Geocentricity is the theory that the Earth is located at the center of the cosmos. The theory was espoused first by Parmenides in the fifth century BCE and then became the standard view from the fourth century BCE onward. Eudoxus and his student Callippus devised geometrical models based on the geocentric hypothesis. In these models, the heavens consist of a series of homocentric spheres, centred on the Earth, that account for the movements of the Sun, Moon, stars, and five planets visible with the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. Aristotle modified this cosmological system and developed an element theory consistent with geocentricity. Some ancient Greek intellectuals offered alternative cosmologies—notably Philolaus, Heraclides of Pontus, and Aristarchus of Samos—but the geocentric conception of the cosmos remained the standard view. Cleomedes and Claudius Ptolemy offer proofs of the geocentric hypothesis in their astronomical texts. In addition, Ptolemy puts forward complex astronomical models that incorporate the eccentric and epicyclic hypotheses as well as the equant, a feature of his own invention. Ptolemy’s geocentric astronomy was authoritative until the seventeenth century, when mathematicians came to espouse a Copernican conception of a heliocentric cosmos, where the Sun is situated at the centre of the cosmos and the Earth orbits the Sun. Isaac Newton developed a physics consistent with heliocentricity and thus prepared the way for the demise of the geocentric theory and the widespread acceptance of heliocentricity.
