Copernican theory Until the 16th century people bilieved in Ptolemaic theory. That is because people watch the move of sun with the eyes of themselves and the other heavenly bodys move like the sun. However, Copernicus advocated Copernican theory. Copernicus noticed that there are some mistakes in the Ptolemaic theory advocated by Ptolemaios. The Ptolemaic theory can not explained the complex movements of the planets such as retrograde motion. So Copernicus thought that all planets, including the earth, are 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 a turning point from Ptolemaic theory to Copernican theory and affected a lot of researcher. For example, Kepler proved that the planet has elliptical motion with the focus as the sun and Galileo Galiei discovered Jupiter in 1610 and raised the reliability of Copernicus's theory. In modern times, the Copernican theory is becoming common.