Aristarchus was a Greek astronomer and mathematician who lived from 310 B.C. until 230 B.C. He is perhaps best known for advancing the idea that the Sun, not the Earth, was at the center of known universe.
In the ancient world, Aristarchus' idea of a heliocentric solar system was often rejected. The geocentric theories put forward by Aristotle and Ptolemy were considered the correct model of the universe until almost 1800 years later. Only one astronomer in the ancient world, Seleucus of Seleucia, is known to have supported the heliocentric model of the universe Aristarchus proposed.
Only one work credited to Aristarchus has survived until today. It is called "On the Sizes and Distances of the Sun and Moon." In this short treatise, he shows that the Sun and stars are very far away from the Earth. He also tried to determine the size of both the Sun and Moon. His calculations showed that the Sun was 19 times larger than the Moon and, because the two bodies have the same angular size, that the Sun was 19 times further away from Earth than the Moon. While his reasoning was correct, his calculations about the sizes and distances of the bodies were incorrect.
The peak in the center of a crater on the Moon is named for Aristarchus. It is the brightest formation on the Moon.