certainly did not provide any rational demonstration of the fact.
Copernicus, by a strict train of reasoning, convinced those who would
listen to him that the sun was the centre of the system. It is
useful for us to consider the arguments which he urged, and by which
he effected that intellectual revolution which is always connected
with his name.

The first of the great discoveries which Copernicus made relates to
the rotation of the earth on its axis. That general diurnal
movement, by which the stars and all other celestial bodies appear to
be carried completely round the heavens once every twenty-four hours,
had been accounted for by Ptolemy on the supposition that the
apparent movements were the real movements. 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