analogous character, which is thus expressed
(the mean longitude being the angle between a fixed line and the radius
to the mean place of the satellite): If to the mean longitude of the
first satellite we add twice the mean longitude of the third, and
subtract three times the mean longitude of the second, the difference is
always 180 deg..

It was from observation that these principles were first discovered.
Laplace, however, showed that if the satellites revolved nearly in this
way, then their mutual perturbations, in accordance with the law of
gravitation, would preserve them in this relative position for ever.

We shall conclude with the remark, that the discovery of Jupiter's
satellites afforded the great confirmation of the Copernican theory.
Copernicus had asked the world to believe that our sun was a great
globe, and that the earth and all the other planets were small bodies
revolving round the great one. This doctrine, so repugnant to the
theories previously held, and to the immediate evidence of our senses,
could only be established by a refined course of reasoning. The
discovery of Jupiter's satellites was very opportune. Here we had an
exquisite ocular demonstration of a system, though, of course, on a much
smaller scale, precisely identical with that which Copernicus had
proposed. The astronomer who had watched Jupiter's moons circling around
their primary, who had noticed their eclipses and all the interesting
phenomena attendant on them, saw before his eyes, in a manner wholly
unmistakable, that the great planet controlled these small bodies, and
forced them to revolve around him, and thus exhibited a miniature of the
great solar system itself. "As in the case of the spots on the sun,
Galileo's announcement of this discovery was received with incredulity
by those philosophers of the day who believed that everything in nature
was described in the writings of Aristotle. One eminent astronomer,
Clavius, said that to see the satellites one must have a telescope which
would produce them; but he changed his mind as soon as he saw them
himself. Another philosopher, more prudent, refused to put his eye to
the telescope lest he should see them and be convinced. He died shortly
afterwards. 'I hope,' said the caustic Galileo, 'that he saw them while
on his way to heaven'"[24]

CHAPTER XIII.

SATURN.

The Position of Saturn in the System--Saturn one of the Three most
Interesting Objects in the