What Did Ptolemy Discover?

While Greek mathematician and astronomy Claudius Ptolemy's geocentric theory was incorrect, he was able to explain the motions of heavenly bodies, according to Iowa State University's Polaris Project. The Central Rappahannock Regional Library also says Ptolemy used geometry to predict the movement of the planets (though he believed they revolved around the Earth). He cataloged 1,022 stars. His book "The Almagest" was the cornerstone of astronomy for many centuries.

His geocentric, or Earth-centered, idea was termed the Ptolemic system. For many centuries after Ptolemy, astronomers accepted his flawed theory as correct. While his work came from his empirical observations, perhaps his greatest contribution to astronomy was his summary of knowledge of the heavens. His book "The Mathematical Collection" displayed his astronomical work and eventually became known as "The Great Astronomer." This book, divided into 13 parts, was also the source of knowledge for another Greek astronomer named Hipparchus. He said the planets must move in epicycles, or smaller circles, and that the Earth moved along an equant. The equant was necessary for explaining the retrograde motion of the nearby planets. Ptolemy's theory was accepted proof until Polish astronomer Nicolaus Copernicus debuted his heliocentric (sun centered) theory in the 15th century.