cardinal doctrine of his system a stationary earth poised at the
centre of the celestial sphere, which stretched around on all sides
at a distance so vast that the diameter of the earth was an
inappreciable point in comparison therewith.

Ptolemy having thus deliberately rejected the doctrine of the earth's
rotation, had to make certain other entirely erroneous suppositions.
It was easily seen that each star required exactly the same period
for the performance of a complete revolution of the heavens. Ptolemy
knew that the stars were at enormous distances from the earth, though
no doubt his notions on this point came very far short of what we
know to be the reality. If the stars had been at very varied
distances, then it would be so wildly improbable that they should all
accomplish their revolutions in the same time, that Ptolemy came to
the conclusion that they must be all at the same distance, that is,
that they must be all on the surface of a sphere. This view, however
erroneous, was corroborated by the obvious fact that the stars in the
constellations preserved their relative places unaltered for
centuries. Thus it was that Ptolemy came to the conclusion that they
were all fixed on one spherical surface, though we are not informed
as to the material of this marvellous setting which sustained the
stars like jewels.

Nor should we hastily pronounce this doctrine to be absurd. The
stars do appear to lie on the surface of a sphere, of which the
observer is at the centre; not only is this the aspect which the
skies present to the untechnical observer, but it is the aspect in
which the skies are presented to the most experienced astronomer of
modern days. No doubt he knows well that the stars are at the most
varied distances from him; he knows that certain stars are ten times,