It served as the basic guide for Islamic and European astronomers until about the beginning of the 17th century.
It came from a hybrid of Arabic and Greek ("the greatest"); however, Ptolemy's name for it was Mathematike Syntaxis, "The Mathematical Collection" because he believed that its subject, the motions of the heavenly bodies, could be explained in mathematical terms.
The opening chapters present empirical arguments for the basic cosmological framework within which Ptolemy worked. The earth, he argued, is a stationary sphere at the center of a vastly larger celestial sphere that revolves at a perfectly uniform rate around the earth, carrying with it the stars, planets, sun, and moon; thereby, causing their daily risings and settings.
Through the course of a year the sun slowly traces out a great circle, known as the ecliptic, against the rotation of the celestial sphere.
The moon and planets similarly travel backward; hence, the planets were also known as "wandering stars" against the "fixed stars" found in the ecliptic.
The fundamental assumption of the Almagest is that the apparently irregular movements of the heavenly bodies are in reality combinations of regular, uniform, and circular motions.
The Almagest arose as an Arabic corruption of the Greek word for greatest (megiste). It was translated into Arabic about 827 and then from Arabic to Latin in the last half of the 12th century.
Subsequently, the Greek text was circulated widely in Europe, although the Latin translations from Arabic continued to be more influential.