Almagest

The Almagest is a 2nd-century mathematical and astronomical treatise on the apparent motions of the stars and planetary paths. Written in Greek by Claudius Ptolemy, a Roman era scholar of Egypt, it is one of the most influential scientific texts of all time, with its geocentric model accepted for more than twelve hundred years from its origin in Hellenistic Alexandria, in the medieval Byzantine and Islamic worlds, and in Western Europe through the Middle Ages and early Renaissance until Copernicus.

The Almagest is a critical source of information on ancient Greek astronomy. It has also been valuable to students of mathematics because it documents the ancient Greek mathematician Hipparchus's work, which has been lost. Hipparchus wrote about trigonometry, but because his works are no longer extant, mathematicians use Ptolemy's book as their source for Hipparchus' works and ancient Greek trigonometry in general.

The treatise's conventional Greek title is Μαθηματικὴ Σύνταξις (Mathēmatikē Syntaxis), and the treatise is also known by the Latin form of this, Syntaxis mathematica. It was later titled Hē Megalē Syntaxis (The Great Treatise), and the superlative form of this (Ancient Greek: μεγίστη, "greatest") lies behind the Arabic name al-majisṭī (المجسطي), from which the English name Almagest derives.

Ptolemy set up a public inscription at Canopus, Egypt, in 147 or 148. The late N. T. Hamilton found that the version of Ptolemy's models set out in the Canopic Inscription was earlier than the version in the Almagest. Hence the Almagest cannot have been completed before about 150, a quarter century after Ptolemy began observing.