c. 310-c. 230 B.C.

Greek astronomer and mathematician who applied geometric theory to calculate the relative sizes, and distance between, the Moon and Sun. The first scientist to propose a heliocentric, or Sun-centered, model of the universe, Aristarchus has often been regarded solely as an astronomer, but in fact much of his work was in pure mathematics. With regard to his Sun–Moon measurements, he used the angle between the half-illuminated Moon and the Sun to estimate that the Sun is about 20 times as large as the Moon, and about 20 times as distant from Earth. That both estimates are too small is the fault of his measuring instruments rather than of his methodology.