Ptolemy
(Greek in Alexandria, ~100 A.D.)

Claudius Ptolemaeus believed that the universe was geocentric (earth centered) and that it moved in a perfectly uniform circular motion. There was a problem with this uniform circular motion in that it did not coincide with the motions of the planets in the sky. The planets, when observed from the naked eye, appear to move at different speeds at different times and sometimes they appear to completly stop and then move backward tracing a retrograde loop.

Ptolemy built a model which was basically a system of wheels within wheels. He placed the planets on a small circle called an epicycle. The center of this epicycle, in turn, moved along a much larger circle around the earth called the deferent. By placing the earth and the equant on either side of the deferent's center, Ptolemy felt that he had predicted the positions of the planets.

Ptolemy put his work together in a book he called, Mathematical Syntaxis. This book after many translations by various nationalities became what is now known as the Almagest.

When Ptolemy died, the classical form of astonomy ended. However his model of the universe worked well for hundreds of years but the errors did accumulate. Arabian and European astronomers began updating and revising the Ptolemaic System. These revisions eventually led to the Alfonsine Tables, the final revision of Ptolemy's system.

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