motion. When we merely consider the immensity of the starry
sphere in comparison with the smallness of the terrestrial ball,
which is contained many million times in the former, and then
think of the rapidity of the motion which completes a whole
rotation in one day and night, I cannot persuade myself how any
one can hold it to be more reasonable and credible that it is the
heavenly sphere which rotates, while the earth stands still.

"Simplicio. I do not well understand how that powerful motion may
be said to as good as not exist for the sun, the moon, the other
planets, and the innumerable host of fixed stars. Do you call
that nothing when the sun goes from one meridian to another,
rises up over this horizon and sinks behind that one, brings now
day, and now night; when the moon goes through similar changes,
and the other planets and fixed stars in the same way?

"Salviati. All the changes you mention are such only in respect
to the earth. To convince yourself of it, only imagine the earth
out of existence. There would then be no rising and setting of
the sun or of the moon, no horizon, no meridian, no day, no
night--in short, the said motion causes no change of any sort in
the relation of the sun to the moon or to any of the other
heavenly bodies, be they planets or fixed stars. All changes are
rather in respect to the earth; they may all be reduced to the
simple fact that the sun is first visible in China, then in
Persia, afterwards in Egypt, Greece, France, Spain, America,
etc., and that the same thing happens with the moon and the other
heavenly bodies. Exactly the same thing happens and in exactly
the same way if, instead of disturbing so large a part of the
universe, you let the earth revolve about itself. The difficulty