What specific observation by Galileo demonstrated that Venus must orbit the Sun?

Answer

Venus goes through a complete set of phases, including a full phase.

The fact that Venus displayed a full phase proved that it must, at times, be on the far side of the Sun relative to the Earth, a configuration only possible if the Earth itself was orbiting the Sun, contradicting the Ptolemaic model.

What specific observation by Galileo demonstrated that Venus must orbit the Sun?
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