What aspect of Venus's appearance, observed through the telescope, proved it must orbit the Sun?
Answer
It displayed a full range of illumination phases, including gibbous and full.
In the strict Ptolemaic model, Venus could never appear more than crescent or new because it stayed between Earth and the Sun; appearing 'full' or 'gibbous' requires Venus to swing to the far side of the Sun relative to Earth, which only happens if Venus orbits the Sun.
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