sentences:  The Copernican model displaced the geocentric model of Ptolemy that had prevailed for centuries, which had placed Earth at the center of the Universe.Copernican heliocentrism is often regarded as the launching point to modern astronomy and the Scientific Revolution.Copernicus was aware that the ancient Greek Aristarchus had already proposed a heliocentric theory, and cited him as a proponent of it in a reference that was deleted before publication; however, there is no evidence that Copernicus had knowledge of, or access to, the specific details of Aristarchus' theory.