As we have already seen,
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 deliberations, 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 seem to be
moving past.
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