Aristarchus of Samos

Aristarchus was a third century BCE Greek philosopher. He first proposed that the planets of the Solar system orbit the Sun.

Aristarchus tried to measure the sizes of the Moon and the Sun using geometric calculations. Though his measuring instruments were unable to provide accurate results (he determined the angle between the Sun and a half-Moon to be 87 degrees; the actual angle is ~89.833 degrees), he still figured out that the sun is very much larger and further from Earth than the moon, and hypothesized that the smaller Earth orbits the larger Sun. Aristarchus is our only ancient reference to heliocentrism, which we know about because of a vague reference in Archimedes. He says: "Aristarchus of Samos brought out a book consisting of some hypotheses, ... His hypotheses are that the fixed stars and the sun remain unmoved, that the earth revolves about the sun in the circumference of a circle, the sun lying in the middle of the orbit . That book is lost, and how he came to those hypotheses remains unknown.

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Aristarchus's hypothesis is the first known challenge to the dominant geocentric model, though it ultimately failed to convince many. One of Aristarchus' opponents was the Stoic philosopher Cleanthes, who regarded the Sun as a divine entity. At some point, Aristarchus claimed that Cleanthes should be indicted for impiety, though there is a common myth that it was Aristarchus being accused by Cleanthes (which arises from a 17th-century translation of Plutarch, where the translator reversed the subject and object of the sentence; older Latin manuscripts have Aristarchus stating that Cleanthes ought to be indicted).

Aristarchus had a small following including but the theory of Aristarchus remained unpopular. Instead, Aristotelian and later Hipparchan/Ptolemaic geocentric ideas dominated science until the second half of the 17th century.

In 1514, nearly two millennia later, Copernicus took up heliocentrism, originally citing Aristarchus. The discoveries of Galileo firmly discredited the Ptolemaic system, Kepler later developed the heliocentric model that accurately describes our solar system, while Newton put it on a physically justified basis at the end of the 17th century.

Today some recognize Aristarchus as a pioneer in astronomy though later heliocentrists are much better known. Greece named an astronomical telescope after Aristarchus. , and one of the most prominent in the Moon was named after him.