THE SOLAR SYSTEM.
GRAVITATION—THE PLANETS—SIZE AND MEASUREMENT OF THE
PLANETS—SATELLITES—FALLING STARS—COMETS—AEROLITES.
[Illustration: Fig. 526.—Planets compared with a quarter of the sun.]
GRAVITATION is the force which keeps the planets in their orbits, and
this theory, perfected by Newton, was partially known to Kepler. Newton
brought this idea into practical shape, and applied it mathematically.
We know that every object in the world tends to attract every other
object in proportion to the quantity of matter of which each consists.
So the sun attracts the planets, and they influence him in a minor
degree. Likewise the moon and our earth reciprocally attract each
other. But as the sun’s mass is far greater than the masses of the
planets he influences them more, and could absorb them all without
inconvenience or disturbance from his centre of gravity. We have,
in a former portion of this work, spoken of the law of universal
gravitation, which is the mutual attraction of any two bodies to each
other, is directly proportioned to their _masses_ (not size), and
inversely to the square of their distances apart.
This law operates amongst the heavenly bodies, and it is to the
never-changing action of gravitation that the planets are kept in
their places. Let us see how this is effected. We have read of force,
and motion, and rest. Every body will remain at rest unless force
compels it to change its position, and it will then go on for ever in
a straight path unless something stops it. But if this body be acted
on simultaneously by two forces in different directions, it will go in
the direction of the greater force. Two equal forces will tend to give
it an intermediate direction, and an equal opposing force will stop
it. The last axiom but one—viz., the two equal forces in different,
not opposing directions, gives us the key to the curving line of the
planetary motions. Were it not for the attraction of the sun the
planets would fly off at a tangent; while, on the other hand, were not
the impelling force as great as it is, they would fall into the sun.
Thus they take an intermediate line, and circle round the centre of the
solar system—the SUN.
The solar system consists of the sun and the planets which revolve in
space around him. These stars are called planets because they move
in the heavens. We shall see that they have certain motions—going
from east to west, from west to east, and sometimes they appear to be
quite motionless. This change of place, appearing now at one side of
the sun and now at another, has given them their title of “wanderers”
(planets). Besides the planets there are comets and meteors, asteroids
and satellites, all circling round the sun in more or less regular
orbits. And there must be families of comets, and whole systems of
meteors that have not yet appeared to us, and which make up the comets,
as has been lately suggested.
Five planets were known to the ancients, and were named after gods and
a goddess: MERCURY, VENUS, MARS, JUPITER, SATURN. In later years a
great number were discovered. We must, however, confine ourselves to
the consideration of the principal ones, eight in number, including
our own EARTH, URANUS and NEPTUNE completing the list. Of these VENUS
and MERCURY are the inferior, or interior planets moving between us
and the sun. MARS, JUPITER, SATURN, URANUS, and NEPTUNE are superior,
or exterior, and pass quite round the heavens. All the planets are
spheroids, and vary greatly in their magnitude, as will be seen by the
illustration (fig. 528), the largest body being the sun. Mercury, Mars,
and Venus, are not so large as the Earth. The other principal planets
are considerably larger than our globe.
[Illustration: Fig. 527.—The Moon.]
Mercury is the smallest of the planets, Venus being nearly as large
as the earth. Then comes Mars, which, though smaller than Venus, is
larger than Mercury. Jupiter is the largest of all—the giant amongst
planets, as Jove was the ruler of the gods of mythology. Saturn comes
next, though much smaller than Jupiter, but bigger than all the rest
together. Next Uranus, then Neptune, larger than Uranus, but farther
away from us. We shall speak more in detail about these in their order
separately.
[Illustration: Fig. 528.—Comparative size of the sun seen from the
planets.]
Taking the earth as 1, the comparative VOLUMES of the planets are as
follows:—
Mercury 1/25, Venus 4/5, Mars 1/5, Jupiter 1300, Saturn 900, Uranus
80, Neptune 230. Sir John Herschel gives the following illustration of
magnitudes and distances:—
“Choose any well-levelled field or bowling green; on it place a globe
two feet in diameter; this will represent the sun. Mercury will be
represented by a grain of mustard seed on the circumference of a
circle 164 feet in diameter for its orbit; Venus a pea, on a circle
284 feet in diameter; the Earth also a pea on a circle 430 feet; Mars
a rather large pin’s head on a circle of 654 feet; Juno, Ceres, Vesta,
and Pallas grains of sand in orbits of 1,000 to 1,200 feet; Jupiter a
moderate-sized orange on a circle nearly half a mile across; Saturn a
small orange on a circle four-fifths of a mile; and Uranus a full-sized
cherry, or small plum, upon the circumference of a circle more than a
mile and a half in diameter”
[Illustration: Fig. 529.—Sizes of the planets.]
[Illustration: Fig 530.—Sizes of planets.]
[Illustration: Fig. 531.—Orbits of planets.]
From an inspection of the following table the relative distances of the
principal planets from the sun, their diameters, and other information
respecting them may approximately be obtained. The dates of the
discovery of the more modern pair are added:—
+-------+---------+-------------+------------+---------+-----------+
|Names. |Diameters|Distance from| Sidereal | Time of | Date of |
| | in | the sun | period of | rotation| discovery.|
| | English | (about). |revolution. | on their| |
| | miles | | | axis. | |
| | (about).| | | | |
+-------+---------+-------------+------------+---------+-----------+
| | | | d. h. m.| d. h. m.| |
|MERCURY| 3,000 | 35,000,000| 87 23 16| 24 5 --| Antiquity |
|VENUS | 7,500 | 66,000,000| 224 16 50| 23 21 21| do. |
|EARTH | 8,000 | 91,000,000| 365 6 9| 23 56 4| do. |
|MARS | 4,500 | 139,000,000| 686 23 31| 24 37 20| do. |
|JUPITER| 85,000 | 476,000,000| 4,332 14 2| 9 55 21| do. |
|SATURN | 70,000 | 872,000,000|10,759 5 16| 10 16 --| do. |
|URANUS | 33,000 |1,753,000,000|30,686 17 21| 9 30 --| Herschel, |
| | | | | | 1781. |
|NEPTUNE| 37,000 |2,746,000,000|60,126 17 5| . . . | Leverrier |
| | | | | | and Galle,|
| | | | | | 1846. |
|THE SUN| 850,000 | . . . . | . . . . | 25 7 48| |
+-------+---------+-------------+------------+---------+-----------+
Altogether there are a great number of planets and asteroids, which
latter are minor planets circulating outside the orbit of Mars. They
have nearly all classical names, such as Juno, Ceres, Vesta, Flora,
Ariadne, Pallas, Pomona, Thalia, etc., and are all at distances from
the sun ranging between 200,000,000 and 300,000,000 of miles, the
periods of sidereal revolution ranging from 1,100 to 3,000 days, so
their years must be from four times to nine times as long as ours.
Altogether about two hundred of the minor planets have been discovered,
and they are all very much smaller than the earth; some, indeed, being
very tiny—only a few miles in diameter, but very massive. They do not
appear to possess any satellites—at least, none have been discovered,
for such very small bodies as they must be, supposing they exist, would
be quite invisible even with our perfected instruments.
[Illustration: Fig. 532.—Mars.]
SATELLITES, however, or “planetary moons,” as they are sometimes
designated, are plainly perceived attending upon the great planets.
There are twenty of these at present under observation. One we are all
familiar with, and the moon, _par excellence_, lends a beauty to our
nights which no other light that we can enjoy or command can ever
do. It is remarkable that only this moon is specially mentioned in
the Bible in connection with the sun. The stars are usually grouped,
although, of course, the sun and moon are equally “stars” in the
firmament. Mars possesses two moons and Jupiter four; Uranus also
rejoices in the latter number; Neptune, like the Earth, has only one.
It is reserved for Saturn to outstrip all the rest in his attendants,
for no less than eight satellites wait upon that enormous planet. No
doubt there are many more of these moons to be found, and every year
will doubtless bring us further knowledge respecting them. Mars’ moons
were only discovered very lately (in 1877), although they were known
to exist; but being very small, unlike the others, they were missed.
So we may conclude that the remaining satellites will remain for some
time undiscovered, even if they actually are in existence. Jupiter’s
moons are supposed to be as large as our own moon; Neptune and Uranus
can boast of equally-sized attendants. But it is impossible to estimate
the riches of astronomical lore which are beyond our ken. Millions of
tiny planets are believed to exist, but their immense distance from us
precludes all investigation. We are but mites in the scale.
[Illustration: Fig. 533.—Jupiter.]
[Illustration: Fig. 534.—Saturn.]
[Illustration: Fig. 535.—Meteor shower.]
METEORS, to which we have already referred, are small erratic bodies
rushing through the planetary system, and getting hot in the process,
appear in the atmosphere surrounding our earth as “shooting stars.”
Some of these falling bodies have reached the earth, and several can
be seen in the British Museum. Numbers, of course, are burnt up before
they reach us, and who can tell what destruction such a catastrophe
may represent, or whether it be or be not an inhabited world which has
thus plunged to destruction by fire? They are of a metallic or stony
nature. On certain nights in August and November it has been calculated
that these meteors will appear. They fall from certain constellations
apparently on these occasions, and are called after their names—as
Leonides, from Leo, in the November displays.
The star-showers at times attain the dimensions of a very beautiful
display of rockets. Millions of them rush round the sun; and when, as
occasionally happens, our earth comes near them, we have (as in 1866) a
grand display of celestial “fireworks.” But the individuals composing
the mass of falling stars are very small. These meteors are very much
like the comets we last year had an example of, and it has been lately
suggested that there is a great degree of affinity between the comets
and the meteors;—in fact, that a comet is merely an aggregation of
meteors. They have been supposed to be bodies of burning gas. Their
mass is very great, and their brilliant tails are many millions of
miles in extent.
Comets are thus distinguished by their tails, and differ very much
in their orbits from the planets. The latter are direct in their
wanderings, but comets are most irregular and eccentric. The name
bestowed upon comets is from the Greek _Kome_, hair; for when the comet
recedes from the sun the “tail” may be said to come out of the head,
and appear as a hair in front, so to speak. But though all comets have
tails, there are many luminous bodies (classed with comets) which have
no tails.
[Illustration: Fig. 536.—Star shower.]
[Illustration: Fig. 537.—Halley’s Comet.]
The comet which created the most excitement was Halley’s in 1456, of
which we append an illustration (fig. 537). A comet had been observed
in 1607, and Halley made a calculation that it would reappear in 1757.
The calculation for its actual appearance was made by Claivant, and
the expected visitor passed the perihelion in March 1759. This comet,
on its appearance at Constantinople, is said to have caused much
consternation, and Christians regarded it as a “sign,” for the Turks
had just then captured Constantinople, and were threatening Europe.
Pious people included it in their supplications for deliverance from
their most dreaded enemies, and “Lord, save us from the Turks, the
devil, and the comet,” was a common prayer.
[Illustration: Fig. 538.—Great comet of 1811.]
There have been several very beautiful comets. Encke’s, Coggia’s,
etc., and the comet of 1858 (Donatis) must be in the recollection of
middle-aged readers. Others came in 1861 and 1874. In 1881 two comets
appeared. Some comets of antiquity were very remarkable, and are
reputed to have equalled the sun in magnitude. One tail is usually
supposed to be the distinguishing mark of a comet, but in 1774 one
appeared with six tails, arranged something like a fan. Sometimes the
tail is separated from the head. Of the actual consistency of comets we
cannot give any lengthened details. They apparently consist of elements
similar to the meteors—namely, of solid masses, and have been supposed
to be aggregations of meteors. Some appear at regular intervals, and
their approach can be determined with accuracy. Of course we only see
those which are attracted by the sun, or those which revolve in the
solar system. There must be thousands of other comets which we never
see at all.
The diagram (fig. 540 in the next page) represents a portion of the
path of the comet of 1680. This visitor pursued its course for two
months at a velocity of 800,000 miles an hour. The tail was estimated
to extend 123,000,000 of miles, and a length of 60,000,000 of miles was
emitted in two days. When this great comet was approaching the sun, or
its perihelion, as such approach is termed, three minutes more would
have seen it rush into the orb had its enormous pace been slackened,
but as it was proceeding so rapidly, and being just then 144,000
miles away, it escaped. We can scarcely estimate the results of such
a collision. This comet appeared B.C. 34, and again at intervals of
about 575 years. It may be expected in 2255. It is to Halley that the
discovery of the elliptical orbits of comets is due.
M. Biela’s comet was the cause of much anxiety in 1832, for a collision
with the earth was apprehended. Fortunately a month intervened between
the period at which the comet was expected at a certain place in the
system and the earth’s arrival at that spot, so, as it happened, about
60,000,000 of miles intervened. We cannot say what the exact effect of
such a collision would be, but some wonderful atmospheric phenomena and
increased temperature would certainly result from the contact. Now the
comet is supposed to have an effect upon the vintage, as “comet” wines
are regarded with much favour. If comets, as is believed, do consist
partly of solid particles, a collision might be unpleasant; but the
weight is, as a rule, a mere nothing compared to their vapoury volume,
which is enormous. That the tails must be of a very attenuated medium
is evident, as we can see the stars through them, and we know that a
very thin cloud will obscure a star. The “menacing” comet, mentioned in
the _Spectator_ February 1881, will not do much damage, so the scare
was needless, as Mr. Proctor has explained.
[Illustration: Fig. 539.—Path of Biela’s Comet.]
[Illustration: Fig. 540.—Path of comet, 1680.]
AEROLITES, or “Meteorites,” are falling bodies (meteors), which reach
the earth in solid form. The greater mass of falling stars are burnt up
ere they reach us, or are dissipated in space. But many instances of
aerolites descending might be adduced. They usually consist of metals,
such as iron and nickel mixed with sulphur, magnesia, and silica. The
theory concerning falling stars has been already mentioned.
[Illustration: Fig. 541.—The heavens as seen from Saturn.]
We have thus far taken a brief general view of the solar system, with a
few of the phenomena of the heavens. Our next step will be to consider
the sun, the planets, and the asteroids, according to the order of
magnitude. The asteroids we cannot consider separately, but the sun,
moon, earth, and the principal planets will yield us much interesting
information as we examine them more closely. We shall then, as far as
possible, look into the domain of the fixed stars, constellations, and
the nebulæ, commenting, as we proceed, upon the varied celestial and
terrestrial phenomena connected with the movements of the heavenly
bodies. As is due to the great centre of our system, we will commence
with the SUN. But before proceeding to do so, we must say a few words
about the motion of the heavenly bodies—that is, the _apparent_ motion
of the rising and setting of the sun and stars.
The attentive observation of the starry heavens, even during a single
night, will convince us that all the visible stars describe circles
which are the smaller, the nearer the stars are to a certain point of
the heavens, P (fig. 542). In close proximity to this point there is a
tolerably bright star, called the _Pole Star_, which has scarcely any
motion, but appears to the eye as always occupying the same position.
Hence a line, PP´, drawn from this star, through the centre of the
earth, _c_, represents the axis around which all the heavenly bodies
perform their apparent motions. The part of the celestial axis, PP´,
passing through the earth, is the earth’s axis; the north pole, of
which _p_ is on the same side as the pole star, and the south pole,
_p´_, is on the opposite side.
[Illustration: Fig. 542.—Celestial axis.]
We have, therefore, by the aid of the stars, determined the position
of the earth’s axis, and by this latter we can assign to the equator
its proper place. For if _pp´_ be the earth’s axis, _aq´_ is the
greatest circle drawn round the earth, equally distant from both poles,
and the plane of which cuts the earth’s axis at right angles.
Furthermore, let us suppose the plane of the equator to be extended
till it reach the celestial concave; we thus find the place of the
celestial equator, A Q, or _equinoctial_, as it is generally termed in
opposition to the equator, which always means the terrestrial equator.
The equinoctial divides the heavens into the northern and southern
hemispheres. We cannot actually describe the equinoctial and make it
visible, but we can imagine its line of direction by observing those
stars through which it passes.
We are now in a condition to assign to an observer different stations
in relation to the earth’s axis on the earth’s surface, which will
essentially modify the aspects under which celestial phenomena are
represented. One of these stations may be supposed to be at one of the
two poles, for example, at _p_, or at any one point of the equator, as
at _a_, or, finally, on any portion of the surface of the earth which
lies between the pole and the equator, as, for example, _o_.
[Illustration: Fig 543.—Great Nebulæ in Orion.]