Ptolemaic system: Quick Facts

Written and fact-checked by

The Information Architects of Encyclopaedia Britannica

Photos and Videos

Diagram illustrating Ptolemy's equant model, showing a stationary Earth at the center with a planet moving in a small circle called an epicycle, which itself moves along a larger circle, the deferent. The equant point is marked off-center from Earth, and the positions of apogee and perigee are labeled. Small stars are scattered in the background.

Geocentric system

How did Ptolemy explain retrograde motion?

See All Videos and Images →

Topics

deferent theory of homocentric spheres equant epicycle Geocentric model Solar system Heliocentrism Tychonic system Copernican system Comet Sun Kuiper belt Asteroid Meteorite Meteor and meteoroidmeteoroid Space weather

Key People

How did Ptolemy explain retrograde motion?

Ptolemy

Egyptian astronomer, mathematician, and geographer

default image

Georg von Peuerbach

Austrian mathematician

Nicolaus Copernicus. Nicolas Copernicus (1473-1543) Polish astronomer. In 1543 he published, forward proof of a Heliocentric (sun centered) universe. Coloured stipple engraving published London 1802. De revolutionibus orbium coelestium libri vi.

All About Astronomy