opposition to the Copernican theory, that the inferior planets do
not seem many times brighter when nearer the earth than when in
the most distant parts of their orbits; the explanation being, of
course, that when the planets are between the earth and the sun
only a small portion of their illumined surfaces is visible from
the earth.

On inspecting the planet Jupiter, a still more striking
revelation was made, as four tiny stars were observed to occupy
an equatorial position near that planet, and were seen, when
watched night after night, to be circling about the planet,
precisely as the moon circles about the earth. Here, obviously,
was a miniature solar system--a tangible object-lesson in the
Copernican theory. In honor of the ruling Florentine house of the
period, Galileo named these moons of Jupiter, Medicean stars.

Turning attention to the sun itself, Galileo observed on the
surface of that luminary a spot or blemish which gradually
changed its shape, suggesting that changes were taking place in
the substance of the sun--changes obviously incompatible with the
perfect condition demanded by the metaphysical theorists. But
however disquieting for the conservative, the sun's spots served
a most useful purpose in enabling Galileo to demonstrate that the
sun itself revolves on its axis, since a given spot was seen to
pass across the disk and after disappearing to reappear in due
course. The period of rotation was found to be about twenty-four
days.

It must be added that various observers disputed priority of
discovery of the sun's spots with Galileo. Unquestionably a