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,