The term "geocentric" means "earth-centered," while "heliocentric" means "sun-centered." In medieval astronomy, two models were proposed regarding the structure of the universe: the geocentric model claimed that Earth was the center of the cosmos, while the heliocentric model suggested that the sun was the center of the universe.
The geocentric and heliocentric models were simple representations of the observed heavenly bodies during the Middle Ages. Both models factored in all the known astronomical data at that time, and reliably estimated the future placements of the major celestial objects.
The geocentric model was devised by Ptolemy, a Greek philosopher who compiled the historical astral observations of the ancient Greeks and Egyptians, which formed the framework for his theory. Ptolemy's model illustrated that the Earth was stationary and that the Sun, Moon, planets and stars moved around it in perfect circles at constant speeds. Aristotle, one of the most prominent Greek philosophers, advocated the Ptolemaic geocentric model, which was widely accepted until the heliocentric model emerged in the 1500s.
Nicolaus Copernicus was a Polish cleric who debunked the Ptolemaic model by proposing the heliocentric model. Copernicus argued that the Sun was motionless and that all planets, including Earth, revolved around it. He further suggested that the stars were stationary and that the Moon orbited Earth. Although initially rejected by the public and the majority of the scientific community, the Copernican theory garnered support from Galileo and was later proven by Johannes Kepler.