Aristarchos (310 BC - 230 BC) was a mathematician, philosopher and astronomer sometimes called the Greek Copernicus. He was born in the island of Samos and was the first person known to have proposed a heliocentric model of the solar system, a hypothesis violently rejected by colleagues and fellow citizens since it was displacing the Earth from the center of the universe (geocentric model). The latter was largely affected by the ideas of Aristotle and Ptolemy. Aristarchos model was put aside for almost 18 centuries for many reasons (Big Bang: The Origin of the Universe, Simon Singh, Fourth Estate, 2005) when it was finally revived, developed and fully confirmed by Copernicus and Brahe.

Aristarchos was a copious writer but only one of his works has survived and this is based on a geocentric view of the Cosmos. The title of his treatise was "On the sizes and distances of the Sun and the Moon" and it was published in Oxford in 1688. Using correct geometric arguments in there, he deduced that the Moon's diameter was one third that of the Earth and the Sun's 20 times that of the Moon, since their apparent angular sizes seen from the Earth were just the same. However correct his reasoning may have been, his findings were erroneously computed due to lack of accurate observations. Following Eratosthenes calculations about the circumference of the Earth, he concluded that the Moon's circumference was 14000 km, its true value being only 11000 km approximately. The fact that the Sun is larger than the Earth was taken as a mere indication of a Sun-centred model.

The work "On a system of the Cosmos" was ascribed to "Aristarchos", the issue not yet fully examined. We know via citations in the writings of other scientists that he presented another book in favour of a heliocentric universe, though little evidence can be found about the origin of his ideas on such a model.