Copernican theory

Up until the 16th century people believed in Ptolemaic theory. That is because people observed the movement of the Sun only using the naked eye, and noticed that the other heavenly bodies too moved like the Sun. However, Copernicus advocated Copernican theory.
Copernicus noticed that there were some mistakes in the Ptolemaic theory advocated by Ptolemaios. The Ptolemaic theory could not explain the complex movements of the planets such as retrograde motion. So Copernicus thought that all planets, including the Earth, were moving in a circular orbit around the Sun and he advocated the planetary system model. This theory was able to physically describe the complex planetary movements in an orderly fashion.
At the time that this theory was advocated, it was not accepted by people and it was theoretically inadequate, but this discovery became the turning point from Ptolemaic theory to Copernican theory and affected a lot of later researchers. For example, Kepler proved that the planets had an elliptical motion with the focus being the Sun, and Galileo Galiei discovered Jupiter in 1610 which proved the credibility of Copernicus' theory. In modern times, Copernican theory has become common.