# Aristarchus of Samos

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Aristarchus (Greek: Ἀρίσταρχος, Arístarchos; 310 BC – ca. 230 BC) was a Greek astronomer and mathematician, born on the island of Samos, in Greece. He presented the first known 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 Croton, but, in contrast to Philolaus, he identified the "central fire" with the Sun, and put the other planets in their correct order of position around the Sun.[1] His astronomical ideas were often 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.

## Contents

### Heliocentrism

The only surviving work usually attributed to Aristarchus, On the Sizes and Distances of the Sun and Moon, is based on a geocentric world view. It states that the angle subtended by the Sun's diameter is 2 degrees: the correct value is about ½ degree. The latter diameter is known from Archimedes to have been Aristarchus's actual value[clarification needed].

Though the original text has been lost, a reference in Archimedes' book The Sand Reckoner (Archimedis Syracusani Arenarius & Dimensio Circuli) describes another work by Aristarchus in which he advanced the heliocentric model as an alternative hypothesis. Archimedes wrote:

You (King Gelon) are aware the 'universe' is the name given by most astronomers to the sphere the center of which is the center of the Earth, while its radius is equal to the straight line between the center of the Sun and the center of the Earth. This is the common account as you have heard from astronomers. But Aristarchus has brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the 'universe' just mentioned. His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the circumference of a circle, the Sun lying in the middle of the Floor, and that the sphere of the fixed stars, situated about the same center as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface.[2]