ments
were the real movements. Ptolemy himself felt the extraordinary
difficulty involved in the supposition that so stupendous a fabric as
the celestial sphere should spin in the way supposed. Such movements
required that many of the stars should travel with almost inconceivable
velocity. Copernicus also saw that the daily rising and setting of the
heavenly bodies could be accounted for either by the supposition that
the celestial sphere moved round and that the earth remained at rest, or
by the supposition that the celestial sphere was at rest while the earth
turned round in the opposite direction. He weighed the arguments on both
sides as Ptolemy had done, and as the result of his deliberation
Copernicus came to an opposite conclusion from Ptolemy. To Copernicus it
appeared that the difficulties attending the supposition that the
celestial sphere revolved were vastly greater than those which appeared
so weighty to Ptolemy as to force him to deny the earth's rotation.

Copernicus shows clearly how the observed phenomena could be accounted
for just as completely by a rotation of the earth as by a rotation of
the heavens. He alludes to the fact that, to those on board a vessel
which is moving through smooth water, the vessel itself appears to be at
rest, while the objects on shore appear to be moving past. If,
therefore, the earth were rotating uniformly, we dwellers upon the
earth, oblivious of our own movement, would wrongly attribute to the
stars the displacement which was actually the consequence of our own
motion.

Copernicus saw the futility of the arguments by which Ptolemy had
endeavored to demonstrate that a revolution of the earth was impossible.
It was plain to him that there was nothing whatever to warrant refusal
to believe in the rotation of the earth. In his clear-sightedness on
this matter we have specially to admire the sagacity of Copernicus as a
natural philosopher. It had been urged that, if the earth moved round,
its motion would not be imparted to the air, and that therefore the
earth would be uninhabitable by the terrific winds which would be the
result of our being carried through the air. Copernicus convinced
himself that this deduction was preposterous. He proved that the air
must accompany the earth, just as one's coat remains round him,
notwithstanding the fact that he is walking down the street. In this way
he was able to show that all _a priori_ objections to the earth's
movements were ab