Ptolemy

The last great astronomical figure in Greek antiquity was Claudius Ptolemy who lived in Alexandria about AD 100-178. He collected the astronomy of the time into his book best known by its later Arabic name Almagest (The Great Book). Adding their own elaborations, Islamic astronomers preserved this work through the Middle Ages, until the time was ripe in Europe for a new start in astronomy. Translations were made from Arabic to Latin, with no translation from the Greek until the fifteenth century.

Ptolemy developed the epicyclic theory. Hipparchus had added to the model the eccentric circle: the epicycle moves uniformly along the large deferent circle, whose center is somewhat off-side of the center of the Earth. With this invention he could quite accurately describe the observed variable speed of the Sun during its yearly path. Ptolemy made another innovation: the equant, a point inside the eccentric circle. The epicycle center is required to move along the eccentric circle with a variable speed so that when looked at from the equant point, the apparent angular speed is constant. This trick further improved the ability of the model to describe planetary motions. However, it meant the abandonment of the traditional uniform circular motion. Later Copernicus, otherwise a great admirer of Ptolemy, could not accept the equant and remained faithful to the idea of uniform circular motion.

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