c. 100-c. 170

Greek Astronomer and Geographer

Ptolemy is known historically through his written works. His earliest and most noted treatise is the 13-volume set commonly known as the Almagest, which he probably wrote around 150. In these books, he pinpointed the location of more than 1,000 stars, identified the so-called "classical 48" constellations, explained how to calculate latitude and longitude, and predicted solar and lunar eclipses. He also used often complicated mathematical models to help explain the movements of the various celestial bodies. The complexity, in part, derived from his belief that Earth was at the center of the universe, and all stars and planets revolved around Earth.

For example, Ptolemy developed an interesting system to explain why the planets usually, but not always, appear to move forward in their paths across the night sky. Under the erroneous assumption that the planets revolve around Earth, Ptolemy resorted to planetary movements known...