First in importance to us in this immense space filled with stars is
what astronomers call the Solar System, so-called because the sun is its
center. It contains the planets, eight in number, of which our earth is
one. They have been named after the ancient deities; the two interior
ones, Mercury and Venus, and the exterior ones, Mars, Jupiter, Saturn,
Uranus, and Neptune; the first three being smaller than our earth, and
the remainder a great deal larger.
Mercury and Venus are known to be _interior_ planets, that is, planets
between us and the sun, because they appear to swing on either side of
the sun. Mercury very seldom leaves the sun sufficiently to rise so
early before the sun, or set so late after him, as to be visible. Venus,
however, gets so far away as to be seen long after sunset or before
sunrise, and is called the Evening or Morning star, accordingly.
Besides the planets there are other members of the system, namely,
_comets_ and _falling stars_, which will be mentioned again more fully
hereafter. All these bodies form a sort of family, having the sun for
their head. The illustrations and drawings on separate pages give a view
of the entire system.
COMPARATIVE SIZE. The size of the planets, in general, increases with
their distance from the sun. The four composing the first group are all
comparatively small, the earth being the largest. Those of the second
group are all of great size. Jupiter, the largest, is not less than
1,390 times as large as the earth; but as it is much less dense, the
amount of matter it contains is only a trifle more than 337 times that
of the earth. All the planets together equal but one seven-hundredth
part of the mass of the sun.
The SATELLITES, except our moon, and the two satellites of Mars, belong
wholly to the second group of planets. Jupiter has eight; Saturn eight
and several revolving rings; Uranus has four, and possibly more; while
Neptune, so far as known with certainty, has but one.