An Objection to the Theory of Concentric Circles

Sosigenes in Simplicius, Commentary on Aristotle’s De Caelo, pp. 504-505 (Heiburg)



            Nevertheless the theories of Eudoxus and his followers fail to save the phenomena, and not only those which were first noticed at a later date, but even those which were before known and actually accepted by the authors themselves. What need is there for me to mention the generality of these, some of which, after Eudoxus had failed to account for them, Callippus tried to save – if indeed we can regard him as so far successful? I confine myself to one fact which is actually evident to the eye; this fact no one before Autolycus of Pitane even tried to explain by means of hypotheses, and not even Autolycus was able to do so, as clearly appears from his controversy with Aristotherus. I refer to the fact that the planets appear at times to be near to us and at times to have receded. This is indeed obvious to our eyes in the case of some of them; for the star called after Aphrodite and also the star of Ares seem, in the middle of their retrogradations, to be many times as large, so much so that the star of Aphrodite actually makes bodies cast shadows on moonless nights. The moon also, even in the perception of our eye, is clearly not always at the same distance from us, because it does not always seem to be the same size under the same conditions as to medium. The same fact is moreover confirmed if we observe the moon by means of an instrument; for it is at one time a disc of eleven fingerbreadths, and again at another time a disc of twelve fingerbreadths, which when placed at the same distance from the observer hides the moon (exactly) so that his eye does not see it. In addition to this, there is evidence for the truth of what I have stated in the observed facts with regard to total eclipses of the sun; for when the centre of the sun, the centre of the moon, and our eye happen to be in a straight line, what is seen is not always alike; but at one time the cone which comprehends the moon and has its vertex at our eye comprehends the sun itself at the same time, and the sun even remains invisible to us for a certain time, while again at another time this is so far from being the case that a rim of a certain breadth on the outside edge is left visible all round it at the middle of the duration of the eclipse. Hence we must conclude that the apparent difference in the size of the two bodies observed under the same atmospheric conditions is due to the inequality of their distances (at different times)… But indeed this inequality in the distances of each star at different times cannot even be said to have been unknown to the authors of the concentric theory themselves. For Polemarchus of Cyzicus appears to be aware of it, but to minimize it as being imperceptible, because he preferred the theory which placed the spheres themselves about the very centre in the universe. Aristotle too, shows that he is conscious of it when, in the Physical Problems, he discusses objections to the hypotheses of astronomers arising from the fact that even the sizes of the planets do not appear to be the same always. In this respect Aristotle was not altogether satisfied with the revolving spheres, although the supposition that, being concentric with the universe, they move about its centre, attracted him. Again, it is clear from what he says in Book Lambda of the Metaphysics that he thought that the facts about the movements of the planets had not been sufficiently explained by the astronomers who came before him or were contemporary with him.