er--Jupiter not Incandescent--The Satellites--Their
Discovery--Telescopic Appearance--Their Orbits--The Eclipses and
Occultations--A Satellite in Transit--The Velocity of Light
Discovered--How is this Velocity to be Measured
Experimentally?--Determination of the Sun's Distance by the
Eclipses of Jupiter's Satellites--Jupiter's Satellites
demonstrating the Copernican System.

In our exploration of the beautiful series of bodies which form the
solar system, we have proceeded step by step outwards from the sun. In
the pursuit of this method we have now come to the splendid planet
Jupiter, which wends its majestic way in a path immediately outside
those orbits of the minor planets which we have just been considering.
Great, indeed, is the contrast between these tiny globes and the
stupendous globe of Jupiter. Had we adopted a somewhat different method
of treatment--had we, for instance, discussed the various bodies of our
planetary system in the order of their magnitude--then the minor planets
would have been the last to be considered, while the leader of the host
would be Jupiter. To this position Jupiter is entitled without an
approach to rivalry. The next greatest on the list, the beautiful and
interesting Saturn, comes a long distance behind. Another great descent
in the scale of magnitude has to be made before we reach Uranus and
Neptune, while still another step downwards must be made before we
reach that lesser group of planets which includes our earth. So
conspicuously does Jupiter tower over the rest, that even if Saturn were
to be augmented by all the other globes of our system rolled into one,
the united mass would still not equal the great globe of Jupiter.

[Illustration: Fig. 56.--The Relative Dimensions of Jupiter and the
Earth.]

The adjoining picture (Fig. 56) shows the relative dimensions of Jupiter
and the earth, and it conveys to the eye a more vivid impression of the
enormous bulk of Jupiter than we can readily obtain by merely
considering the numerical statements by which his bulk is to be
accurately estimated. As, however, it will be necessary to place the
numerical facts before our readers, we do so at the outset of this
chapter.

Jupiter revolves in an elliptic orbit around the sun in the focus, at a
mean distance of 483,000,000 miles. The path of Jupiter is thus about
5.2 times as great in diameter as the path pursued by the earth. The
shape of Jupiter's orbit dep