[11] We have little definite knowledge of his life. He was born in the earlier part of the 6th century B.C., and died at the end of the same century or beginning of the next.
[12] Theophrastus was born about half a century, Plutarch nearly five centuries, later than Plato.
[13] Republic, VII. 529, 530.
[14] Confused, because the mechanical knowledge of the time was quite unequal to giving any explanation of the way in which these spheres acted on one another.
[15] I have introduced here the familiar explanation of the phases of the moon, and the argument based on it for the spherical shape of the moon, because, although probably known before Aristotle, there is, as far as I know, no clear and definite statement of the matter in any earlier writer, and after his time it becomes an accepted part of Greek elementary astronomy. It may be noticed that the explanation is unaffected either by the question of the rotation of the earth or by that of its motion round the sun.
[16] See, for example, the account of Galilei’s controversies, in chapter VI.
[17] The poles of a great circle on a sphere are the ends of a diameter perpendicular to the plane of the great circle. Every point on the great circle is at the same distance, 90°, from each pole.
[18] The word “zenith” is Arabic, not Greek: cf. chapter III., § 64.
[19] Most of these names are not Greek, but of later origin.
[20] That of M. Paul Tannery: Recherches sur l’Histoire de l’Astronomie Ancienne, chap. V.