490?-420? B.C.

Greek mathematician and astronomer credited by some sources with discovering that the Sun moves in an oblique circle from east to west (the obliquity of the zodiac). He also calculated the length of the Great Year of the Sun and Moon, the time it takes for them to return to the same relative position in the sky. He speculated, incorrectly, that the cause of the Nile river floods was underground temperature changes, and may have written on geometrical constructions.