consideration that,
whatever motion may be attributed to the earth, yet we, as dwellers upon
it, and hence as participators in its motion, cannot possibly perceive
anything of it, presupposing that we are to consider only earthly
things. On the other hand, it is just as necessary that this same motion
belong apparently to all other bodies and visible objects, which, being
separated from the earth, do not take part in its motion. The correct
method to discover whether one can ascribe motion to the earth, and what
kind of motion, is, therefore, to investigate and observe whether in
bodies outside the earth a perceptible motion may be discovered which
belongs to all alike. Because a movement which is perceptible only in
the moon, for instance, and has nothing to do with Venus or Jupiter or
other stars, cannot possibly be peculiar to the earth, nor can its
seat be anywhere else than in the moon. Now there is one such universal
movement which controls all others--namely, that which the sun, moon,
the other planets, the fixed stars--in short, the whole universe, with
the single exception of the earth--appears to execute from east to west
in the space of twenty-four hours. This now, as it appears at the first
glance anyway, might just as well be a motion of the earth alone as of
all the rest of the universe with the exception of the earth, for the
same phenomena would result from either hypothesis. Beginning with the
most general, I will enumerate the reasons which seem to speak in favor
of the earth's 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