OUR SOLAR SYSTEM
Now let us go upward in imagination, far, far beyond the tops of the
highest mountains, beyond the moon and sun, and outward in space until
we reach a point in the northern heavens millions and millions of miles
away, directly above and equally distant from all points in the
ecliptic, or path in which our earth travels yearly round the sun. Then
we should have that sort of comprehensive view of the solar system which
is necessary if we are to visualize as a whole the working of the vast
machine, and the motions, sizes, and distances of all the bodies that
comprise it. Of such stupendous mechanism our earth is part.
Or in lieu of this, let us attempt to get in mind a picture of the solar
system by means of Sir William Herschel's apt illustration: "Choose any
well-leveled field. 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 of 284 feet in diameter; the Earth also
a pea, on a circle of 430 feet; Mars a rather larger pin's head on a
circle of 654 feet; the asteroids, grains of sand in orbits of 1,000 to
1,200 feet; Jupiter, a moderate sized orange in a circle of nearly half
a mile across; Saturn, a small orange on a circle of four-fifths of a
mile; Uranus, a full-sized cherry or small plum upon the circumference
of a circle more than a mile and a half; and finally Neptune, a
good-sized plum on a circle about two miles and a half in diameter....
To imitate the motions of the planets in the above mentioned orbits,
Mercury must describe its own diameter in 41 seconds; Venus in 4
minutes, 14 seconds; the Earth in 7 minutes; Mars in 4 minutes 48
seconds; Jupiter in 2 minutes 56 seconds; Saturn in 3 minutes 13
seconds; Uranus in 2 minutes 16 seconds; and Neptune in 3 minutes 30
seconds."
Now, let us look earthward from our imaginary station near the north
pole of the ecliptic. All these planetary bodies would be seen to be
traveling eastward round the sun, that is, in a counter-clockwise
direction, or contrary to the motions of the hands of a timepiece. Their
orbits or paths of motion are very nearly circular, and the sun is
practically at the center of all of them except Mercury and Mars; of
Venus and Neptune, almost at the absolute center. The planes of all
their orbits are very nearly the same as that of the ecliptic, or plane
in which the earth moves. These and many other resemblances and
characteristics suggest a uniformity of origin which comports with the
idea of a family, and so the whole is spoken of as the solar system, or
the sun and his family of planets.
In addition to the nine bodies already specified, the solar system
comprises a great variety of other and lesser bodies; no less than
twenty-six moons or satellites tributary to the planets and traveling
round them in various periods as the moon does round our earth. Then
between the orbits of Mars and Jupiter are many thousands of asteroids,
so called, or minor planets (about 1,000 of them have actually been
discovered, and their paths accurately calculated). And at all sorts of
angles with the planetary orbits are the paths of hundreds of comets,
delicate filmy bodies of a wholly different constitution from the
planets, and which now and then blaze forth in the sky, their tails
appearing much like the beam of a searchlight, and compelling for the
time the attention of everybody. Connected with the comets and doubtless
originally parts of them are uncounted millions of millions of meteors,
which for the time become a part of the solar system, their minute
masses being attracted to the planets, upon which they fall, those
hitting the earth being visible to us as familiar shooting stars.
We next follow the story of astronomy through the solar system,
beginning with the sun itself and proceeding outward through his family
of planets, now much more numerous and vastly more extended than it was
to the ancient world, or indeed till within a century and a half of our
own day.