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On The Heavens   


whatever size, moves towards the centre. Clearly it will not stop when

its edge touches the centre. The greater quantity must prevail until

the body's centre occupies the centre. For that is the goal of its

impulse. Now it makes no difference whether we apply this to a clod or

common fragment of earth or to the earth as a whole. The fact

indicated does not depend upon degrees of size but applies universally

to everything that has the centripetal impulse. Therefore earth in

motion, whether in a mass or in fragments, necessarily continues to

move until it occupies the centre equally every way, the less being

forced to equalize itself by the greater owing to the forward drive of

the impulse.

If the earth was generated, then, it must have been formed in this

way, and so clearly its generation was spherical; and if it is

ungenerated and has remained so always, its character must be that

which the initial generation, if it had occurred, would have given it.

But the spherical shape, necessitated by this argument, follows also

from the fact that the motions of heavy bodies always make equal

angles, and are not parallel. This would be the natural form of

movement towards what is naturally spherical. Either then the earth is

spherical or it is at least naturally spherical. And it is right to

call anything that which nature intends it to be, and which belongs to

it, rather than that which it is by constraint and contrary to nature.

The evidence of the senses further corroborates this. How else would

eclipses of the moon show segments shaped as we see them? As it is,

the shapes which the moon itself each month shows are of every kind

straight, gibbous, and concave-but in eclipses the outline is always

curved: and, since it is the interposition of the earth that makes the

eclipse, the form of this line will be caused by the form of the

earth's surface, which is therefore spherical. Again, our observations

of the stars make it evident, not only that the earth is circular, but

also that it is a circle of no great size. For quite a small change of

position to south or north causes a manifest alteration of the

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