was a
fixed body; it possessed neither rotation round an axis nor translation
through space, but remained constantly at rest in what he supposed to be
the centre of the universe. According to Ptolemy's theory the sun and
the moon moved in circular orbits around the earth in the centre. The
explanation of the movements of the planets he found to be more
complicated, because it was necessary to account for the fact that a
planet sometimes advanced and that it sometimes retrograded. The ancient
geometers refused to believe that any movement, except revolution in a
circle, was possible for a celestial body: accordingly a contrivance was
devised by which each planet was supposed to revolve in a circle, of
which the centre described another circle around the earth.

Although the Ptolemaic doctrine is now known to be framed on quite an
extravagant estimate of the importance of the earth in the scheme of the
heavens, yet it must be admitted that the apparent movements of the
celestial bodies can be thus accounted for with considerable accuracy.
This theory is described in the great work known as the "Almagest,"
which was written in the second century of our era, and was regarded for
fourteen centuries as the final authority on all questions of astronomy.

Such was the system of Astronomy which prevailed during the Middle Ages,
and was only discredited at an epoch nearly simultaneous with that of
the discovery of the New World by Columbus. The true arrangement of the
solar system was then expounded by Copernicus in the great work to which
he devoted his life. The first principle established by these labours
showed the diurnal movement of the heavens to be due to the rotation of
the earth on its axis. Copernicus pointed out the fundamental difference
between real motions and apparent motions; he proved that the
appearances presented in the daily rising and setting of the sun and the
stars could be accounted for by the supposition that the earth rotated,
just as satisfactorily as by the more cumbrous supposition of Ptolemy.
He showed, moreover, that the latter supposition must attribute an
almost infinite velocity to the stars, so that the rotation of the
entire universe around the earth was clearly a preposterous supposition.
The second great principle, which has conferred immortal glory on
Copernicus, assigned to the earth its true position in the universe.
Copernicus transferred the centre, about which all the planets revolve,