(Οἰνοπίδης), a distinguished astronomer and mathematician, a native of Chios. Plato (Erastae, 100.1) mentions him in conjunction with Anaxagoras, from which it has been concluded that he was a contemporary of the latter. It may have been so, but there is nothing else to confirm the conjecture. He is spoken of in connection with Pythagoras and his followers, so that he seems to have been regarded as a Pythagorean. Oenopides derived most of his astronomical knowledge from the priests and astronomers of Egypt, with whom he lived for some time. Diodorus (1.98) mentions in particular that he derived from this source his knowledge of the obliquity of the ecliptic, the discovery of which he is said to have claimed (in the treatise de Plac. Phil. 2.12, ascribed to Plutarch). Aelian (Ael. VH 10.7) attributes to Oenopides the invention of the cycle of fifty-nine years for bringing the lunar and solar years into accordance, of which Censorinus (100.19) makes Philolaus to have been the originator. The length of the solar year was fixed by Oenopides at 365 days, and somewhat less than nine hours. (As Censorinus expresses it, the fifty-ninth part of twenty-two days.) Oenopides set up at Olympia a brazen tablet containing an explanation of his cycle. He had a notion that the milky-way was the original path of the sun, from which he had been frightened into his present path by the spectacle of the banquet of Thyestes. (Achilles Tatius, Isag. in Arat. 100.24.) Proclus, in his commentary on Euclid, attributes to Oenopides the discovery of the twelfth and twentythird propositions of the first book of Euclid, and the quadrature of the meniscus. Oenopides is also mentioned more than once by Sextus Empiricus. (Hypot. 3.4, ad v. Math. p. 367.) He had a theory of his own about the rise of the Nile, which was this, that in the summer the waters beneath the earth are cold, in the winter warm; a fact which he said was proved by the temperature of deep wells. So that in the winter the heat shut up in the earth carries off the greater part of the moisture, while there are no rains in Egypt. In the summer, on the contrary, the moisture is no longer carried off in that way, so that there is enough to fill the bed of the Nile and cause it to overflow. Diodorus (1.41) objects to that theory, that other rivers of Libya, which correspond in position and direction to the Nile, are not so affected. (Fabric. Bibl. Graec. vol. i. p. 860; Ideler, Handbuch der Chronologie, vol. i. p. 302.)