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This article is about the geographer Ptolemy. To read about Ptolemy, Alexander the Great's general, see Ptolemy I Soter.


Ptolemy (c. 90 – c. 168 AD) was a Greek mathematician, geographer, astronomer, and astrologer.

Nothing is known about where Ptolemy was born or lived during his life. But he is known for his study of the heavens and for his maps. He was one of the first to try and make a complete description of the stars and planets. His model was geocentric and showed the other planets and stars as they appear from Earth.

This theory was popular for nearly 1500 years. It accurately predicted the motion of the planets better than any alternative. In his Algamest he disproves any alternatives provided at that time, and lays out a framework for his astronomy. It was eventually replaced by heliocentric theory. Copernicus eventually challenges the Ptolemaic model on philosophical grounds though no new data had surfaced to justify such a change. Due to the many devices that Ptolemy used to account for the phenomena, such as minor epicycles and equants, many of the Copernican diagrams of the universe in a heliocentric (sun-centered) universe seemed far simpler in comparison. However, many scholars for the next few hundred years after Copernicus held to the geocentric (earth-centered) universe due to tradition, and kept on using Ptolemaic models, though they began to accept certain pieces of Copernicus' work, particularly his mathematical tables.