On The Heavens
separately given special movements of its own, there is in the first
place this reason for regarding the arrangement as a natural one. In
thinking of the life and moving principle of the several heavens one
must regard the first as far superior to the others. Such a
superiority would be reasonable. For this single first motion has to
move many of the divine bodies, while the numerous other motions
move only one each, since each single planet moves with a variety of
motions. Thus, then, nature makes matters equal and establishes a
certain order, giving to the single motion many bodies and to the
single body many motions. And there is a second reason why the other
motions have each only one body, in that each of them except the last,
i.e. that which contains the one star, is really moving many bodies.
For this last sphere moves with many others, to which it is fixed,
each sphere being actually a body; so that its movement will be a
joint product. Each sphere, in fact, has its particular natural
motion, to which the general movement is, as it were, added. But the
force of any limited body is only adequate to moving a limited body.
The characteristics of the stars which move with a circular
motion, in respect of substance and shape, movement and order, have
now been sufficiently explained.
It remains to speak of the earth, of its position, of the question
whether it is at rest or in motion, and of its shape.
I. As to its position there is some difference of opinion. Most
people-all, in fact, who regard the whole heaven as finite-say it lies
at the centre. But the Italian philosophers known as Pythagoreans take
the contrary view. At the centre, they say, is fire, and the earth
is one of the stars, creating night and day by its circular motion
about the centre. They further construct another earth in opposition
to ours to which they give the name counterearth. In all this they are