# Aristarchus of Samos

Updated on January 2, 2010

Aristarchus of Samos, Greek astronomer and mathematician. Born about 310 B.C. Died about 250 B.C.

He was the chief exponent of the heliocentric theory of the universe- that the sun stands still and the earth and the rest of the universe revolve around it. Because of this belief he was accused of impiety.

He is supposed to have written extensively on astronomy, but all that survives is a short essay on the sizes of the sun and the moon and his estimates of their distances from each other and from the earth. He was the first to attempt to work out this problem by trigonometry. Though his theory was sound, his estimates were inaccurate because he did not have accurate instruments.

Aristarchus was the first man to propose a heliocentric theory of the universe. By geometrical calculations based on astronomical observations, he concluded that the sun is considerably larger than the earth. Aristarchus reasoned that the sun, as the larger body, must be in a fixed position and that the earth must revolve around it once a year. He realized that the changes of seasons are caused by the tilt of the earth's axis with respect to the plane in which the earth revolves around the sun. He also adopted the theory, held by some philosophers of the 4th century B.C., that day and night are caused by the earth's daily rotation on its axis.

Aristarchus was one of the few ancient astronomers to compute the dimensions of the universe. He held that the universe was much larger than his predeces sors had imagined, and that the stars were immeasurably more distant than the known planets. He calculated that the diameter of the sun was about seven times larger than that of the earth and that the sun was 18 to 20 times as distant as the moon. (The diameter of the sun is actually more than 100 times larger than that of the earth, and the sun is about 360 times more distant than the moon.) His errors in calculation were probably due to the crudity of his instruments. None of the writings of Aristarchus have survived except the short essay On the Sizes and Distances of the Sun and Moon.