Ptolemy's geocentric model was the dominant astronomical system in Europe before Copernicus. Claudius Ptolemy, a Greek-speaking astronomer and mathematician working in Roman Egypt, described his system in the Almagest around 150 CE.
In the Ptolemaic model, Earth remained stationary at the center while the Sun, Moon, planets, and stars moved around it. The system used deferents and epicycles—large and small circular motions—to account for observations such as planets temporarily appearing to move backward against the stars. Its mathematical complexity allowed astronomers to predict planetary positions with useful accuracy.
Ptolemy built on earlier Greek ideas, especially the work associated with Aristotle and other geocentric thinkers. This is why Aristotle may sometimes appear in discussions of ancient geocentrism, but the specific, influential mathematical model that Copernicus challenged was Ptolemy's. Copernicus's 1543 work replaced Earth's central role with a Sun-centered arrangement, beginning a major shift in astronomy.
Ptolemy's system remained influential for roughly 1,400 years. Later observations by Galileo, Kepler's laws, and Newtonian gravity eventually established the modern heliocentric framework.