Jupiter, Saturn, Uranus, Neptune--Jupiter--The Satellites of
Jupiter--Saturn--The Nature of the Rings--William Herschel--The
Discovery of Uranus--The Satellites of Uranus--The Discovery of
Neptune.
Our lecture to-day ought to make us take a very humble view of the size
of our earth. Mercury, Venus, and Mars may be regarded as the earth’s
peers, though we are slightly larger than Venus, and a good deal larger
than Mercury or Mars; but all these four globes are insignificant in
comparison with the gigantic planets which lie in the outer parts of
our system. These great bodies do not enjoy the benefits of the sun to
the same extent that we are permitted to do; they are so far off that
the sun’s rays become greatly enfeebled before they can traverse the
distance; but the gloom of their situation seems to matter but little,
for it is highly improbable that any of these bodies could be inhabited.
A view of parts of the paths of these four great planets is shown
in Fig. 65. The innermost is Jupiter, which completes a circuit in
about twelve years; then comes Saturn, revolving in an orbit so great
that twenty-nine years and a half are required before the complete
journey is finished. Still further outside is Uranus, which has a
longer journey than Saturn, and moves so much more slowly that a man
would have to live to the ripe old age of eighty-four if a complete
revolution of Uranus was to be accomplished during his lifetime. At the
boundary of our system revolves the planet Neptune, and though it is a
mighty globe, yet we cannot see it without a telescope. It is invisible
to the naked eye for two reasons: first of all, because it is so far
from the sun that the light which illuminates it is excessively feeble;
and, secondly, because it is so far from us that whatever brilliancy it
has is largely reduced.
[Illustration: FIG. 65.--The Orbits of the Four Giant Planets.]