PTOLEMY (c. 100–170), Alexandrian astronomer, geographer, and mathematician. The last of the great astronomers of antiquity, Claudius Ptolemaeus (Ptolemy) compiled works that remained the standard astronomical textbooks until the Copernican revolution in astronomy in the sixteenth century. Almost nothing is known of the details of Ptolemy's life. His Hē mathēmatikē syntaxis (Mathematical Compilation) was written about 150 CE; the title by which this work is better known, the Almagest, is a medieval Latin derivation from an Arabic corruption of the Greek title under which the work came to be known in later antiquity, Ho megale syntaxis (The Great Compilation). The Almagest sums up the mathematical astronomy of the ancient world; it became the basis of Latin and Arabic astronomy.

Ptolemy's work follows in the Greek philosophical tradition, in which the sacred nature of the heavens is expressed by the incorruptibility of the celestial realm, the divinity of the heavenly...