On The Heavens
happens, however, that the centre of the earth and of the whole is the
same. Thus they do move to the centre of the earth, but
accidentally, in virtue of the fact that the earth's centre lies at
the centre of the whole. That the centre of the earth is the goal of
their movement is indicated by the fact that heavy bodies moving
towards the earth do not parallel but so as to make equal angles,
and thus to a single centre, that of the earth. It is clear, then,
that the earth must be at the centre and immovable, not only for the
reasons already given, but also because heavy bodies forcibly thrown
quite straight upward return to the point from which they started,
even if they are thrown to an infinite distance. From these
considerations then it is clear that the earth does not move and
does not lie elsewhere than at the centre.
From what we have said the explanation of the earth's immobility
is also apparent. If it is the nature of earth, as observation
shows, to move from any point to the centre, as of fire contrariwise
to move from the centre to the extremity, it is impossible that any
portion of earth should move away from the centre except by
constraint. For a single thing has a single movement, and a simple
thing a simple: contrary movements cannot belong to the same thing,
and movement away from the centre is the contrary of movement to it.
If then no portion of earth can move away from the centre, obviously
still less can the earth as a whole so move. For it is the nature of
the whole to move to the point to which the part naturally moves.
Since, then, it would require a force greater than itself to move
it, it must needs stay at the centre. This view is further supported
by the contributions of mathematicians to astronomy, since the
observations made as the shapes change by which the order of the stars
is determined, are fully accounted for on the hypothesis that the
earth lies at the centre. Of the position of the earth and of the
manner of its rest or movement, our discussion may here end.
Its shape must necessarily be spherical. For every portion of