Using a telescope, Galileo was able to observe Venus going through a full set of phases, something prohibited by the Ptolemaic system. (The Ptolemaic system would never allow Venus to be fully lit from the perspective of the Earth, as this would require it to be on the far side of the Sun, which is impossible if Venus's orbit in its entirety is between the Earth and the Sun, as the Ptolemaic system requires[3]). This observation essentially ruled out the Ptolemaic system, and was compatible only with the Copernican system and the Tychonic system and other heliocentric models such as the Capellan and Riccioli's extended Capellan model.