Aristarchus (Ἀρίσταρχος; 310 BC - ca. 230 BC) was a Greek astronomer and mathematician, born on the island of Samos, in Greece. He was the first person to present an explicit argument for a heliocentric model of the solar system, placing the Sun, not the Earth, at the center of the known universe (hence he is sometimes known as the "Greek Copernicus"). He was influenced by the Pythagorean Philolaus of Kroton, but in contrast to Philolaus he had both identified the central fire with the Sun, as well as putting other planets in correct order from the Sun. His astronomical ideas were rejected in favor of the geocentric theories of Aristotle and Ptolemy until they were successfully revived nearly 1800 years later by Copernicus and extensively developed and built upon by Johannes Kepler and Isaac Newton.
Though the original text has been lost, a reference in Archimedes' book The Sand Reckoner describes another work by Aristarchus in which he advanced an alternative hypothesis of the heliocentric model. Archimedes wrote: (translated into English)
Aristarchus thus believed the stars to be very far away, and saw this as the reason why there was no visible parallax, that is, an observed movement of the stars relative to each other as the Earth moved around the Sun. The stars are in fact much farther away than the distance that was generally assumed in ancient times, which is why stellar parallax is only detectable with telescopes. The geocentric model, consistent with planetary parallax, was assumed to be an explanation for the unobservability of the parallel phenomenon, stellar parallax. The rejection of the heliocentric view was common, as the following passage from Plutarch suggests (On the Apparent Face in the Orb of the Moon):
The only other astronomer from antiquity who is known by name and who is known to have supported Aristarchus' heliocentric model was Seleucus of Seleucia, a Mesopotamian astronomer who lived a century after Aristarchus.
Aristarchus claimed that at half moon (first or last quarter moon), the angle between sun and moon was 87°. Possibly he proposed 87° as a lower bound since gauging the lunar terminator's deviation from linearity to 1° accuracy is beyond the unaided human eye's ocular limit, (that limit being about 3° accuracy). Aristarchus is known to have also studied light and vision.
Using correct geometry, but the insufficiently accurate 87° datum, Aristarchus concluded that the Sun was 19 times farther away than the Moon. (The true value of this angle is close to 89° 50', and the Sun's distance is actually about 390 times the Moon's.) The implicit false solar parallax of slightly under 3' was used by astronomers up to and including Tycho Brahe, ca. 1600 A. D. Aristarchus pointed out that the Moon and Sun have nearly equal apparent angular sizes and therefore their diameters must be in proportion to their distances from Earth. He thus concluded that the diameter of the Sun was about 20 times larger than the diameter of the Moon; which, although wrong, follows logically from his data. It also leads to the conclusion that the Sun's diameter is almost seven times greater than the Earth's; the volume of Aristarchus's Sun would be almost 300 times greater than the Earth. Perhaps this difference in sizes inspired the heliocentric model.
Aristarchus's lunar conception represents an advance of science in several respects. Previous estimates of the length of the month were in error by 114 seconds (Meton, 432 B. C.) and 22 seconds (Callippus, 330 B. C.). The attribution of a mean motion to so complex a motion as the moon's was possibly new.