Greek Astronomy by Sir Thomas L. Heath

Author:Sir Thomas L. Heath [Sir Thomas L. Heath]
Language: eng
Format: epub

Seeing that the stars on the one hand, and the whole heaven on the other, appear to change their positions, there are three possible hypotheses, namely (1) that both are at rest, (2) that both are in motion, (3) that one is at rest and the other in motion. Now it is impossible that both should be at rest, granted that the earth is at rest; for on this hypothesis the observed phenomena could not have taken place. Let us assume that the earth is at rest. Two alternatives then remain: either the stars and the heaven both move or one moves and the other is at rest. If now both move, it is absurd to suppose that the stars and the circles have the same speeds; yet each of the stars must have the same speed as the circle in which it is carried, since the stars appear to return again to the same position at the same time as the circles do, and it follows that the star has traversed its own circle, and the circle has in completing its own revolution traversed its own circumference, in the same time. But it is not reasonable that the speeds of the stars and the sizes of the circles should have the same ratio. For, while it is in no way absurd, nay, it is essential that the speeds of circles should be proportional to their sizes, it is the reverse of reasonable to expect this of the several stars in them. Suppose, on the one hand, that that which is carried in the larger circle necessarily moves more swiftly; then it is clear that, if the stars be transferred to each other’s circles, the one will be swifter, and the other slower, than before, and this would mean that they have not a movement of their own but are carried by the circles. On the other hand, if their movements had spontaneously coincided, even then it would not be reasonable to expect that greater size in the circles and greater swiftness in the revolution of the star upon it would in all cases go together; that it should be so with one or two would not be impossible, but to assume it for all stars alike is fantastic. Besides, there is in things natural no happening by chance, nor is that which applies to everything everywhere the result of chance.

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