Ptolemaic system

An Earth-centered model of the cosmos in which the Sun, planets, and stars move around Earth. It used combinations of circles to account for observed planetary motions.

Deferent and epicycle: A model of planetary motion combining a large circle around Earth with a smaller circle carried along it. Their combined motions form the system’s main device for matching planetary paths.
Eudoxus of Cnidus: An ancient Greek mathematician who modeled celestial motions with nested concentric spheres. His sphere-based approach was an early mathematical attempt to preserve an Earth-centered cosmos.
Alfonsoine Tables: Medieval astronomical tables based largely on Ptolemaic models and produced under Alfonso X of Castile. They supplied widely used predictions of planetary positions in medieval Europe.
Copernican heliocentrism: A model placing the Sun near the center and making Earth a moving planet. It explains planetary retrograde motion through Earth’s changing viewpoint rather than epicycles alone.
Equant: A point offset from a circle’s center from which motion along that circle appears uniform. Ptolemy used it to improve predictions of changing planetary speed.
Hipparchus: A Greek astronomer of the second century BCE known for precise observations and mathematical astronomy. His work on planetary theory and observations helped prepare the foundations Ptolemy developed.
Prutenic Tables: Astronomical tables published by Erasmus Reinhold in 1551 using Copernican parameters. Their comparison with older tables shows how prediction methods changed during the system’s decline.
Tychonic system: A geoheliocentric model in which the Sun orbits a stationary Earth while other planets orbit the Sun. It retained a stationary Earth while adopting a planetary arrangement that differed from Ptolemy’s.
Retrograde motion: The apparent temporary reversal of a planet’s motion against the background stars. Epicycles reproduce this conspicuous pattern without Earth orbiting the Sun.
Claudius Ptolemy: A second-century CE Greco-Egyptian mathematician and astronomer who wrote the Almagest. He assembled and refined the system’s models into a durable astronomical framework.

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