Greek philosopher who theorized the radical view that Earth and planets revolve around the Sun. This model was too revolutionary to be accepted by his contemporaries who debunked the theory because it conflicted with geocentric religious principles, as well as Aristotle's principle that all objects move toward the center of the Earth. Aristarchus' model also predicted stellar parallax and seemed to imply that falling bodies would be swept westward, neither of which were observed. Aristarchus explained the lack of observed parallax by postulating that the stars were infinitely far away.

Aristarchus also applied modern geometric methods to measuring the size of celestial bodies. From a lunar eclipse, he concluded that the radius of the Moon was 0.5 times the radius of the Earth (actually 0.28 times). He measured the Moon's angular diameter to be 2° (later 0.5°) and calculated the Earth -Moon distance to be 114.6 Earth radii (actually 60.4). By noticing that the Sun and Moon have equal angular diameters during a solar eclipse, he calculated that the distance to the Sun was 19.1 times the distance to the Moon (actually 390 times). Although his angular measurements were inaccurate, his geometric procedure was correct.

Additional biographies: Bonn, Greek and Roman Science and Technology