Aristarchus of Samos, a Greek astronomer and mathematician living in the third century BCE, developed several important theories about the universe. He is best known for promoting the idea that the Earth revolves around the sun. This hypothesis, which is known as the heliocentric theory, accurately describes our solar system as it is understood today. In Aristarchus's time, however, the heliocentric theory was rejected by most other astronomers. At the time, most astronomers did not believe that the Earth revolved around the sun. They supported the more familiar geocentric theory, which asserts that the sun revolves around the Earth. Aristarchus's heliocentric model would not gain popularity for another seventeen hundred years, when it was revived by famed Polish astronomer Nicolaus Copernicus.