tive evidence that the planets are dark bodies
reflecting the light of the sun, and an explanation was given of the
fact, hitherto urged in 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 sun-spot had been
seen by earlier observers, and by them mistaken for the transit of an
inferior planet. Kepler himself had made this mistake. Before the day of
the telescope, he had viewed the image of the sun as thrown on a screen
in a camera-obscura, and had observed a spot on the disk which be
interpreted as representing the planet Mercury, but which, as is now
known, must have been a sun-spot, since the planetary disk is too
small to have been revealed by this method. Such observations as these,
however interesting, cannot be claimed as discoveries of the sun-spots.
It is probable, however, that several discoverers (notably Johann
Fabricius) made the telescopic observation of the spots, and recognized
them as