Ptolemy Writes the Almagest

Around 150 AD, the astronomer and mathematician Claudius Ptolemy, working in Alexandria in Roman Egypt, completed a great work on astronomy, known today as the Almagest, from an Arabic version of its Greek title. It brought together centuries of Greek astronomical knowledge, including the work of Hipparchus, and presented a complete mathematical model of the universe. In Ptolemy's system, the Earth sat motionless at the center, while the Sun, Moon, planets and stars moved around it. To explain the complex movements of the planets, including their apparent backward motion, Ptolemy used circles within circles called epicycles, along with other geometric devices. Although the model was wrong, it predicted the positions of the planets with impressive accuracy. The Almagest also included a catalog of over a thousand stars. It became the standard work on astronomy in the Byzantine Empire, the Islamic world and medieval Europe, where it was translated into Arabic and Latin. Its Earth-centered model dominated astronomy for about 1,400 years, until Copernicus proposed a Sun-centered system in 1543.