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How many books are included in the Almagest?

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How did Ptolemy explain retrograde motion? — Ptolemy Ptolemy's theory of the solar system.

The Almagest, a foundational astronomical manual written by the Egyptian astronomer Ptolemy around 150 ce, is divided into 13 books.12

These books provide a comprehensive guide to the geocentric model of the universe, covering topics such as trigonometry, the motions of the Sun, Moon, and planets, as well as a star catalog of 1,022 stars.1