e, to which also we must ascribe a complete rotation in
twenty-four hours. If, however, we assume a motion of the earth, the
rapidity of the periods is very well preserved; from the slowest sphere
of Saturn we come to the wholly motionless fixed stars. We also escape
thereby a fourth difficulty, which arises as soon as we assume that
there is motion in the sphere of the stars. I mean the great unevenness
in the movement of these very stars, some of which would have to revolve
with extraordinary rapidity in immense circles, while others moved very
slowly in small circles, since some of them are at a greater, others at
a less, distance from the pole. That is likewise an inconvenience,
for, on the one hand, we see all those stars, the motion of which is
indubitable, revolve in great circles, while, on the other hand, there
seems to be little object in placing bodies, which are to move in
circles, at an enormous distance from the centre and then let them
move in very small circles. And not only are the size of the different
circles and therewith the rapidity of the movement very different in the
different fixed stars, but the same stars also change their orbits and
their rapidity of motion. Therein consists the fifth inconvenience.
Those stars, namely, which were at the equator two thousand years ago,
and hence described great circles in their revolutions, must to-day
move more slowly and in smaller circles, because they are many degrees
removed from it. It will even happen, after not so very long a time,
that one of those which have hitherto been continually in motion will
finally coincide with the pole and stand still, but after a period of
repose will again begin to move. The other stars in the mean while,
which unquestionably move, all have, as was said, a great circle for an
orbit and keep this unchangeably.

"The improbability is further increased--this may be considered the
sixth inconvenience--by the fact that it is impossible to conceive what
degree of solidity those immense spheres must have, in the depths of
which so many stars are fixed so enduringly that they are kept revolving
evenly in spite of such difference of motion without changing their
respective positions. Or if, according to the much more probable theory,
the heavens are fluid, and every star describes an orbit of its own,
according to what law then, or for what reason, are their orbits
so arranged that, when looked at from the earth, they appear to b