The Almagest ˈælmədʒɛst is a 2nd-century mathematical and astronomical treatise on the apparent motions of the stars and planetary paths, written by Claudius Ptolemy ( AD 100 – 170) in Koine Greek. One of the most influential scientific texts in history, it canonized a geocentric model of the Universe that was accepted for more than 1,200 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. It is also a key source of information about ancient Greek astronomy.
Ptolemy set up a public inscription at Canopus, Egypt, in 147 or 148. 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 could not have been completed before about 150, a quarter-century after Ptolemy began observing.
The name comes from Arabic اَلْمَجِسْطِيّ , with اَل al meaning "the", and magesti being a corruption of Greek μεγίστη megístē 'greatest'.
The work was originally titled "Μαθηματικὴ Σύνταξις" (Mathēmatikē Syntaxis) in Koine Greek, and also called Syntaxis Mathematica in Latin. The treatise was later titled Hē Megalē Syntaxis (Ἡ Μεγάλη Σύνταξις, "The Great Treatise"; Magna Syntaxis), and the superlative form of this (μεγίστη, megiste, "greatest") lies behind the Arabic name (المجسطي), from which the English name Almagest derives. The Arabic name is important due to the popularity of a Latin translation known as Almagestum made in the 12th century from an Arabic translation, which would endure until original Greek copies resurfaced in the 15th century.
The Syntaxis Mathematica consists of thirteen sections, called books. As with many medieval manuscripts that were handcopied or, particularly, printed in the early years of printing, there were considerable differences between various editions of the same text, as the process of transcription was highly personal.