its axis once in twenty-four hours, or that thousands of mighty
stars should circle round the earth in the same time, many of them
having to describe circles many thousands of times greater in
circumference than the circuit of the earth at the equator? The obvious
answer pressed upon Copernicus with so much force that he was compelled
to reject Ptolemy's theory of the stationary earth, and to attribute the
diurnal rotation of the heavens to the revolution of the earth on its
axis.

Once this tremendous step had been taken, the great difficulties which
beset the monstrous conception of the celestial sphere vanished, for the
stars need no longer be regarded as situated at equal distances from the
earth. Copernicus saw that they might lie at the most varied degrees of
remoteness, some being hundreds or thousands of times farther away than
others. The complicated structure of the celestial sphere as a material
object disappeared altogether; it remained only as a geometrical
conception, whereon we find it convenient to indicate the places of the
stars. Once the Copernican doctrine had been fully set forth, it was
impossible for anyone, who had both the inclination and the capacity to
understand it, to withhold acceptance of its truth. The doctrine of a
stationary earth had gone forever.

Copernicus having established a theory of the celestial movements which
deliberately set aside the stability of the earth, it seemed natural
that he should inquire whether the doctrine of a moving earth might not
remove the difficulties presented in other celestial phenomena. It had
been universally admitted that the earth lay unsupported in space.
Copernicus had further shown that it possessed a movement of rotation.
Its want of stability being thus recognized, it seemed reasonable to
suppose that the earth might also have some other kinds of movements as
well. In this, Copernicus essayed to solve a problem far more difficult
than that which hitherto occupied his attention. It was a comparatively
easy task to show how the diurnal rising and setting could be accounted
for by the rotation of the earth. It was a much more difficult
undertaking to demonstrate that the planetary movements, which Ptolemy
had represented with so much success, could be completely explained by
the supposition that each of these planets revolved uniformly round the
sun, and that the earth was also a planet, accomplishing a complete
circuit of the sun once in the co