Why did Venus exhibiting a full phase contradict the traditional Ptolemaic model?

Answer

The model placed Venus between the Earth and the Sun, allowing only crescent or new views.

If Venus orbited between the Earth and Sun, an observer on Earth could never see the side facing the Sun fully illuminated; seeing a full phase requires observing Venus on the opposite side of the Sun relative to Earth.

Why did Venus exhibiting a full phase contradict the traditional Ptolemaic model?
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