Galileo's observations of the entire phase cycle of Venus proved that Ptolemy's epicyclescould not be correct in keeping Venus between us and the Sun.

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Galileo discovered that Venus showed a full range of phases, which could only occur if Venus were orbiting the Sun and not the Earth. This contradicted Ptolemy's geocentric model of the universe, which included epicycles to explain Venus's movements. Galileo's observations showed that Venus went through phases, just like the Moon, and therefore must orbit the Sun and not the Earth. This contradicted Ptolemy's geocentric model of the universe, which relied on epicycles to explain the movement of the planets.

Galileo's Observations of the Entire Phase Cycle of Venus Proved