Around 290 B.C.E., astronomer and mathematician Aristarchus of Samos (c. 310 B.C.E.-c. 230 B.C.E.) used geometric methods to calculate the distances to and sizes of the Moon and Sun. Based on his observations and calculations, he suggested that the Sun was about 20 times as distant from the Earth as the Moon (it is actually 390 times); he also determined that the Moon’s radius was 0.5 times the radius of the Earth (it is actually 0.28 times). The numbers differ not because Aristarchus had no geometric knowledge, but because of the poor instruments used at that time.

These calculations were not the only contribution made by Aristarchus. He was also the first to propose that the Earth orbits the Sun—many centuries before Nicolaus Copernicus (see below). This concept was radical for his time, because it conflicted with geocentric religious beliefs and Aristotle’s principle that all objects move toward the center of the Earth.