the way across the
heavens from east to west, thus forming a complete ring.
The general appearance of this column of light, as seen
in the morning, in the latitude of Europe, is shown in
Fig. 360.
.if h
.il fn=fig360.png w=70% alt='Zodiacal Light'
.ca Fig. 360.
.if-
.if t
[Illustration: Fig. 360.]
.if-
Taking all these appearances together, they indicate that
it is due to a lens-shaped appendage surrounding the sun,
and extending a little
beyond the earth's
orbit. It lies nearly
in the plane of the
ecliptic; but its exact
position is not easily
determined. Fig.
361 shows the general
form and position
of this solar
appendage, as seen
in the west.
.if h
.il fn=fig361.png w=70% alt='Zodiacal Light'
.ca Fig. 361.
.if-
.if t
[Illustration: Fig. 361.]
.if-
320. The Visibility
of the Zodiacal Light.--The
reason why the
zodiacal light is more
favorably seen in the
evening during the
winter and spring than
in the summer and fall is evident from Fig. 362, which shows
the position of the ecliptic and the zodiacal light with reference
to the western horizon at the time of sunset in March and in
September. It will be seen that in September the axis of the
light forms a small angle with the horizon, so that the phenomenon
is visible only a short time after sunset and low down
where it is difficult to distinguish it from the glimmer of the twilight;
while in March, its axis being nearly perpendicular to the
horizon, the light may be observed for some hours after sunset
// File: psp_321.png
.pn +1
and well up in the sky. Fig. 363 gives the position of the
ecliptic and of the zodiacal light with reference to the eastern
horizon at the time of
sunrise, and shows why
the zodiacal light is
seen to better advantage
in the morning
during the summer and
fall than during the
winter and spring. It
will be observed that
here the angle made by
the axis of the light
with the horizon is small
in March, while it is
large in September; the
conditions represented
in the preceding figure
being thus reversed.
.if h
.il fn=fig362.png w=70% alt='Zodiacal Light'
.ca Fig. 362.
.if-
.if t
[Illustration: Fig. 362.]
.if-
.if h
.il fn=fig363.png w=70% alt='Zodiacal Light'
.ca Fig. 363.
.if-
.if t
[Illustration: Fig. 363.]
.if-
321. Nature of the Zodiacal Light.--Various attempts have
been made to explain
the phenomena of the
zodiacal light; but the
most probable theory
is, that it is due to
an immense number of
meteors which are revolving
around the sun,
and which lie mostly
within the earth's orbit.
Each of these meteors
reflects a sensible portion
of sunlight, but is
far too small to be separately
visible. All of
these meteors together
would, by their combined
reflection, produce a kind of pale, diffused light.
// File: psp_322.png
.pn +1
.sp 4
.h2 id='stellar' title='III. The Stellar Universe.'
III. THE STELLAR UNIVERSE.
.sp 2
.h3 id='aspect'
I. GENERAL ASPECT OF THE HEAVENS.
.sp 2
322. The Magnitude of the Stars.--The stars that are
visible to the naked eye are divided into six classes, according
to their brightness. The brightest stars are called stars
of the first magnitude; the next brightest, those of the
second magnitude; and so on to the sixth magnitude. The
last magnitude includes the faintest stars that are visible to
the naked eye on the most favorable night. Stars which
are fainter than those of the sixth magnitude can be seen
only with the telescope, and are called telescopic stars.
Telescopic stars are also divided into magnitudes; the division
extending to the sixteenth magnitude, or the faintest
stars that can be seen with the most powerful telescopes.
The classification of stars according to magnitudes has
reference only to their brightness, and not at all to their
actual size. A sixth magnitude star may actually be larger
than a first magnitude star; its want of brilliancy being due
to its greater distance, or to its inferior luminosity, or to
both of these causes.
None of the stars present any sensible disk, even in the
most powerful telescope: they all appear as mere points of
light. The larger the telescope, the greater is its power
of revealing faint stars; not because it makes these stars
appear larger, but because of its greater light-gathering
// File: psp_323.png
.pn +1
power. This power increases with the size of the object-glass
of the telescope, which plays the part of a gigantic
pupil of the eye.
The classification of the stars into magnitudes is not
made in accordance with any very accurate estimate of
their brightness. The stars which are classed together in
the same magnitude are far from being equally bright.
The stars of each lower magnitude are about two-fifths as
bright as those of the magnitude above. The ratio of diminution
is about a third from the higher magnitude down to the
fifth. Were the ratio two-fifths exact, it would take about
.pm verse-start
2-1/2 stars of the 2d magnitude to make one of the 1st.
6 stars of the 3d magnitude to make one of the 1st.
16 stars of the 4th magnitude to make one of the 1st.
40 stars of the 5th magnitude to make one of the 1st.
100 stars of the 6th magnitude to make one of the 1st.
10,000 stars of the 11th magnitude to make one of the 1st.
1,000,000 stars of the 16th magnitude to make one of the 1st.
.pm verse-end
323. The Number of the Stars.--The total number
of stars in the celestial sphere visible to the average naked
eye is estimated, in round numbers, at five
thousand; but the number varies much with
the perfection and the training of the eye
and with the atmospheric conditions. For
every star visible to the naked eye, there are
thousands too minute to be seen without
telescopic aid. Fig. 364 shows a portion of the constellation
of the Twins as seen with the naked eye; and Fig. 365
shows the same region as seen in a powerful telescope.
.if h
.il fn=fig364.png w=70% alt='Constellation'
.ca Fig. 364.
.if-
.if t
[Illustration: Fig. 364.]
.if-
.if h
.il fn=fig365.png w=70% alt='Constellation'
.ca Fig. 365.
.if-
.if t
[Illustration: Fig. 365.]
.if-
Struve has estimated that the total number of stars visible
with Herschel's twenty-foot telescope was about twenty
million. The number that can be seen with the great telescopes
of modern times has not been carefully estimated,
but is probably somewhere between thirty million and fifty
million.
// File: psp_324.png
.pn +1
The number of stars between the north pole and the circle
thirty-five degrees south of the equator is about as follows:--
.pm verse-start
Of the 1st magnitude about 14 stars.
Of the 2d magnitude about 48 stars.
Of the 3d magnitude about 152 stars.
Of the 4th magnitude about 313 stars.
Of the 5th magnitude about 854 stars.
Of the 6th magnitude about 2010 stars.
----
Total visible to naked eye 3391 stars.
.pm verse-end
The number of stars of the several magnitudes is approximately
in inverse proportion to that of their brightness, the
ratio being a little greater in the higher magnitudes, and probably
a little less in the lower ones.
// File: psp_325.png
.pn +1
324. The Division of the Stars into Constellations.--A
glance at the heavens is sufficient to show that the stars
are not distributed uniformly over the sky. The larger ones
especially are collected into more or less irregular groups.
The larger groups are called constellations. At a very early
period a mythological figure was allotted to each constellation;
and these figures were drawn in such a way as to
include the principal stars of each constellation. The
heavens thus became covered, as it were, with immense
hieroglyphics.
There is no historic record of the time when these figures
were formed, or of the principle in accordance with which they
were constructed. It is probable that the imagination of the
earlier peoples may, in many instances, have discovered some
fanciful resemblance in the configuration of the stars to the
forms depicted. The names are still retained, although the
figures no longer serve any astronomical purpose. The constellation
Hercules, for instance, no longer represents the figure
of a man among the stars, but a certain portion of the heavens
within which the ancients placed that figure. In star-maps
intended for school and popular use it is still customary to
give these figures; but they are not generally found on maps
designed for astronomers.
325. The Naming of the Stars.--The brighter stars have
all proper names, as Sirius, Procyon, Arcturus, Capella,
Aldebaran, etc. This method of designating the stars was
adopted by the Arabs. Most of these names have dropped
entirely out of astronomical use, though many are popularly
retained. The brighter stars are now generally designated
by the letters of the Greek alphabet,--alpha, beta, gamma,
etc.,--to which is appended the genitive of the name of
the constellation, the first letter of the alphabet being used
for the brightest star, the second for the next brightest, and
so on. Thus Aldebaran would be designated as Alpha
Tauri. In speaking of the stars of any one constellation,
// File: psp_326.png
.pn +1
we simply designate them by the letters of the Greek alphabet,
without the addition of the name of the constellation,
which answers to a person's surname, while the Greek letter
answers to his Christian name. The names of the seven
stars of the "Dipper" are given in Fig. 366. When the
letters of the Greek alphabet are exhausted, those of the
Roman alphabet are employed. The fainter stars in a
constellation are usually designated by some system of
numbers.
.if h
.il fn=fig366.png w=70% alt='Dipper'
.ca Fig. 366.
.if-
.if t
[Illustration: Fig. 366.]
.if-
326. The Milky-Way, or Galaxy.--The Milky-Way is
a faint luminous band, of irregular outline, which surrounds
the heavens with a great circle, as shown in Fig. 367.
Through a considerable portion of its course it is divided
into two branches, and there are various vacant spaces at
different points in this band; but at only one point in the
southern hemisphere is it entirely interrupted.
.if h
.il fn=fig367.png w=80% alt='Milky Way'
.ca Fig. 367.
.if-
.if t
[Illustration: Fig. 367.]
.if-
The telescope shows that the Galaxy arises from the
light of countless stars too minute to be separately visible
with the naked eye. The telescopic stars, instead of being
uniformly distributed over the celestial sphere, are mostly
// File: psp_327.png
.pn +1
// File: psp_328.png
.pn +1
condensed in the region of the Galaxy. They are fewest
in the regions most distant from this belt, and become
thicker as we approach it. The greater the telescopic
power, the more marked is the condensation. With the
naked eye the condensation is hardly noticeable; but with
the aid of a very small telescope,
we see a decided thickening
of the stars in and near the
Galaxy, while the most powerful
telescopes show that a large
majority of the stars lie actually
in the Galaxy. If all the stars
visible with a twelve-inch telescope
were blotted out, we
should find that the greater part
of those remaining were in the
Galaxy.
.if h
.il fn=fig368.png w=60% alt='Star Distribution'
.ca Fig. 368.
.if-
.if t
[Illustration: Fig. 368.]
.if-
The increase in the number
of the stars of all magnitudes as
we approach the plane of the
Milky-Way is shown in Fig. 368.
The curve acb shows by its
height the distribution of the
stars above the ninth magnitude,
and the curve ACB those of
all magnitudes.
327. Star-Clusters.--Besides
this gradual and regular condensation
towards the Galaxy,
occasional aggregations of stars
into clusters may be seen. Some of these are visible to the
naked eye, sometimes as separate stars, like the "Seven
Stars," or Pleiades, but more commonly as patches of diffused
light, the stars being too small to be seen separately.
The number visible in powerful telescopes is, however, much
// File: psp_329.png
.pn +1
// File: psp_330.png
.pn +1
greater. Sometimes hundreds or even thousands of stars
are visible in the field of view at once, and sometimes the
number is so great that they cannot be counted.
328. Nebulæ.--Another class of objects which are found
in the celestial spaces are irregular masses of soft, cloudy
light, known as nebulæ. Many objects which look like
nebulæ in small telescopes are shown by more powerful
instruments to be really star-clusters. But many of these
objects are not composed of stars at all, being immense
masses of gaseous matter.
.if h
.il fn=fig369.png w=90% alt='Nebula Distribution'
.ca Fig. 369.
.if-
.if t
[Illustration: Fig. 369.]
.if-
The general distribution of nebulæ is the reverse of that
of the stars. Nebulæ are thickest where stars are thinnest.
While stars are most numerous in the region of the Milky-Way,
nebulæ are most abundant about the poles of the
Milky-Way. This condensation of nebulæ about the poles
of the Milky-Way is shown in Figs. 367 and 369, in which
the points represent, not stars, but nebulæ.
.sp 2
.h3 id='stars'
II. THE STARS.
.sp 2
.h4 id='constellations'
The Constellations.
.sp 2
.if h
.il fn=fig370.png w=70% alt='Great Bear'
.ca Fig. 370.
.if-
.if t
[Illustration: Fig. 370.]
.if-
.if h
.il fn=fig371.png w=70% alt='Great Bear'
.ca Fig. 371.
.if-
.if t
[Illustration: Fig. 371.]
.if-
329. The Great Bear.--The Great Bear, or Ursa Major,
is one of the circumpolar constellations (4), and contains one
of the most familiar asterisms, or groups of stars, in our
sky; namely, the Great Dipper, or Charles's Wain. The
positions and names of the seven prominent stars in it are
shown in Fig. 370. The two stars Alpha and Beta are called
the Pointers. This asterism is sometimes called the Butcher's
Cleaver. The whole constellation is shown in Fig. 371. A
rather faint star marks the nose of the bear, and three equidistant
pairs of faint stars mark his feet.
330. The Little Bear, Draco, and Cassiopeia.--These are
all circumpolar constellations. The most important star of the
Little Bear, or Ursa Minor, is Polaris, or the Pole Star. This
star may be found by drawing a line from Beta to Alpha of the
Dipper, and prolonging it as shown in Fig. 372. This explains
why these stars are called the Pointers. The Pole Star, with
// File: psp_331.png
.pn +1
the six other chief stars of the Little Bear, form an asterism
called the Little Dipper. These six stars are joined with
Polaris by a dotted line in Fig. 372.
.if h
.il fn=fig372.png w=90% alt='Little Bear'
.ca Fig. 372.
.if-
.if t
[Illustration: Fig. 372.]
.if-
The stars in a serpentine line between the two Dippers are
the chief stars of Draco, or the Dragon; the trapezium marking
// File: psp_332.png
.pn +1
its head. Fig. 373 shows the constellations of Ursa Minor
and Draco as usually figured.
.if h
.il fn=fig373.png w=70% alt='Ursa Minor and Draco'
.ca Fig. 373.
.if-
.if t
[Illustration: Fig. 373.]
.if-
To find Cassiopeia, draw a line from Delta of the Dipper to
// File: psp_333.png
.pn +1
Polaris, and prolong it about an equal distance beyond, as
shown in Fig. 372. This line will pass near Alpha of Cassiopeia.
// File: psp_334.png
.pn +1
The five principal stars of this constellation form an
irregular W, opening towards the pole. Between Cassiopeia
and Draco are five rather faint stars, which form an irregular
K. These are the principal stars of the constellation Cepheus.
These two constellations are shown in Fig. 374.
.if h
.il fn=fig374.png w=90% alt='Cassiopeia and Draco'
.ca Fig. 374.
.if-
.if t
[Illustration: Fig. 374.]
.if-
.if h
.il fn=fig375.png w=70% alt='Lion'
.ca Fig. 375.
.if-
.if t
[Illustration: Fig. 375.]
.if-
331. The Lion, Berenice's Hair, and the Hunting-Dogs.--A
line drawn from Alpha to Beta of the Dipper, and prolonged
as shown in Fig. 375, will pass between the two stars Denebola
and Regulus of Leo, or the Lion. Regulus forms a sickle with
several other faint stars, and marks the heart of the lion.
Denebola is at the apex of a right-angled triangle, which it
forms with two other stars, and marks the end of the lion's
tail. This constellation is visible in the evening from February
to July, and is figured in Fig. 376.
.if h
.il fn=fig376.png w=70% alt='Lion'
.ca Fig. 376.
.if-
.if t
[Illustration: Fig. 376.]
.if-
In a straight line between Denebola and Eta, at the end of
the Great Bear's tail, are, at about equal distances, the two
small constellations of Coma Berenices, or Berenice's Hair,
and Canes Venatici, or the Hunting-Dogs. These are shown
in Fig. 377. The dogs are represented as pursuing the bear,
urged on by the huntsman Boötes.
.if h
.il fn=fig377.png w=70% alt='Dogs'
.ca Fig. 377.
.if-
.if t
[Illustration: Fig. 377.]
.if-
// File: psp_335.png
.pn +1
332. Boötes, Hercules, and the Northern Crown.--Arcturus,
the principal star of Boötes, may be found by drawing
// File: psp_336.png
.pn +1
a line from Zeta to Eta of the Dipper, and then prolonging
it with a slight bend, as shown in Fig. 378. Arcturus and
Polaris form a large isosceles triangle with a first-magnitude
star called Vega. This triangle encloses at one corner the
principal stars of Boötes, and the head of the Dragon near
the opposite side. The side running from Arcturus to Vega
passes through Corona Borealis, or the Northern Crown, and
the body of Hercules, which is marked by a quadrilateral of
four stars.
.if h
.il fn=fig378.png w=70% alt='Triangle'
.ca Fig. 378.
.if-
.if t
[Illustration: Fig. 378.]
.if-
Boötes, who is often represented as a husbandman, Corona
Borealis, and Hercules, are delineated in Fig. 379. These
constellations are visible in the evening from May to September.
.if h
.il fn=fig379.png w=90% alt='Triangle'
.ca Fig. 379.
.if-
.if t
[Illustration: Fig. 379.]
.if-
.if h
.il fn=fig380.png w=70% alt='Lyre, Swan, Dolphin'
.ca Fig. 380.
.if-
.if t
[Illustration: Fig. 380.]
.if-
333. The Lyre, the Swan, the Eagle, and the Dolphin.--Altair,
the principal star of Aquila, or the Eagle, lies on the
opposite side of the Milky-Way from Vega. Altair is a first-magnitude
// File: psp_337.png
.pn +1
star, and has a faint star on each side of it, as
shown in Fig. 380. Vega, also of the first magnitude, is the
// File: psp_338.png
.pn +1
principal star of Lyra, or the Lyre. Between these two stars,
and a little farther to the north, are several stars arranged in
the form of an immense cross. The bright star at the head
of this cross is called Deneb. The cross lies in the Milky-Way,
and contains the chief stars of the constellation Cygnus, or
the Swan. A little to the north of Altair are four stars in the
form of a diamond. This asterism is popularly known as Job's
Coffin. These four stars are the chief stars of Delphinus, or
the Dolphin. These four constellations are shown together in
Fig. 381. The Swan is visible from June to December, in the
evening.
.if h
.il fn=fig381.png w=90% alt='Lyre, Swan, Dolphin'
.ca Fig. 381.
.if-
.if t
[Illustration: Fig. 381.]
.if-
334. Virgo.--A line drawn from Alpha to Gamma of the
Dipper, and prolonged with a slight bend at Gamma, will reach
to a first-magnitude star called Spica (Fig. 382). This is the
chief star of the constellation Virgo, or the Virgin, and forms
a large isosceles triangle with Arcturus and Denebola.
// File: psp_339.png
.pn +1
.if h
.il fn=fig382.png w=70% alt='Lyre, Swan, Dolphin'
.ca Fig. 382.
.if-
.if t
[Illustration: Fig. 382.]
.if-
Virgo is represented in Fig. 383. To the right of this constellation,
as shown in the figure, there are four stars which
form a trapezium, and mark the constellation Corvus, or the
// File: psp_340.png
.pn +1
Crow. This bird is represented as standing on the body of
Hydra, or the Water-Snake. Virgo is visible in the evening,
from April to August.
.if h
.il fn=fig383.png w=70% alt='Virgo'
.ca Fig. 383.
.if-
.if t
[Illustration: Fig. 383.]
.if-
.if h
.il fn=fig384.png w=70% alt='Twins'
.ca Fig. 384.
.if-
.if t
[Illustration: Fig. 384.]
.if-
.if h
.il fn=fig385.png w=70% alt='Virgo'
.ca Fig. 385.
.if-
.if t
[Illustration: Fig. 385.]
.if-
335. The Twins.--A line drawn from Delta to Beta of the
Dipper, and prolonged as shown in Fig. 384, passes between
two bright stars called Castor and Pollux. The latter of these
is usually reckoned as a first-magnitude star. These are the
principal stars of the constellation Gemini, or the Twins,
which is shown in Fig. 385. The constellation Canis Minor,
or the Little Dog, is shown in the lower part of the figure.
There are two conspicuous stars in this constellation, the
brightest of which is of the first magnitude, and called Procyon.
The region to which we have now been brought is the
richest of the northern sky, containing no less than seven first-magnitude
stars. These are Sirius, Procyon, Pollux, Capella,
Aldebaran, Betelgeuse, and Rigel. They are shown in Fig. 386.
.if h
.il fn=fig386.png w=70% alt='Virgo'
.ca Fig. 386.
.if-
.if t
[Illustration: Fig. 386.]
.if-
// File: psp_341.png
.pn +1
Betelgeuse and Rigel are in the constellation Orion, being
// File: psp_342.png
.pn +1
about equally distant to the north and south from the three
stars forming the belt of Orion. Betelgeuse is a red star.
Sirius is the brightest star in the heavens, and belongs to the
constellation Canis Major, or the Great Dog. It lies to the
east of the belt of Orion. Aldebaran lies at about the same
distance to the west of the belt. It is a red star, and belongs
to the constellation Taurus, or the Bull. Capella is in the
constellation Auriga, or the Wagoner. These stars are visible
in the evening, from about December to April.
336. Orion and his Dogs, and Taurus.--Orion and his
Dogs are shown in Fig. 387, and Orion and Taurus in Fig. 388.
Aldebaran marks one of the eyes of the bull, and is often called
the Bull's Eye. The irregular V in the face of the bull is
called the Hyades, and the cluster on the shoulder the Pleiades.
.if h
.il fn=fig387.png w=70% alt='Orion'
.ca Fig. 387.
.if-
.if t
[Illustration: Fig. 387.]
.if-
.if h
.il fn=fig388.png w=70% alt='Orion'
.ca Fig. 388.
.if-
.if t
[Illustration: Fig. 388.]
.if-
// File: psp_343.png
.pn +1
.if h
.il fn=fig389.png w=70% alt='Wagoner'
.ca Fig. 389.
.if-
.if t
[Illustration: Fig. 389.]
.if-
337. The Wagoner.--The constellation Auriga, or the
Wagoner (sometimes called the Charioteer), is shown in Fig.
389. Capella marks the Goat, which he is represented as
carrying on his back, and the little right-angled triangle of
stars near it the Kids. The five chief stars of this constellation
form a large, irregular pentagon. Gamma of Auriga is
also Beta of Taurus, and marks one of the horns of the Bull.
.if h
.il fn=fig390.png w=70% alt='Pegasus'
.ca Fig. 390.
.if-
.if t
[Illustration: Fig. 390.]
.if-
338. Pegasus, Andromeda, and Perseus.--A line drawn
from Polaris near to Beta of Cassiopeia will lead to a bright
second-magnitude star at one corner of a large square (Fig. 390).
Alpha belongs both to the Square of Pegasus and to Andromeda.
Beta and Gamma, which are connected with Alpha in
the figure by a dotted line, also belong to Andromeda. Algol,
// File: psp_344.png
.pn +1
which forms, with the last-named stars and with the Square of
// File: psp_345.png
.pn +1
Pegasus, an asterism similar in configuration to the Great
// File: psp_346.png
.pn +1
Dipper, belongs to Perseus. Algenib, which is reached by
bending the line at Gamma in the opposite direction, is the
principal star of Perseus.
.if h
.il fn=fig391.png w=70% alt='Pegasus'
.ca Fig. 391.
.if-
.if t
[Illustration: Fig. 391.]
.if-
.if h
.il fn=fig392.png w=70% alt='Andromeda'
.ca Fig. 392.
.if-
.if t
[Illustration: Fig. 392.]
.if-
.if h
.il fn=fig393.png w=70% alt='Cetus'
.ca Fig. 393.
.if-
.if t
[Illustration: Fig. 393.]
.if-
Pegasus is shown in Fig. 391, and Andromeda in Fig. 392.
Cetus, the Whale, or the Sea Monster, shown in Fig. 393,
belongs to the same mythological group of constellations.
// File: psp_347.png
.pn +1
.if h
.il fn=fig394.png w=70% alt='Scorpio'
.ca Fig. 394.
.if-
.if t
[Illustration: Fig. 394.]
.if-
339. Scorpio, Sagittarius, and Ophiuchus.--During the
summer months a brilliant constellation is visible, called Scorpio,
or the Scorpion. The configuration of the chief stars of
this constellation is shown in Fig. 394. They bear some
resemblance to a boy's kite. The brightest star is of the first
magnitude, and called Antares (from anti, instead of, and Ares,
the Greek name of Mars), because it rivals Mars in redness.
The stars in the tail of the Scorpion are visible in our latitude
only under very favorable circumstances. This constellation
is shown in Fig. 395, together with Sagittarius and Ophiuchus.
Sagittarius, or the Archer, is to the east of Scorpio. It contains
no bright stars, but is easily recognized from the fact
that five of its principal stars form the outline of an inverted
dipper, which, from the fact of its being partly in the Milky-Way,
is often called the Milk Dipper.
.if h
.il fn=fig395.png w=70% alt='Scorpio'
.ca Fig. 395.
.if-
.if t
[Illustration: Fig. 395.]
.if-
Ophiuchus, or the Serpent-Bearer, is a large constellation,
filling all the space between the head of Hercules and Scorpio.
It is difficult to trace, since it contains no very brilliant
stars. This constellation and Libra, or the Balances, which is
// File: psp_348.png
.pn +1
// File: psp_349.png
.pn +1
the zodiacal constellation to the west of Scorpio, are shown
in Fig. 396.
.if h
.il fn=fig396.png w=70% alt='Libra'
.ca Fig. 396.
.if-
.if t
[Illustration: Fig. 396.]
.if-
.if h
.il fn=fig397.png w=70% alt='Aquarius'
.ca Fig. 397.
.if-
.if t
[Illustration: Fig. 397.]
.if-
340. Capricornus, Aquarius, and the Southern Fish.--The
two zodiacal constellations to the east of Sagittarius are Capricornus
// File: psp_350.png
.pn +1
and Aquarius. Capricornus contains three pairs of
small stars, which mark the head, the tail, and the knees of the
animal.
Aquarius is marked by no conspicuous stars. An irregular
line of minute stars marks the course of the stream of water
which flows from the Water-Bearer's Urn into the mouth of the
Southern Fish. This mouth is marked by the first-magnitude
star Fomalhaut. These constellations are shown in Fig. 397.
.if h
.il fn=fig398.png w=70% alt='Pisces and Aries'
.ca Fig. 398.
.if-
.if t
[Illustration: Fig. 398.]
.if-
341. Pisces and Aries.--The remaining zodiacal constellations
are Pisces, or the Fishes, Aries, or the Ram (Fig. 398),
and Cancer, or the Crab.
The Fishes lie under Pegasus and Andromeda, but contain
no bright stars. Aries (between Pisces and Taurus) is marked
by a pair of stars on the head,--one of the second, and one
of the third magnitude. Cancer (between Leo and Gemini) has
no bright stars, but contains a remarkable cluster of small stars
called Præsepe, or the Beehive.
.sp 2
.h4 id='clusters'
Clusters.
.sp 2
342. The Hyades.--The Hyades are a very open cluster in
the face of Taurus (334). The three brightest stars of this
cluster form a letter V, the point of the V being on the nose,
and the open ends at the eyes. This cluster is shown in Fig.
// File: psp_351.png
.pn +1
399. The name, according to the most probable etymology,
means rainy; and
they are said to have
been so called because
their rising was
associated with wet
weather. They were
usually considered
the daughters of
Atlas, and sisters of
the Pleiades, though
sometimes referred
to as the nurses of
Bacchus.
.if h
.il fn=fig399.png w=70% alt='Hyades Cluster'
.ca Fig. 399.
.if-
.if t
[Illustration: Fig. 399.]
.if-
343. The Pleiades.--The
Pleiades constitute
a celebrated
group of stars, or a miniature constellation, on the shoulder
of Taurus. Hesiod
mentions them as
"the seven virgins
of Atlas born," and
Milton calls them
"the seven Atlantic
sisters." They are
referred to in the
Book of Job. The
Spaniards term them
"the little nanny-goats;"
and they are
sometimes called "the
hen and chickens."
.if h
.il fn=fig400.png w=70% alt='Pleiades Cluster'
.ca Fig. 400.
.if-
.if t
[Illustration: Fig. 400.]
.if-
.if h
.il fn=fig401.png w=70% alt='Pleiades Cluster'
.ca Fig. 401.
.if-
.if t
[Illustration: Fig. 401.]
.if-
Usually only six
stars in this cluster
can be seen with the
naked eye, and this
fact has given rise
to the legend of the
"lost Pleiad." On a clear, moonless night, however, a good
// File: psp_352.png
.pn +1
eye can discern seven or eight stars, and some observers have
distinguished as many as eleven. Fig. 400 shows the Pleiades
as they appear to the naked eye under the most favorable
circumstances. Fig. 401 shows this cluster as it appears in
// File: psp_353.png
.pn +1
a powerful telescope. With such an instrument more than five
hundred stars are visible.
344. Cluster in the Sword-handle of Perseus.--This is a
somewhat dense double cluster.
It is visible to the naked
eye, appearing as a hazy star.
A line drawn from Algenib, or
Alpha of Perseus (338), to Delta
of Cassiopeia (330), will pass
through this cluster at about
two-thirds the distance from
the former. This double cluster
is one of the most brilliant
objects in the heavens, with a
telescope of moderate power.
.if h
.il fn=fig402.png w=70% alt='Hercules Cluster'
.ca Fig. 402.
.if-
.if t
[Illustration: Fig. 402.]
.if-
345. Cluster of Hercules.--The
celebrated globular cluster of Hercules can be seen only
with a telescope of considerable power, and to resolve it into
distinct stars (as shown in Fig. 402) requires an instrument of
the very highest class.
// File: psp_354.png
.pn +1
.if h
.il fn=fig403.png w=70% alt='Aquarius Cluster'
.ca Fig. 403.
.if-
.if t
[Illustration: Fig. 403.]
.if-
346. Other Clusters.--Fig. 403 shows a magnificent globular
cluster in the constellation
Aquarius.
Herschel describes it
as appearing like a
heap of sand, being
composed of thousands
of stars of the
fifteenth magnitude.
.if h
.il fn=fig404.png w=70% alt='Toucan Cluster'
.ca Fig. 404.
.if-
.if t
[Illustration: Fig. 404.]
.if-
Fig. 404 shows a
cluster in the constellation
Toucan,
which Sir John Herschel
describes as a
most glorious globular
cluster, the stars
of the fourteenth
magnitude being immensely numerous. There is a marked condensation
of light at the centre.
.if h
.il fn=fig405.png w=70% alt='Centaur Cluster'
.ca Fig. 405.
.if-
.if t
[Illustration: Fig. 405.]
.if-
// File: psp_355.png
.pn +1
.if h
.il fn=fig406.png w=70% alt='Scorpio Cluster'
.ca Fig. 406.
.if-
.if t
[Illustration: Fig. 406.]
.if-
Fig. 405 shows a cluster in the Centaur, which, according
to the same astronomer,
is beyond comparison
the richest and
largest object of the
kind in the heavens,
the stars in it being
literally innumerable.
Fig. 406 shows a cluster
in Scorpio, remarkable
for the peculiar
arrangement of its component
stars.
Star clusters are
especially abundant in
the region of the Milky-Way,
the law of their
distribution being the reverse of that of the nebulæ.
.sp 2
.h4 id='double-stars'
Double and Multiple Stars.
.sp 2
347. Double Stars.--The telescope shows that many
stars which appear single to the naked eye are really double,
or composed of a pair of stars lying side by side. There
are several pairs of stars in the heavens which lie so near
together that they almost seem to touch when seen with
the naked eye.
.if h
.il fn=fig407.png w=70% alt='Pair of Stars'
.ca Fig. 407.
.if-
.if t
[Illustration: Fig. 407.]
.if-
.if h
.il fn=fig408.png w=70% alt='Pair of Stars'
.ca Fig. 408.
.if-
.if t
[Illustration: Fig. 408.]
.if-
Pairs of stars are not considered double unless the components
are so near together that they both appear in the
// File: psp_356.png
.pn +1
field of view when examined with a telescope. In the
majority of the pairs classed as double stars the distance
between the components ranges from half a second to
fifteen seconds.
.if h
.il fn=fig409.png w=70% alt='Epsilon Lyrae'
.ca Fig. 409.
.if-
.if t
[Illustration: Fig. 409.]
.if-
Epsilon Lyræ is a good
example of a pair of
stars that can barely be
separated with a good
eye. Figs. 407 and 408
show this pair as it appears
in telescopes magnifying
respectively four
and fifteen times; and
Fig. 409 shows it as seen
in a more powerful telescope,
in which each of
the two components of the pair is seen to be a truly double
star.
.if h
.il fn=fig410.png w=70% alt='Multiple Star'
.ca Fig. 410.
.if-
.if t
[Illustration: Fig. 410.]
.if-
.if h
.il fn=fig411.png w=70% alt='Multiple Star'
.ca Fig. 411.
.if-
.if t
[Illustration: Fig. 411.]
.if-
348. Multiple Stars.--When a star is resolved into
more than two components by a telescope, it is called a
multiple star. Fig. 410 shows a triple star in Pegasus.
Fig. 411 shows a quadruple star in Taurus. Fig. 412
shows a sextuple star, and Fig. 413 a septuple star. Fig.
414 shows the celebrated septuple star in Orion, called
Theta Orionis, or the trapezium of Orion.
// File: psp_357.png
.pn +1
349. Optically Double and Multiple Stars.--Two or
more stars which are really very distant from each other,
and which have no physical connection whatever, may
appear to be near together, because they happen to lie in
the same direction, one behind the other. Such accidental
combinations are called optically double or multiple stars.
.if h
.il fn=fig412.png w=70% alt='Multiple Star'
.ca Fig. 412.
.if-
.if t
[Illustration: Fig. 412.]
.if-
.if h
.il fn=fig413.png w=70% alt='Multiple Star'
.ca Fig. 413.
.if-
.if t
[Illustration: Fig. 413.]
.if-
350. Physically Double and Multiple Stars.--In the
majority of cases the components of double and multiple
stars are in reality comparatively near together, and are
bound together by gravity into a physical system. Such
combinations are called physically
double and multiple stars.
The components of these systems
all revolve around their
common centre of gravity. In
many instances their orbits and
periods of revolution have been
ascertained by observation and
calculation. Fig. 415 shows the orbit of one of the components
of a double star in the constellation Hercules.
.if h
.il fn=fig414.png w=70% alt='Multiple Star'
.ca Fig. 414.
.if-
.if t
[Illustration: Fig. 414.]
.if-
351. Colors of Double and Multiple Stars.--The components
of double and multiple stars are often highly colored,
and frequently the components of the same system
are of different colors. Sometimes one star of a binary
system is white, and the other red; and sometimes a white
// File: psp_358.png
.pn +1
star is combined with a blue one. Other colors found in
combination in these systems are red and blue, orange and
green, blue and green, yellow and blue, yellow and red, etc.
.if h
.il fn=fig415.png w=70% alt='Multiple Star Orbits'
.ca Fig. 415.
.if-
.if t
[Illustration: Fig. 415.]
.if-
If these double and multiple stars are accompanied by
planets, these planets will sometimes have two or more suns
in the sky at once. On alternate days they may have suns
of different colors, and perhaps on the same day two
suns of different colors. The effect of these changing
colored lights on the landscape must be very remarkable.
.sp 2
.h4 id='new-stars'
New and Variable Stars.
.sp 2
352. Variable Stars.--There are many stars which
undergo changes of brilliancy, sometimes slight, but occasionally
very marked. These changes are in some cases
apparently irregular, and in others periodic. All such stars
are said to be variable, though the term is applied especially
to those stars whose variability is periodic.
.if h
.il fn=fig416.png w=70% alt='Algol'
.ca Fig. 416.
.if-
.if t
[Illustration: Fig. 416.]
.if-
353. Algol.--Algol, a star of Perseus, whose position is
// File: psp_359.png
.pn +1
shown in Fig. 416, is a remarkable variable star of a short
period. Usually it shines as a faint second-magnitude star;
but at intervals of a little less than three days it fades to
the fourth magnitude for a few hours, and then regains its
former brightness. These changes were first noticed some
two centuries ago, but it was not till 1782 that they were
accurately observed. The period is now known to be two
days, twenty hours, forty-nine minutes. It takes about four
hours and a half to fade away, and four hours more to
recover its brilliancy. Near the beginning and end of the
variations, the change is very
slow, so that there are not more
than five or six hours during
which an ordinary observer
would see that the star was
less bright than usual.
This variation of light was at
first explained by supposing that
a large dark planet was revolving
round Algol, and passed
over its face at every revolution,
thus cutting off a portion of its
light; but there are small irregularities
in the variation, which
this theory does not account for.
354. Mira.--Another remarkable variable star is Omicron
Ceti, or Mira (that is, the wonderful star). It is generally
invisible to the naked eye; but at intervals of about eleven
months it shines forth as a star of the second or third
magnitude. It is about forty days from the time it becomes
visible until it attains its greatest brightness, and is then
about two months in fading to invisibility; so that its
increase of brilliancy is more rapid than its waning. Its
period is quite irregular, ranging from ten to twelve months;
so that the times of its appearance cannot be predicted
// File: psp_360.png
.pn +1
with certainty. Its maximum brightness is also variable,
being sometimes of the second magnitude, and at others
only of the third or fourth.
.if h
.il fn=fig417.png w=70% alt='Eta Argus'
.ca Fig. 417.
.if-
.if t
[Illustration: Fig. 417.]
.if-
355. Eta Argus.--Perhaps the most extraordinary variable
star in the heavens is Eta Argus, in the constellation
Argo, or the Ship, in the southern hemisphere (Fig. 417).
The first careful observations of its variability were made
by Sir John Herschel while at the Cape of Good Hope.
He says, "It was on the 16th of December, 1837, that,
resuming the photometrical comparisons, my astonishment
was excited by the appearance of a new candidate for distinction
among the very brightest stars of the first magnitude
in a part of the heavens
where, being perfectly familiar
with it, I was certain that no
such brilliant object had before
been seen. After a momentary
hesitation, the natural consequence
of a phenomenon so
utterly unexpected, and referring
to a map for its configuration
with other conspicuous
stars in the neighborhood, I
became satisfied of its identity with my old acquaintance,
Eta Argus. Its light was, however, nearly tripled. While
yet low, it equalled Rigel, and, when it attained some
altitude, was decidedly greater." It continued to increase
until Jan. 2, 1838, then faded a little till April following,
though it was still as bright as Aldebaran. In 1842 and
1843 it blazed up brighter than ever, and in March of the
latter year was second only to Sirius. During the twenty-five
years following it slowly but steadily diminished. In
1867 it was barely visible to the naked eye; and the next
year it vanished entirely from the unassisted view, and has
not yet begun to recover its brightness. The curve in
// File: psp_361.png
.pn +1
Fig. 418 shows the change in brightness of this remarkable
star. The numbers at the bottom show the years of the
century, and those at the side the brightness of the star.
.if h
.il fn=fig418.png w=70% alt='Eta Argus'
.ca Fig. 418.
.if-
.if t
[Illustration: Fig. 418.]
.if-
356. New Stars.--In several cases stars have suddenly
appeared, and even become very brilliant; then, after a
longer or shorter time, they have faded away and disappeared.
Such stars are called new or temporary stars.
For a time it was supposed that such stars were actually
new. They are now, however, classified by astronomers
among the variable stars, their changes being of a very
irregular and fitful character. There is scarcely a doubt
that they were all in the heavens as very small stars before
they blazed forth in so extraordinary a manner, and that
they are in the same places still. There is a wide difference
between these irregular variations, or the breaking-forth of
light on a single occasion in the course of centuries, and
the regular and periodic changes in the case of a star like
Algol; but a long series of careful observation has resulted
in the discovery of stars of nearly every degree of irregularity
between these two extremes. Some of them change
gradually from one magnitude to another, in the course of
years, without seeming to follow any law whatever; while
in others some slight tendency to regularity can be traced.
Eta Argus may be regarded as a connecting link between
new and variable stars.
357. Tycho Brahe's Star.--An apparently new star
// File: psp_362.png
.pn +1
suddenly appeared in Cassiopeia in 1572. It was first seen
by Tycho Brahe, and is therefore associated with his name.
Its position in the constellation is shown in Fig. 419. It
was first seen on Nov. 11, when it had already attained the
first magnitude. It became rapidly brighter, soon rivalling
Venus in splendor, so that good eyes could discern it in
full daylight. In December it began to wane, and gradually
faded until the following May, when it disappeared
entirely.
.if h
.il fn=fig419.png w=70% alt='Tycho Brahe'
.ca Fig. 419.
.if-
.if t
[Illustration: Fig. 419.]
.if-
A star showed itself in the same part of the heavens in
945 and in 1264. If these were three appearances of the
same star, it must
be reckoned as a
periodic star with
a period of a little
more than three
hundred years.
358. Kepler's
Star.--In 1604 a
new star was seen
in the constellation
Ophiuchus. It was
first noticed in
October of that
year, when it was of the first magnitude. In the following
winter it began to fade, but remained visible during the
whole year 1605. Early in 1606 it disappeared entirely.
A very full history of this star was written by Kepler.
One of the most remarkable things about this star was
its brilliant scintillation. According to Kepler, it displayed
all the colors of the rainbow, or of a diamond cut with
multiple facets, and exposed to the rays of the sun. It is
thought that this star also appeared in 393, 798, and 1203;
if so, it is a variable star with a period of a little over four
hundred years.
// File: psp_363.png
.pn +1
359. New Star of 1866.--The most striking case of
this kind in recent times was in May, 1866, when a star
of the second magnitude suddenly appeared in Corona
Borealis. On the 11th and 12th of that month it was
observed independently by at least five observers in Europe
and America. The fact that none of these new stars were
noticed until they had nearly or quite attained their greatest
brilliancy renders it probable that they all blazed up very
suddenly.
360. Cause of the Variability of Stars.--The changes in
the brightness of variable and temporary stars are probably
due to operations similar to those which produce the spots and
prominences in our sun. We have seen (188) that the frequency
of solar spots shows a period of eleven years, during
one portion of which there are few or no spots to be seen, while
during another portion they are numerous. If an observer so
far away as to see our sun like a star could from time to time
measure its light exactly, he would find it to be a variable star
with a period of eleven years, the light being least when we
see most spots, and greatest when few are visible. The variation
would be slight, but it would nevertheless exist. Now,
we have reason to believe that the physical constitution of the
sun and the stars is of the same general nature. It is therefore
probable, that, if we could get a nearer view of the stars,
we should see spots on their disks as we do on the sun. It
is also likely that the varying physical constitution of the stars
might give rise to great differences in the number and size of
the spots; so that the light of some of these suns might vary
to a far greater degree than that of our own sun does. If the
variations had a regular period, as in the case of our sun, the
appearances to a distant observer would be precisely what we
see in the case of a periodic variable star.
The spectrum of the new star of 1866 was found to be a
continuous one, crossed by bright lines, which were apparently
due to glowing hydrogen. The continuous spectrum was also
crossed by dark lines, indicating that the light had passed
through an atmosphere of comparatively cool gas. Mr. Huggins
// File: psp_364.png
.pn +1
infers from this that there was a sudden and extraordinary outburst
of hydrogen gas from the star, which by its own light,
as well as by heating up the whole surface of the star, caused
the extraordinary increase of brilliancy. Now, the spectroscope
shows that the red flames of the solar chromosphere
(197) are largely composed of hydrogen; and it is not unlikely
that the blazing-forth of this star arose from an action similar
to that which produces these flames, only on an immensely
larger scale.
.sp 2
.h4 id='star-distance'
Distance of the Stars.
.sp 2
361. Parallax of the Stars.--Such is the distance of
the stars, that only in a comparatively few instances has any
displacement of these bodies been detected when viewed
from opposite parts of the earth's orbit, that is, from points
a hundred and eighty-five million miles apart; and in no
case can this displacement be detected except by the most
careful and delicate measurement. Half of the above displacement,
or the displacement of the star as seen from
the earth instead of the sun, is called the parallax of the
star. In no case has a parallax of one second as yet been
detected.
362. The Distance of the Stars.--The distance of a star
whose parallax is one second would be 206,265 times the
distance of the earth from the sun, or about nineteen million
million miles. It is quite certain that no star is nearer than
this to the earth. Light has a velocity which would carry
it seven times and a half around the earth in a second; but
it would take it more than three years to reach us from
that distance. Were all the stars blotted out of existence
to-night, it would be at least three years before we should
miss a single one.
Alpha Centauri, the brightest star in the constellation
of the Centaur, is, so far as we know, the nearest of the
fixed stars. It is estimated that it would take its light about
three years and a half to reach us. It has also been estimated
// File: psp_365.png
.pn +1
that it would take light over sixteen years to reach
us from Sirius, about eighteen years to reach us from Vega,
about twenty-five years from Arcturus, and over forty years
from the Pole-Star. In many instances it is believed that
it would take the light of stars hundreds of years to make
the journey to our earth, and in some instances even thousands
of years.
.sp 2
.h4 id='star-proper'
Proper Motion of the Stars.
.sp 2
363. Why the Stars appear Fixed.--The stars seem to
retain their relative positions in the heavens from year to
year, and from age to age; and hence they have come
universally to be denominated as fixed. It is, however, now
well known that the stars, instead of being really stationary,
are moving at the rate of many miles a second; but their
distance is so enormous, that, in the majority of cases, it
would be thousands of years before this rate of motion
would produce a sufficient displacement to be noticeable
to the unaided eye.
.if h
.il fn=fig420.png w=70% alt='Star Displacement'
.ca Fig. 420.
.if-
.if t
[Illustration: Fig. 420.]
.if-
// File: psp_366.png
.pn +1
364. Secular Displacement of the Stars.--Though the
proper motion of the stars is apparently slight, it will, in
the course of many ages, produce a marked change in the
configuration of the stars. Thus, in Fig. 420, the left-hand
portion shows the present configuration of the stars of
the Great Dipper. The small arrows attached to the stars
show the direction and comparative magnitudes of their
motion. The right-hand portion of the figure shows these
stars as they will appear thirty-six thousand years from the
present time.
.if h
.il fn=fig421.png w=70% alt='Star Motion'
.ca Fig. 421.
.if-
.if t
[Illustration: Fig. 421.]
.if-
Fig. 421 shows in a similar way the present configuration
and proper motion of the stars of Cassiopeia, and
also these stars as they will appear thirty-six thousand years
hence.
.if h
.il fn=fig422.png w=70% alt='Star Motion'
.ca Fig. 422.
.if-
.if t
[Illustration: Fig. 422.]
.if-
Fig. 422 shows the same for the constellation Orion.
365. The Secular Motion of the Sun.--The stars in all
parts of the heavens are found to move in all directions
and with all sorts of velocities. When, however, the motions
// File: psp_367.png
.pn +1
of the stars are averaged, there is found to be an apparent
proper motion common to all the stars. The stars in the
neighborhood of Hercules appear to be approaching us,
and those in the
opposite part of
the heavens appear
to be receding
from us.
In other words,
all the stars
appear to be
moving away
from Hercules,
and towards the
opposite part of
the heavens.
.if h
.il fn=fig423.png w=70% alt='Star Motion'
.ca Fig. 423.
.if-
.if t
[Illustration: Fig. 423.]
.if-
This apparent motion common to all the stars is held by
astronomers to be due to the real motion of the sun
// File: psp_368.png
.pn +1
through space. The point in the heavens towards which
our sun is moving at the present time is indicated by the
small circle in the constellation Hercules in Fig. 423. As
the sun moves, he carries the earth and all the planets along
with him. Fig. 424 shows the direction of the sun's motion
with reference to the ecliptic
and to the axis of the
earth. Fig. 425 shows the
earth's path in space; and
Fig. 426 shows the paths of
the earth, the moon, Mercury,
Venus, and Mars in
space.
.if h
.il fn=fig424.png w=70% alt='Star Motion'
.ca Fig. 424.
.if-
.if t
[Illustration: Fig. 424.]
.if-
.if h
.il fn=fig425.png w=70% alt='Star Motion'
.ca Fig. 425.
.if-
.if t
[Illustration: Fig. 425.]
.if-
.if h
.il fn=fig426.png w=70% alt='Star Motion'
.ca Fig. 426.
.if-
.if t
[Illustration: Fig. 426.]
.if-
Whether the sun is actually
moving in a straight
line, or around some distant centre, it is impossible to determine
at the present time. It is estimated that the sun is
moving along his path at the rate of about a hundred and
fifty million miles a year. This is about five-sixths of the
diameter of the earth's
orbit.
366. Star-Drift.--In
several instances, groups
of stars have a common
proper motion entirely different
from that of the
stars around and among
them. Such groups probably
form connected systems,
in the motion of
which all the stars are carried along together without any
great change in their relative positions. The most remarkable
case of this kind occurs in the constellation
Taurus. A large majority of the brighter stars in the
region between Aldebaran and the Pleiades have a common
// File: psp_369.png
.pn +1
proper motion of about ten seconds per century towards
the east. Proctor has shown that five out of the seven
stars which form the Great Dipper have a common proper
motion, as shown in Fig. 427 (see also Fig. 420). He proposes
for this phenomenon the name of Star-Drift.
.if h
.il fn=fig427.png w=70% alt='Star Motion'
.ca Fig. 427.
.if-
.if t
[Illustration: Fig. 427.]
.if-
367. Motion of Stars along the Line of Sight.--A motion
of a star in the direction of the line of sight would produce
no displacement of the star that could be detected with the
// File: psp_370.png
.pn +1
telescope; but it would cause a change in the brightness of
the star, which would become gradually fainter if moving from
us, and brighter if approaching us. Motion along the line
of sight has, however, been detected by the use of the tele-spectroscope
(152), owing to the fact that it causes a displacement
of the spectral lines. As has already been explained
(169), a displacement of a spectral line towards the red end
of the spectrum indicates a motion away from us, and a displacement
towards the violet end, a motion towards us.
.tb
By means of these displacements of the spectral lines,
Huggins has detected motion in the case of a large number
of stars, and calculated its rate:--
STARS RECEDING FROM US.
.pm verse-start
Sirius 20 miles per second.
Betelgeuse 22 miles per second.
Rigel 15 miles per second.
Castor 25 miles per second.
Regulus 15 miles per second.
.pm verse-end
STARS APPROACHING US.
.pm verse-start
Arcturus 55 miles per second.
Vega 50 miles per second.
Deneb 39 miles per second.
Pollux 49 miles per second.
Alpha Ursæ Majoris 46 miles per second.
.pm verse-end
These results are confirmed by the fact that the amount
of motion indicated is about what we should expect the
stars to have, from their observed proper motions, combined
with their probable distances. Again: the stars in the
neighborhood of Hercules are mostly found to be approaching
the earth, and those which lie in the opposite direction
to be receding from it; which is exactly the effect which
would result from the sun's motion through space. The
five stars in the Dipper, which have a common proper
// File: psp_371.png
.pn +1
motion, are also found to have a common motion in the
line of sight. But the displacement of the spectral lines
is so slight, and its measurement so difficult, that the velocities
in the above table are to be accepted as only an
approximation to the true values.
.sp 2
.h4 id='star-chemical'
Chemical and Physical Constitution of the Stars.
.sp 2
368. The Constitution of the Stars Similar to that of
the Sun.--The stellar spectra bear a general resemblance
to that of the sun, with characteristic differences. These
spectra all show Fraunhofer's lines, which indicate that
their luminous surfaces are surrounded by atmospheres containing
absorbent vapors, as in the case of the sun. The
positions of these lines indicate that the stellar atmospheres
contain elements which are also found in the sun's, and
on the earth.
.if h
.il fn=fig428.png w=50% alt='Spectra'
.ca Fig. 428.
.if-
.if t
[Illustration: Fig. 428.]
.if-
369. Four Types of Stellar Spectra.--The spectra of
the stars have been carefully observed by Secchi and Huggins.
They have found that stellar spectra may be reduced
to four types, which are shown in Fig. 428. In the spectrum
of Sirius, a representative of Type I., very few lines
are represented; but the lines are very thick.
Next we have the solar spectrum, which is a representative
of Type II., one in which more lines are represented.
In Type III. fluted spaces begin to appear,
and in Type IV., which is that of the red stars, nothing
but fluted spaces is visible; and this spectrum shows that
something is at work in the atmosphere of those red
stars different from what there is in the simpler atmosphere
of Type I.
Lockyer holds that these differences of spectra are due
simply to differences of temperature. According to him,
the red stars, which give the fluted spectra, are of the
lowest temperature; and the temperature of the stars of
// File: psp_372.png
.pn +1
the different types gradually rises till we reach the first
type, in which the temperature is so high that the dissociation
(161) of the elements is nearly if not quite
complete.
// File: psp_373.png
.pn +1
.sp 4
.h3 id='nebulae'
III. NEBULÆ.
.sp 2
.h4 id='nebulae-classification'
Classification of Nebulæ.
.sp 2
370. Planetary Nebulæ.--Many nebulæ (328) present a
well-defined circular disk, like that of a planet, and are therefore
called planetary nebulæ. Specimens of planetary nebulæ
are shown in Fig. 429.
.if h
.il fn=fig429.png w=70% alt='Nebulae'
.ca Fig. 429.
.if-
.if t
[Illustration: Fig. 429.]
.if-
371. Circular and Elliptical Nebulæ.--While many
nebulæ are circular in form, others are elliptical. The former
are called circular nebulæ, and the latter elliptical
nebulæ. Elliptical nebulæ have been discovered of every
degree of eccentricity. Examples of various circular and
elliptical nebulæ are given in Fig. 430.
.if h
.il fn=fig430.png w=70% alt='Nebulae'
.ca Fig. 430.
.if-
.if t
[Illustration: Fig. 430.]
.if-
372. Annular Nebulæ.--Occasionally ring-shaped nebulæ
have been observed, sometimes with, and sometimes
without, nebulous matter within the ring. They are called
annular nebulæ. They are both circular and elliptical in
form. Several specimens of this class of nebulæ are given
in Fig. 431.
.if h
.il fn=fig431.png w=70% alt='Nebulae'
.ca Fig. 431.
.if-
.if t
[Illustration: Fig. 431.]
.if-
373. Nebulous Stars.--Sometimes one or more minute
stars are enveloped in a nebulous haze, and are hence
called nebulous stars. Several of these nebulæ are shown
in Fig. 432.
.if h
.il fn=fig432.png w=70% alt='Nebulae'
.ca Fig. 432.
.if-
.if t
[Illustration: Fig. 432.]
.if-
374. Spiral Nebulæ.--Very many nebulæ disclose a
more or less spiral structure, and are known as spiral
nebulæ. They are illustrated in Fig. 433. There are, however,
// File: psp_374.png
.pn +1
// File: psp_375.png
.pn +1
a great variety of spiral forms. We shall have occasion
to speak of these nebulæ again (381-383).
.if h
.il fn=fig433.png w=70% alt='Nebulae'
.ca Fig. 433.
.if-
.if t
[Illustration: Fig. 433.]
.if-
375. Double and Multiple Nebulæ.--Many double and
multiple nebulæ have been observed, some of which are
represented in Fig. 434.
.if h
.il fn=fig434.png w=70% alt='Nebulae'
.ca Fig. 434.
.if-
.if t
[Illustration: Fig. 434.]
.if-
Fig. 435 shows what appears to be a double annular
nebula. Fig. 436 gives two views of a double nebula.
The change of position in the components of this double
nebula indicates a motion of revolution similar to that of
the components of double stars.
.if h
.il fn=fig435.png w=70% alt='Nebulae'
.ca Fig. 435.
.if-
.if t
[Illustration: Fig. 435.]
.if-
.if h
.il fn=fig436.png w=70% alt='Nebulae'
.ca Fig. 436.
.if-
.if t
[Illustration: Fig. 436.]
.if-
// File: psp_376.png
.pn +1
.sp 2
.h4 id='nebulae-irregular'
Irregular Nebulæ.
.sp 2
376. Irregular Forms.--Besides the more or less regular
forms of nebulæ which have been classified as indicated
above, there are many of very irregular shapes, and some
of these are the most remarkable nebulæ in the heavens.
Fig. 437 shows a curiously shaped nebula, seen by Sir
John Herschel in the southern
heavens; and Fig. 438, one in
Taurus, known as the Crab
nebula.
.if h
.il fn=fig437.png w=70% alt='Nebulae'
.ca Fig. 437.
.if-
.if t
[Illustration: Fig. 437.]
.if-
.if h
.il fn=fig438.png w=70% alt='Nebulae'
.ca Fig. 438.
.if-
.if t
[Illustration: Fig. 438.]
.if-
377. The Great Nebula of
Andromeda.--This is one of
the few nebulæ that are visible
to the naked eye. We see at
a glance that it is not a star,
but a mass of diffused light. Indeed, it has sometimes
been very naturally mistaken for a comet. It was first
described by Marius in 1614, who compared its light to
// File: psp_377.png
.pn +1
that of a candle shining through horn. This gives a very
good idea of the impression it produces, which is that of
a translucent object illuminated by a brilliant light behind
it. With a small
telescope it is easy
to imagine it to be
a solid like horn;
but with a large one
the effect is more
like fog or mist with
a bright body in its
midst. Unlike most
of the nebulæ, its
spectrum is a continuous
one, similar
to that from a heated solid, indicating that the light
emanates, not from a glowing gas, but from matter in the
solid or liquid state. This would suggest that it is really
an immense star-cluster, so distant that the highest telescopic
power cannot resolve it; yet in the largest telescopes
it looks less resolvable, and more like a gas, than in
those of moderate size. If it is really a gas, and if the
// File: psp_378.png
.pn +1
spectrum is continuous throughout the whole extent of
the nebula, either it must shine by reflected light, or the
gas must be subjected to a great pressure almost to its
outer limit, which is hardly possible. If the light is reflected,
we cannot determine whether it comes from a single
bright star, or a number of small ones scattered through
the nebula.
With a small telescope this nebula appears elliptical, as
in Fig. 439. Fig. 440 shows it as it appeared to Bond, in
the Cambridge refractor.
.if h
.il fn=fig439.png w=70% alt='Nebulae'
.ca Fig. 439.
.if-
.if t
[Illustration: Fig. 439.]
.if-
.if h
.il fn=fig440.png w=70% alt='Nebulae'
.ca Fig. 440.
.if-
.if t
[Illustration: Fig. 440.]
.if-
378. The Great Nebula of Orion.--The nebula which,
above all others, has occupied the attention of astronomers,
// File: psp_379.png
.pn +1
// File: psp_380.png
.pn +1
// File: psp_381.png
.pn +1
and excited the wonder of observers, is the great
nebula of Orion, which surrounds the middle star of the
three which form the sword of Orion. A good eye will perceive
that this star, instead of looking like a bright point,
has a hazy appearance, due to the surrounding nebula.
This object was first described by Huyghens in 1659, as
follows:--
"There is one phenomenon among the fixed stars worthy
of mention, which, so far as I know, has hitherto been
noticed by no one, and indeed cannot be well observed
except with large telescopes. In the sword of Orion are
three stars quite close together. In 1656, as I chanced
to be viewing the middle one of these with the telescope,
instead of a single star, twelve showed themselves (a not
uncommon circumstance). Three of these almost touched
each other, and with four others shone through a nebula,
so that the space around them seemed far brighter than the
rest of the heavens, which was entirely clear, and appeared
// File: psp_382.png
.pn +1
quite black; the effect being that of an opening in the sky,
through which a brighter region was visible."
.if h
.il fn=fig441.png w=70% alt='Nebulae'
.ca Fig. 441.
.if-
.if t
[Illustration: Fig. 441.]
.if-
The representation
of this nebula in Fig.
441 is from a drawing
made by Bond. In
brilliancy and variety
of detail it exceeds
any other nebula visible
in the northern
hemisphere. In its
centre are four stars,
easily distinguished by
a small telescope with
a magnifying power of forty or fifty, together with two
smaller ones, requiring a nine-inch telescope to be well seen.
Besides these, the whole nebula is dotted with stars.
// File: psp_383.png
.pn +1
In the winter of 1864-65 the spectrum of this nebula
was examined independently by Secchi and Huggins, who
found that it consisted of three bright lines, and hence
concluded that the nebula was composed, not of stars, but
of glowing gas. The position of one of the lines was near
that of a line of nitrogen, while another seemed to coincide
with a hydrogen line. This would suggest that the
nebula is a mixture of hydrogen and nitrogen gas; but of
this we cannot be certain.
.if h
.il fn=fig442.png w=70% alt='Nebulae'
.ca Fig. 442.
.if-
.if t
[Illustration: Fig. 442.]
.if-
379. The Nebula in Argus.--There is a nebula (Fig.
442) surrounding
the variable star
Eta Argus (355),
which is remarkable
as exhibiting
variations of brightness
and of outline.
In many other
nebulæ, changes
have been suspected;
but the indistinctness
of outline
which characterizes
most of these objects,
and the very different aspect they present in telescopes
of different powers, render it difficult to prove a change
beyond a doubt.
380. The Dumb-Bell Nebula.--This nebula was named
from its peculiar shape. It is a good illustration of the
change in the appearance of a nebula when viewed with
different magnifying powers. Fig. 443 shows it as it appeared
in Herschel's telescope, and Fig. 444 as it appears
in the great Parsonstown reflector (20).
.if h
.il fn=fig443.png w=70% alt='Nebulae'
.ca Fig. 443.
.if-
.if t
[Illustration: Fig. 443.]
.if-
.if h
.il fn=fig444.png w=70% alt='Nebulae'
.ca Fig. 444.
.if-
.if t
[Illustration: Fig. 444.]
.if-
// File: psp_384.png
.pn +1
.sp 2
.h4 id='nebulae-spiral'
Spiral Nebulæ.
.sp 2
381. The Spiral Nebula in Canes Venatici.--The great
spiral nebula in the constellation Canes Venatici, or the
Hunting-Dogs, is one of the most remarkable of its class.
Fig. 445 shows this nebula as it appeared in Herschel's
telescope, and Fig. 446 shows it as it appears in the Parsonstown
reflector.
.if h
.il fn=fig445.png w=70% alt='Nebulae'
.ca Fig. 445.
.if-
.if t
[Illustration: Fig. 445.]
.if-
.if h
.il fn=fig446.png w=70% alt='Nebulae'
.ca Fig. 446.
.if-
.if t
[Illustration: Fig. 446.]
.if-
// File: psp_385.png
.pn +1
382. Condensation of Nebulæ.--The appearance of the
nebula just mentioned suggests a body rotating on its axis,
and undergoing condensation at the same time.
It is now a generally received theory that nebulæ are the
material out of which stars are formed. According to this
// File: psp_386.png
.pn +1
theory, the stars originally existed as nebulæ, and all nebulæ
will ultimately become condensed into stars.
.if h
.il fn=fig447.png w=70% alt='Nebulae'
.ca Fig. 447.
.if-
.if t
[Illustration: Fig. 447.]
.if-
.if h
.il fn=fig448.png w=70% alt='Nebulae'
.ca Fig. 448.
.if-
.if t
[Illustration: Fig. 448.]
.if-
.if h
.il fn=fig449.png w=70% alt='Nebulae'
.ca Fig. 449.
.if-
.if t
[Illustration: Fig. 449.]
.if-
383. Other Spiral Nebulæ.--Fig. 447 represents a spiral
// File: psp_387.png
.pn +1
nebula of the Great Bear. This nebula seems to have several
centres of condensation. Fig. 448 is a view of a spiral
nebula in Cepheus, and Fig. 449 of a singular spiral nebula
in the Triangle. This also appears to have several points
of condensation. Figs. 450 and 451 represent oval and
elliptical nebulæ having a spiral structure.
.if h
.il fn=fig450.png w=70% alt='Nebulae'
.ca Fig. 450.
.if-
.if t
[Illustration: Fig. 450.]
.if-
.if h
.il fn=fig451.png w=70% alt='Nebulae'
.ca Fig. 451.
.if-
.if t
[Illustration: Fig. 451.]
.if-
// File: psp_388.png
.pn +1
// File: psp_389.png
.pn +1
THE MAGELLANIC CLOUDS.
.if h
.il fn=fig452.png w=90% alt='Magellanic Clouds'
.ca Fig. 452.
.if-
.if t
[Illustration: Fig. 452.]
.if-
384. Situation and General Appearance of the Magellanic
Clouds.--The Magellanic clouds are two nebulous-looking
bodies near the
southern pole of the heavens,
as shown in the right-hand
portion of Fig. 452.
In the appearance and
brightness of their light
they resemble portions of
the Milky-Way.
.if h
.il fn=fig453.png w=70% alt='Magellanic Clouds'
.ca Fig. 453.
.if-
.if t
[Illustration: Fig. 453.]
.if-
The larger of these
clouds is called the Nubecula
Major. It is visible
to the naked eye in
strong moonlight, and covers
a space about two
hundred times the surface of the moon. It is shown in
Fig. 453. The smaller
cloud is called the Nubecula
Minor. It has
only about a fourth the
extent of the larger cloud,
and is considerably less
brilliant. It is visible to
the naked eye, but it disappears
in full moonlight.
This cloud is shown in
Fig. 454. The region
around this cloud is singularly
bare of stars; but
the magnificent cluster of
Toucan, already described (346), is near, and is shown a
little to the right of the cloud in the figure.
.if h
.il fn=fig454.png w=70% alt='Magellanic Clouds'
.ca Fig. 454.
.if-
.if t
[Illustration: Fig. 454.]
.if-
// File: psp_390.png
.pn +1
.if h
.il fn=fig455.png w=70% alt='Magellanic Clouds'
.ca Fig. 455.
.if-
.if t
[Illustration: Fig. 455.]
.if-
385. Structure of the Nubeculæ.--Fig. 455 shows the
structure of these clouds as revealed by a powerful telescope.
The general ground of both consists of large tracts
and patches of nebulosity in every stage of resolution,--from
that which is irresolvable with eighteen inches of
reflecting aperture, up to perfectly separated stars, like the
Milky-Way and clustering groups. There are also nebulæ
in abundance, both regular and irregular, globular clusters
in every state of condensation, and objects of a nebulous
character quite peculiar, and unlike any thing in other
regions of the heavens. In the area occupied by the
nubecula major two hundred and seventy-eight nebulæ and
clusters have been enumerated, besides fifty or sixty outliers,
which ought certainly to be reckoned as its appendages,
being about six and a half per square degree; which very
far exceeds the average of any other part of the nebulous
heavens. In the nubecula minor the concentration of such
objects is less, though still very striking. The nubeculæ,
// File: psp_391.png
.pn +1
then, combine, each within its own area, characters which
in the rest of the heavens are no less strikingly separated;
namely, those of the galactic and the nebular system.
Globular clusters (except in one region of small extent) and
nebulæ of regular elliptic forms are comparatively rare in
the Milky-Way, and are found congregated in the greatest
abundance in a part of the heavens the most remote possible
from that circle; whereas in the nubeculæ they are
indiscriminately mixed with the general starry ground, and
with irregular though small nebulæ.
.sp 2
.h4 id='nebular-hypothesis'
THE NEBULAR HYPOTHESIS.
.sp 2
.pm letter-start
386. The Basis of the Nebular Hypothesis.--We have seen
that the planets all revolve around the sun from west to east
in nearly the same plane, and that the sun rotates on his axis
from west to east. The planets, so far as known, rotate on
their axes from west to east; and all the moons, except those
of Uranus and Neptune, revolve around their planets from
west to east. These common features in the motion of the
sun, moons, and planets, point to the conclusion that they are
of a common origin.
387. Kant's Hypothesis.--Kant, the celebrated German
philosopher, seems to have the best right to be regarded as the
founder of the modern nebular hypothesis. His reasoning has
been concisely stated thus: "Examining the solar system, we
find two remarkable features presented to our consideration.
One is, that six planets and nine satellites [the entire number
then known] move around the sun in circles, not only in the
same direction in which the sun himself revolves on his axis,
but very nearly in the same plane. This common feature of
the motion of so many bodies could not by any reasonable
possibility have been a result of chance: we are therefore
forced to believe that it must be the result of some common
cause originally acting on all the planets.
"On the other hand, when we consider the spaces in which
the planets move, we find them entirely void, or as good as
void; for, if there is any matter in them, it is so rare as to be
// File: psp_392.png
.pn +1
without effect on the planetary motions. There is, therefore,
no material connection now existing between the planets
through which they might have been forced to take up a
common direction of motion. How, then, are we to reconcile
this common motion with the absence of all material connection?
The most natural way is to suppose that there was once
some such connection, which brought about the uniformity of
motion which we observe; that the materials of which the
planets are formed once filled the whole space between them.
There was no formation in this chaos, the formation of separate
bodies by the mutual gravitation of parts of the mass
being a later occurrence. But, naturally, some parts of the
mass would be more dense than others, and would thus gather
around them the rare matter which filled the intervening spaces.
The larger collections thus formed would draw the smaller
ones into them, and this process would continue until a few
round bodies had taken the place of the original chaotic
mass."
Kant, however, failed to account satisfactorily for the motion
of the sun and planets. According to his system, all the
bodies formed out of the original nebulous mass should have
been drawn to a common centre so as to form one sun, instead
of a system of revolving bodies like the solar system.
388. Herschel's Hypothesis.--The idea of the gradual transmutation
of nebulæ into stars seems to have been suggested
to Herschel, not by the study of the solar system, but by that
of the nebulæ themselves. Many of these bodies he believed
to be immense masses of phosphorescent vapor; and he conceived
that these must be gradually condensing, each around
its own centre, or around the parts where it is most dense,
until it should become a star, or a cluster of stars. On classifying
the nebulæ, it seemed to him that he could see this
process going on before his eyes. There were the large, faint,
diffused nebulæ, in which the condensation had hardly begun;
the smaller but brighter ones, which had become so far condensed
that the central parts would soon begin to form into
stars; yet others, in which stars had actually begun to form;
and, finally, star-clusters in which the condensation was complete.
The spectroscopic revelations of the gaseous nature of
// File: psp_393.png
.pn +1
the true nebulæ tend to confirm the theory of Herschel, that
these masses will all, at some time, condense into stars.
389. Laplace's Hypothesis.--Laplace was led to the nebular
hypothesis by considering the remarkable uniformity in the
direction of the rotation of the planets. Believing that this
could not have been the result of chance, he sought to investigate
its cause. This, he thought, could be nothing else than
the atmosphere of the sun, which once extended so far out as
to fill all the space now occupied by the planets. He begins
with the sun, surrounded by this immense fiery atmosphere.
Since the sum total of rotary motion now seen in the planetary
system must have been there from the beginning, he conceives
the immense vaporous mass forming the sun and his atmosphere
to have had a slow rotation on its axis. As the intensely
hot mass gradually cooled, it would contract towards the centre.
As it contracted, its velocity of rotation would, by the laws of
mechanics, constantly increase; so that a time would arrive,
when, at the outer boundary of the mass, the centrifugal force
due to the rotation would counterbalance the attractive force
of the central mass. Then those outer portions would be left
behind as a revolving ring, while the next inner portions would
continue to contract until the centrifugal and attractive forces
were again balanced, when a second ring would be left behind;
and so on. Thus, instead of a continuous atmosphere, the sun
would be surrounded by a series of concentric revolving rings
of vapor. As these rings cooled, their denser materials would
condense first; and thus the ring would be composed of a
mixed mass, partly solid and partly vaporous, the quantity of
solid matter constantly increasing, and that of vapor diminishing.
If the ring were perfectly uniform, this condensation
would take place equally all around it, and the ring would thus
be broken up into a group of small planets, like the asteroids.
But if, as would more likely be the case, some portions of the
ring were much denser than others, the denser portions would
gradually attract the rarer portions, until, instead of a ring,
there would be a single mass composed of a nearly solid
centre, surrounded by an immense atmosphere of fiery vapor.
This condensation of the ring of vapor around a single point
would not change the amount of rotary motion that had existed
// File: psp_394.png
.pn +1
in the ring. The planet with its atmosphere would therefore
be in rotation; and would be, on a smaller scale, like the
original solar mass surrounded by its atmosphere. In the
same way that the latter formed itself first into rings, which
afterwards condensed into planets, so the planetary atmospheres,
if sufficiently extensive, would form themselves into
rings, which would condense into satellites. In the case of
Saturn, however, one of the rings was so uniform throughout,
that there was no denser portion to attract the rest around it;
and thus the ring of Saturn retained its annular form.
.if h
.il fn=fig456.png w=70% alt='Condensing Mass'
.ca Fig. 456.
.if-
.if t
[Illustration: Fig. 456.]
.if-
Such is the celebrated nebular hypothesis of Laplace. It
starts, not with a purely nebulous mass, but with the sun, surrounded
by an immense atmosphere, out of which the planets
were formed by gradual condensation. Fig. 456 represents
the condensing mass according to this theory.
390. The Modern Nebular Hypothesis.--According to the
nebular hypothesis as held at the present time, the sun, planets,
and meteoroids originated from a purely nebulous mass.
This nebula first condensed into a nebulous star, the star being
the sun, and its surrounding nebulosity being the fiery atmosphere
of Laplace. The original nebula must have been put
into rotation at the beginning. As it contracted and became
// File: psp_395.png
.pn +1
condensed through the loss of heat by radiation into space,
and under the combined attraction of gravity, cohesion, and
affinity, its speed of rotation increased; and the nebulous
envelop became, by the centrifugal force, flattened into a thin
disk, which finally broke up into rings, out of which were
formed the planets and their moons. According to Laplace,
the rings which were condensed into the planets were thrown
off in succession from the equatorial region of the condensing
nebula; and so the outer planets would be the older. According
to the more modern idea, the nebulous mass was first flattened
into a disk, and subsequently broken up into rings, in
such a way that there would be no marked difference in the
ages of the planets. The sun represents the central portion
of the original nebula, and the comets and meteoroids its outlying
portion. At the sun the condensation is still going on,
and the meteoroids appear to be still gradually drawn in to
the sun and planets.
The whole store of energy with which the original solar
nebula was endowed existed in it in the potential form. By
the condensation and contraction this energy was gradually
transformed into the kinetic energy of molar motion and of
heat; and the heat became gradually dissipated by radiation
into space. This transformation of potential energy into heat
is still going on at the sun, the centre of the condensing mass,
by the condensation of the sun itself, and by the impact of
meteors as they fall into it.
It has been calculated, that, by the shrinking of the sun to
the density of the earth, the transformation of potential energy
into heat would generate enough heat to maintain the sun's
supply, at the present rate of dissipation, for seventeen million
years. A shrinkage of the sun which would generate all the
heat he has poured into space since the invention of the telescope
could not be detected by the most powerful instruments
yet constructed.
The least velocity with which a meteoroid could strike the
sun would be two hundred and eighty miles a second; and
it is easy to calculate how much heat would be generated
by the collision. It has been shown, that, were enough meteoroids
to fall into the sun to develop its heat, they would not
// File: psp_396.png
.pn +1
increase his mass appreciably during a period of two thousand
years.
The sun's heat is undoubtedly developed by contraction and
the fall of meteoroids; that is to say, by the transformation of
the potential energy of the original nebula into heat.
It must be borne in mind that the nebular hypothesis is simply
a supposition as to the way in which the present solar
system may have been developed from a nebula endowed with
a motion of rotation and with certain tendencies to condensation.
Of course nothing could have been developed out of
the nebula, the germs of which had not been originally implanted
in it by the Creator.
.pm letter-end
.sp 2
.h3 id='stellar-universe'
IV. THE STRUCTURE OF THE STELLAR UNIVERSE.
.sp 2
391. Sir William Herschel's View.--Sir William Herschel
assumed that the stars are distributed with tolerable uniformity
throughout the space occupied by our stellar system. He
accounted for the increase in the number of stars in the field
of view as he approached the plane of the Milky-Way, not
by the supposition that the stars are really closer together in
and about this plane, but by the supposition that our stellar
system is in the form of a flat disk cloven at one side, and
with our sun near its centre. A section of this disk is shown
in Fig. 457.
.if h
.il fn=fig457.png w=70% alt='Disc'
.ca Fig. 457.
.if-
.if t
[Illustration: Fig. 457.]
.if-
// File: psp_397.png
.pn +1
An observer near S, with his telescope pointed in the direction
of S b, would see comparatively few stars within the field
of view, because looking through a comparatively thin stratum
of stars. With his telescope pointed in the direction S a, he
would see many more stars within his field of view, even though
the stars were really no nearer together, because he would be
looking through a thicker stratum of stars. As he directed
his telescope more and more nearly in the direction S f, he
would be looking through a thicker and thicker stratum of
stars, and hence he would see a greater and greater number of
them in the field of view, though they were everywhere in the
disk distributed at uniform distances. He assumed, also, that
the stars are all tolerably
uniform in size, and that
certain stars appear smaller
than others, only because
they are farther off.
He supposed the faint
stars of the Milky-Way
to be merely the most distant
stars of the stellar
disk; that they are really
as large as the other
stars, but appear small
owing to their great distance.
The disk was assumed
to be cloven on
one side, to account for the division of the Milky-Way through
nearly half of its course. This theory of the structure of the
stellar universe is often referred to as the cloven disk theory.
.if h
.il fn=fig458.png w=70% alt='Cloven Ring'
.ca Fig. 458.
.if-
.if t
[Illustration: Fig. 458.]
.if-
392. The Cloven Ring Theory.--According to Mädler, the
stars of the Milky-Way are entirely separated from the other
stars of our system, belonging to an outlying ring, or system
of rings. To account for the division of the Milky-Way, the
ring is supposed to be cloven on one side: hence this theory
is often referred to as the cloven ring theory. According to
this hypothesis, the stellar system viewed from without would
present an appearance somewhat like that in Fig. 458. The
outlying ring cloven on one side would represent the stars
// File: psp_398.png
.pn +1
of the Milky-Way; and the luminous mass at the centre, the
remaining stars of the system.
393. Proctor's View.--According to Proctor, the Milky-Way
is composed of an irregular spiral stream of minute stars
lying in and among the larger stars of our system, as represented
in Fig. 459. The spiral stream is shown in the inner
circle as it really exists among the stars, and in the outer
circle as it is seen projected upon the sky. According to this
view, the stars of the Milky-Way appear faint, not because
they are distant, but because they are really small.
.if h
.il fn=fig459.png w=70% alt='Spiral'
.ca Fig. 459.
.if-
.if t
[Illustration: Fig. 459.]
.if-
394. Newcomb's View.--According to Newcomb, the stars
of our system are all situated in a comparatively thin zone
lying in the plane of the Milky-Way, while there is a zone
of nebulæ lying on each side of the stellar zone. He believes
// File: psp_399.png
.pn +1
that so much is certain with reference to the structure of our
stellar universe; but he considers that we are as yet comparatively
ignorant of the internal structure of either the stellar
or the nebular zones. The structure of the stellar universe,
according to this view, is shown in Fig. 460.
.if h
.il fn=fig460.png w=70% alt='Structure'
.ca Fig. 460.
.if-
.if t
[Illustration: Fig. 460.]
.if-
// File: psp_400.png
.pn +1
// File: psp_401a.png
.pn +1
.sp 4
.h2 title='Index'
INDEX
.sp 2
.ix
A.
Aberration of light, #38#.
Aerolites, #304#.
Aldebaran, star in Taurus, #340#, #342#.
Algol, a variable star, #343#, #358#.
Almanac, perpetual, #82#.
Alps, lunar mountains, #126#.
Altair, star in Aquila, #336#.
Alt-azimuth instrument, #13#.
Altitude, #12#.
Andromeda (constellation), #343#, #346#.
nebula in, #376#.
Angström's map of spectrum, #164#.
Antares, star in Scorpio, #347#.
Apennines, lunar mountains, #122#, #124#.
Aphelion, #47#.
Apogee, #44#.
Aquarius, or the Water-Bearer, #350#.
cluster in, #354#.
Aquila, or the Eagle, #336#.
Arcturus, star in Boötes, #335#, #365#, #370#.
Argo, or the Ship, #360#.
nebula in, #383#.
variable star in, #360#.
Aries, or the Ram, #350#.
Asteroids, #223#, #241#.
Astræa, an asteroid, #241#.
Auriga, or the Wagoner, #342#.
Azimuth, #13#.
B.
Betelgeuse, star in Orion, #340#, #370#.
Berenice's Hair (constellation), #334#.
Bode's law, #241#.
disproved, #273#.
Boötes (constellation), #334#, #335#.
C.
Calendar, the, #80#.
Callisto, moon of Jupiter, #250#.
Cancer, or the Crab, #350#.
tropic of, #61#.
Canes Venatici, or the Hunting-Dogs, #334#.
Canes Venatici, nebula in, #384#.
Canis Major, or the Great Dog, #342#.
Canis Minor, or the Little Dog, #340#.
Capella, star in Auriga, #340#, #343#.
Capricorn, tropic of, #61#.
Capricornus, or the Goat, #350#.
Cassiopeia (constellation), #332#.
new star in, #362#.
Castor, star in Gemini, #340#, #370#.
Caucasus, a lunar range, #124#.
Centaurus, star-cluster in, #355#.
Cepheus (constellation), #334#.
nebula in, #387#.
Ceres, the planet, #241#.
Cetus, or the Whale, #346#.
variable star in, #359#.
Charles's Wain, #330#.
Circles, great, #4#.
diurnal, #8#.
hour, #16#.
small, #4#.
vertical, #12#.
Clock, astronomical, #18#.
time, #78#.
Coma Berenices, or Berenice's Hair, #334#.
Comet, Biela's, #293#.
and earth, collision of, #316#.
Coggia's, #297#.
Donati's, #296#.
Encke's, #293#.
Halley's, #291#.
of 1680, #290#.
of 1811, #290#.
of 1843, #295#.
of 1861, #297#.
of June, 1881, #300#.
Comets, appearance of, #274#.
and meteors, #313#.
bright, #274#.
chemical constitution of, #318#.
development of, #277#.
number of, #288#.
orbits of, #282#.
origin of, #287#.
periodic, #286#.
physical constitution of, #314#.
tails of, #279#.
telescopic, #275#, #281#.
visibility of, #281#.
Conic sections, #48#.
Conjunction, #91#.
inferior, #130#.
superior, #130#, #136#.
Constellations, #325#.
zodiacal, #32#.
Copernican system, the, #44#, #53#.
Copernicus, a lunar crater, #120#, #129#.
Corona Borealis, or the Northern Crown, #336#.
Corona Borealis, new star in, #363#.
Corvus, or the Crow, #339#.
Crystalline spheres, #41#.
Cycles and epicycles, #42#.
Cygnus, or the Swan, #338#.
D.
Day and night, #57#.
civil, #77#.
lunar, #108#.
sidereal, #74#.
solar, #74#.
Declination, #16#.
Deimos, satellite of Mars, #239#.
Delphinus, or the Dolphin, #338#.
Deneb, star in Cygnus, #338#, #370#.
Dione, satellite of Saturn, #259#.
Dipper, the Great, #330#, #366#, #369#, #370#.
the Little, #331#.
the Milk, #347#.
Dissociation, #163#.
Dominical Letter, the, #81#.
Draco, or the Dragon, #331#.
E.
Earth, density of, #85#.
flattened at poles, #55#.
form of, #53#.
in space, #56#.
seen from moon, #109#.
size of, #55#.
weight of, #83#.
Eccentric, the #43#.
Eccentricity, #46#.
Eclipses, #210#.
annular, #219#.
lunar, #210#, #214#.
solar, #216#.
Ecliptic, the, #27#.
obliquity of, #28#.
Ellipse, the, #45#, #49#.
Elongation, of planet, #130#.
Enceladus, moon of Saturn, #259#.
Epicycles, #42#.
Epicycloid, #107#.
Epsilon Lyræ, a double star, #356#.
Equator, the celestial, #7#.
Equinoctial, the, #7#.
colure, #16#.
elevation of, #9#.
Equinox, autumnal, #29#.
vernal, #16#, #29#.
Equinoxes, precession of, #31#, #85#.
Eta Argus, a variable star, #360#, #383#.
Europa, moon of Jupiter, #250#.
F.
Faculæ, solar, #177#.
Fomalhaut, star in Southern Fish, #350#.
Fraunhofer's lines, #164#, #371#.
G.
Galaxy, the, #326#.
Ganymede, moon of Jupiter, #250#.
Gemini, or the Twins, #340#.
Georgium Sidus, #271#.
H.
Hercules (constellation), #336#.
cluster in, #353#.
orbit of double star in, #357#.
solar system moving towards, #367#.
Herschel, the planet (see #Uranus:index-uranus#).
Herschel's hypothesis, #392#, #396#.
Horizon, the, #5#.
Hyades, the, #342#, #350#.
Hydra, or the Water-Snake, #340#.
Hyperbola, the, #49#.
Hyperion, moon of Saturn, #259#.
I.
Io, moon of Jupiter, #250#.
Irradiation, #90#, #113#.
J.
Japetus, moon of Saturn, #259#.
Job's Coffin (asterism), #338#.
Juno, the planet, #241#.
Jupiter, apparent size of, #245#.
distance of, #245#.
great red spot of, #249#.
orbit of, #244#.
periods of, #246#.
physical constitution of, #246#.
rotation of, #248#.
satellites of, #250#.
eclipses of, #252#.
transits of, #254#.
volume of, #245#.
without satellites, #255#.
K.
Kant's hypothesis, #391#.
Kepler, a lunar crater, #129#.
Kepler's system, #44#.
laws, #46#.
star, #362#.
Kirchhoff's map of spectrum, #164#.
L.
Laplace's hypothesis, #392#.
Latitude, celestial, #30#.
Leap year, #81#.
Leo, or the Lion, #334#.
Leonids (meteors), #312#.
Libra, or the Balances, #347#.
Libration, #102#.
Longitude, celestial, #30#.
Lyra, or the Lyre, #338#.
double star in, #356#.
M.
Magellanic clouds, the, #389#.
Magnetic storms, #190#.
Magnetism and sun-spots, #190#.
Mars, apparent size of, #236#.
brilliancy of, #237#.
distance of, #235#.
orbit of, #235#.
periods of, #237#.
rotation of, #239#.
satellites of, #239#.
volume of, #236#.
Mercury, apparent size of, #226#.
atmosphere of, #228#.
distance of, #225#.
elongation of, #227#.
orbit of, #225#.
periods of, #227#.
volume of, #226#.
Meridian, the, #12#.
Meridian circle, #17#.
Meridians, celestial, #31#.
Meteoric iron, #305#, #307#.
showers, #310#.
stones, #305#.
Meteors, #300#.
August, #311#.
light of, #309#.
November, #312#.
sporadic, #310#.
Meteoroids, #308#.
Micrometers, #20#, #153#.
Milky-Way, the, #326#.
Mimas, moon of Saturn, #259#.
Mira, a variable star, #359#.
Moon, apparent size of, #87#, #89#.
aspects of, #91#.
atmosphere of, #109#.
chasms in, #123#.
craters in, #119#.
day of, #108#.
distance of, #86#.
eclipses of, #210#.
form of orbit, #97#.
harvest, #101#.
hunter's, #102#.
inclination of orbit, #97#.
kept in her path by gravity, #51#.
librations of, #102#.
mass of, #90#.
meridian altitude of, #98#.
mountains of, #116#.
orbital motion of, #91#.
phases of, #93#.
real size of, #88#.
rising of, #99#.
rotation of, #102#.
sidereal period of, #92#.
surface of, #115#.
synodical period of, #92#.
terminator of, #115#.
wet and dry, #98#.
N.
Nadir, the, #6#.
Neap-tides, #72#.
Nebula, in Andromeda, #376#.
crab, #376#.
dumb-bell, #383#.
in Argus, #383#.
in Canes Venatici, #384#.
in Cepheus, #387#.
in Orion, #378#.
in the Triangle, #387#.
in Ursa Major, #386#.
Nebulæ, #281#, #330#, #373#.
annular, #373#.
circular, #373#.
condensation of, #385#.
double, #375#.
elliptical, #373#.
irregular, #376#.
multiple, #375#.
spiral, #373#, #384#.
Nebular hypothesis, the, #391#.
Neptune, discovery of, #271#.
orbit of, #271#.
satellite of, #274#.
New style, #80#.
Newcomb's theory of the stellar universe, #398#.
Newton's system, #48#.
Nodes, #97#.
Nubecula, Major, #389#.
Minor, #389#.
Nutation, #34#.
O.
Olbers's hypothesis, #241#.
Old style, #80#.
Ophiuchus (constellation), #347#.
new star in, #362#.
Opposition, #91#, #136#.
Orion, #341#.
nebula in, #378#.
the trapezium of, #356#.
P.
Pallas, the planet, #241#.
Parabola, the, #49#.
Parallax, #37#.
Pegasus (constellation), #343#, #346#.
triple star in, #356#.
Perigee, #44#.
Perihelion, #47#.
Perseids (meteors), #311#.
Perseus (constellation), #346#.
cluster in, #353#.
Phobos, satellite of Mars, #239#.
Pico, a lunar mountain, #127#.
Pisces, or the Fishes, #350#.
Piscis Australis, or the Southern Fish, #350#.
Planets, #39#.
inferior, #130#.
periods of, #132#.
phases of, #132#.
inner group of, #221#.
intra-Mercurial, #230#.
minor, #223#.
outer group of, #222#, #244#.
superior, #134#.
motion of, #134#.
periods of, #137#.
phases of, #137#.
three groups of, #221#.
Pleiades, the, #328#, #342#, #351#.
Pointers, the, #330#.
Polar distance, #16#.
Pole Star, the, #7#, #330#, #365#.
Poles, celestial, #7#, #9#.
Pollux, star in Gemini, #340#, #370#.
Præsepe, or the Beehive, #350#.
Precession of equinoxes, #31#, #85#.
Prime vertical, the, #12#.
Proctor's theory of the stellar universe, #398#.
Procyon, star in Canis Minor, #340#.
Ptolemaic system, the, #41#.
Q.
Quadrature, #91#, #137#.
R.
Radiant point (meteors), #310#.
Radius vector, #47#.
Refraction, #35#.
Regulus, star in Leo, #334#, #370#.
Rhea, moon of Saturn, #259#.
Rigel, star in Orion, #340#, #370#.
Right ascension, #16#.
S.
Sagittarius, or the Archer, #347#.
Saturn, apparent size of, #256#.
distance of, #256#.
orbit of, #255#.
periods of, #256#.
physical constitution of, #257#.
ring of, #261#.
changes in, #268#.
constitution of, #269#.
phases of, #263#.
rotation of, #258#.
satellites of, #259#.
volume of, #256#.
Scorpio, or the Scorpion, #347#.
cluster in, #355#.
Seasons, the, #64#.
Sirius, the Dog-Star, #340#, #342#, #365#, #370#, #371#.
Solar system, the, #41#.
Solstices, #29#, #59#, #60#.
Sound, effect of motion on, #168#.
Spectra, bright-lined, #158#.
comparison of, #154#.
continuous, #158#.
displacement of lines in, #171#.
of comets, #318#.
reversed, #161#.
sun-spot, #193#.
types of stellar, #371#.
Spectroscope, the, #152#.
diffraction, #157#.
direct-vision, #155#.
dispersion, #152#.
Spectrum analysis, #159#.
solar, #164#.
Sphere, defined, #3#.
the celestial, #5#.
rotation of, #7#.
Spring-tides, #72#.
Stars, circumpolar, #7#.
clusters of, #328#, #350#.
color of, #357#.
constellations of, #325#.
constitution of, #371#.
distance of, #364#.
double, #355#.
drift of, #368#.
four sets of, #10#.
magnitude of, #322#.
motion of, in line of sight, #369#.
multiple, #356#.
names of, #325#.
nebulous, #373#.
new, #361#.
number of, #323#.
parallax of, #364#.
proper motion of, #365#.
secular displacement of, #366#.
temporary, #361#.
variable, #358#.
Sun, atmosphere of, #149#.
brightness of, #151#.
chemical constitution of, #164#.
chromosphere of, #149#, #196#.
corona of, #149#, #196#, #204#.
distance of, #142#.
faculæ of, #177#.
heat radiated by, #150#.
inclination of axis of, #187#.
mass of, #140#.
motion of, among the stars, #26#.
at surface of, #168#.
in atmosphere of, #172#.
secular, #366#.
photosphere of, #149#, #175#.
prominences of, #149#, #197#.
rotation of, #186#.
spectrum of, #164#, #171#.
temperature of, #149#.
volume of, #140#.
winds on, #174#.
Sun-spots, #179#.
and magnetism, #190#.
birth and decay of, #185#.
cause of, #194#.
cyclonic motion in, #182#.
distribution of, #188#.
duration of, #181#.
groups of, #181#.
periodicity of, #189#.
proper motion of, #187#.
size of, #181#.
spectrum of, #193#.
T.
Taurus, or the Bull, #342#.
quadruple star in, #356#.
Telescope, Cassegrainian, #23#.
equatorial, #19#.
front-view, #22#.
Gregorian, #23#.
Herschelian, #22#.
Lord Rosse's, #25#.
Melbourne, #25#.
Newall, #20#.
Newtonian, #22#.
Paris, #26#.
reflecting, #21#.
Washington, #20#.
Vienna, #20#.
Telespectroscope, the, #155#.
Telluric lines of spectrum, #165#.
Tethys, moon of Saturn, #259#.
Tides, #67#.
Time, clock, #78#.
sun, #78#.
Titan, moon of Saturn, #259#, #261#.
Toucan, star cluster in, #354#, #389#.
Transit instrument, #17#.
Transits of Venus, #145#.
Triesneker, lunar formation, #123#.
Tropics, #61#.
Twilight, #62#.
Tycho Brahe's star, #361#.
system, #44#.
Tycho, a lunar crater, #129#.
U.
Universe, structure of the stellar, #396#.
Uranus, discovery of, #271#.
name of, #270#.
orbit of, #269#.
satellites of, #271#.
Ursa Major, or the Great Bear, #330#.
nebula in, #386#.
Ursa Minor, or the Little Bear, #330#.
V.
Vega, star in Lyra, #336#, #365#, #370#.
Venus, apparent size of, #231#.
atmosphere of, #234#.
brilliancy of, #232#.
distance of, #231#.
elongation of, #231#.
orbit of, #230#.
periods of, #232#.
volume of, #231#.
transits of, #145#, #234#.
Vernier, the, #15#.
Virgo, or the Virgin, #338#.
Vesta, the planet, #241#.
Vulcan, the planet, #230#.
Y.
Year, the, #78#.
anomalistic, #79#.
Julian, #80#.
sidereal, #79#.
tropical, #79#.
Z.
Zenith, the, #6#.
distance, #12#.
Zodiac, the, #32#.
Zodiacal constellations, #32#.
light, #318#.
Zones, #61#.
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Transcriber's Notes:
Missing or obscured punctuation was corrected.
Typographical errors were silently corrected.
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