The ancient astronomer Ptolemy was a Greek mathematician, astronomer, geographer, and astrologer who lived in the 2nd century CE. He is best known for his work on the Almagest, a treatise on astronomy that was used as the standard astronomical text for over 1500 years. Ptolemy's model of the universe was based on the idea that the Earth was the center of the universe, and he proposed a system of epicycles to explain the motion of the planets. His work had a major influence on later astronomers and scientists, and his ideas were accepted until the 16th century when they were replaced by the heliocentric model of Copernicus.