Ptolemy, the ancient Greek astronomer and mathematician, is best known for his geocentric model of the universe, which he presented in his work, the Almagest. This model proposed that the Earth was at the center of the universe and that all celestial bodies, including the Sun, Moon, and planets, revolved around it.
While the geocentric model was not a new idea (it had roots in earlier Greek astronomy), Ptolemy's contribution was significant because of his detailed and systematic approach to celestial motion, incorporating observations and developing a mathematical framework using epicycles (small circles along which planets move while simultaneously orbiting around a larger circle) to explain the apparent retrograde motion of planets.
Ptolemy's work shaped astronomical thought for over a thousand years until the Copernican revolution in the 16th century, which introduced the heliocentric model with the Sun at the center.