What Is the Geocentric Theory?
The geocentric theory, or the Ptolemaic system, was one of the earliest theories regarding the origins of the universe, and it posited that the sun, stars and even the other planets revolve around the Earth. Ptolemy was not the first to suggest this theory, though, as documents indicate that Aristotle and Plato discussed this paradigm.
Because the Earth is not the center of the universe, it would make sense that discrepancies in the geocentric theory would begin to show up in some later measurements. Once the Greeks began to notice some anomalies between projected locations of the planets and their current locations, some adjustments to the theory became necessary.
The Ptolemaic version of the geocentric theory featured a complex network of circles. Ptolemy thought that each planet traveled in a circle, known as an epicycle, and each epicycle also orbits around the Earth. This means that the center of the gravitational pull, or the deferent, is different from the Earth. Recognizing this difference was a major step toward understanding the motion of the Earth, sun, planets and stars. This system was popular into the 1500s, because it fit neatly with quite a few of the observations of the earliest Greek scientists.