AlmagestPtolemy

Ptolemy composed the Almagest around 150 CE as a complete mathematical astronomy of the heavens as they appear from Earth. Its Greek title means a mathematical compilation; the name Almagest comes through Arabic as "the greatest." Thirteen books set out a geocentric system in which a spherical Earth sits motionless at the center of spherical heavens.

Book I states the cosmological premises and develops the trigonometry of chords required for the rest. Book II treats spherical astronomy as it varies with the observer's latitude. Book III models the Sun; Books IV and V model the Moon, including parallax and relative distances; Book VI treats eclipses. Books VII and VIII catalogue more than a thousand fixed stars and discuss the precession of the equinoxes. Books IX through XIII construct geometric models for the five planets, combining deferents, epicycles, and the equant to account for stations and retrograde motion.

The Almagest absorbed and organized earlier Greek astronomy, above all Hipparchus, and remained the standard textbook from late antiquity through the Islamic world and medieval Europe. Copernicus designed On the Revolutions of the Heavenly Spheres as its structural rival. Kepler and Galileo still had to answer it. None ignored it.