Ptolemy, or Claudius Ptolemaeus, (90?–168? A.D.), a Greek astronomer and geographer. Ptolemy was born at a Greek settlement in Egypt and became a scholar at Alexandria. He is primarily known for the Almagest, which is on astronomy, and the Geography. Both compilations were the most widely accepted sources of information on those subjects throughout the Middle Ages.

Ptolemy's system of astronomy—called the Ptolemaic system even though much of it was not original with him—was based on the belief held by most Greek thinkers that the universe is circular with the earth at its center. The sun, planets, and stars, they believed, revolve around the earth in circular orbits. To explain their apparent irregular motions, Ptolemy expanded on a system developed by the astronomer Hipparchus (second century B.C.); according to this system, small orbits called epicycles account for these irregularities. The system was proved to be incorrect by Copernicus in the 16th century.

The Geography, based mainly on the works of earlier writers, contained many errors and led to the acceptance of a number of false ideas about the earth.