II.

The Almagest

Ptolemy's earliest and most famous work, originally written in Greek, was translated into Arabic as al-Majisti (Great Work). In Europe, medieval Latin translations reproduced the title as Almagesti, and it has since become known simply as the Almagest. In this work, Ptolemy proposed a geometric theory to account mathematically for the apparent motions and positions of the planets, sun, and moon against a background of unmoving stars. This work did not include any physical descriptions of objects in space.

Ptolemy began by accepting the generally held theory that the earth did not move but was at the center of the universe. The planets and stars were considered, for philosophical reasons, to move continuously in perfectly circular orbits. He then elaborated on the theory in an attempt to account for the astronomical puzzles that the theory presented, such as the apparent backward motions of the planets and apparent variations in size or brightness of the moon and planets. Ptolemy proposed that the planets, sun, and moon moved in small circles around much larger circles, in which the earth was centered. In this way, he made his system fit most of the observations that astronomers had recorded.

Ptolemy used the term epicycle to describe the small circle around which he claimed objects in space move. To make his theory of epicycles appear sound, he had to introduce variations from traditional mathematics. This departure from traditional assumptions was one reason the Polish astronomer Nicolaus Copernicus rejected Ptolemy's system in the 16th century and developed his own heliocentric theory, which correctly stated that the sun was located at the center of the solar system (see Copernican System). Even so, Copernicus retained an elaborate system of epicycles.