c. 370-c. 310 B.C.

Greek astronomer and mathematician who used mathematical astronomy for a number of purposes, including the creation of a 76-year cycle that aligned the solar and lunar years. Later astronomers used the 940-month calendar of Callippus, who accounted for the inequality of the seasons by developing a model in which the Sun had a variable velocity at different times of the year. His 34-sphere system improved on the system of homocentric spheres proposed by his teacher Eudoxus of Cnidus (c. 408-c. 355 B.C.)