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On The Heavens   



as that in which it is situated. The warmth and light which proceed

from them are caused by the friction set up in the air by their

motion. Movement tends to create fire in wood, stone, and iron; and

with even more reason should it have that effect on air, a substance

which is closer to fire than these. An example is that of missiles,

which as they move are themselves fired so strongly that leaden

balls are melted; and if they are fired the surrounding air must be

similarly affected. Now while the missiles are heated by reason of

their motion in air, which is turned into fire by the agitation

produced by their movement, the upper bodies are carried on a moving

sphere, so that, though they are not themselves fired, yet the air

underneath the sphere of the revolving body is necessarily heated by

its motion, and particularly in that part where the sun is attached to

it. Hence warmth increases as the sun gets nearer or higher or

overhead. Of the fact, then, that the stars are neither fiery nor move

in fire, enough has been said.



8



Since changes evidently occur not only in the position of the

stars but also in that of the whole heaven, there are three

possibilities. Either (1) both are at rest, or (2) both are in motion,

or (3) the one is at rest and the other in motion.

(1) That both should be at rest is impossible; for, if the earth

is at rest, the hypothesis does not account for the observations;

and we take it as granted that the earth is at rest. It remains either

that both are moved, or that the one is moved and the other at rest.

(2) On the view, first, that both are in motion, we have the

absurdity that the stars and the circles move with the same speed,

i.e. that the ace of every star is that of the circle in it moves. For

star and circle are seen to come back to the same place at the same

moment; from which it follows that the star has traversed the circle

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