Aristarchus or Aristarch (Greek: 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. 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.

The crater Aristarchus on the Moon is named in his honor