The Greek astronomer Ptolemy (7 letters) was a mathematician, astronomer, and geographer who lived in the 2nd century CE. He is best known for his work on the geocentric model of the universe, which placed Earth at the center of the universe. He also wrote several influential works on astronomy, astrology, and geography. His most famous work is the Almagest, a treatise on the mathematical models of the motions of the stars and planets. Ptolemy's work remained influential in the Islamic world and Europe until the 16th century.