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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

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