Under the Ptolemaic model, why was it impossible to see Venus fully illuminated according to geometric constraints?

Answer

Its orbit was confined to the interior of the Sun’s orbit around Earth

Because the reigning Earth-centered model placed Venus strictly between the Earth and the Sun, geometric constraints meant observers could only ever see Venus appear as a crescent or a slender crescent/new phase, never fully illuminated.

Under the Ptolemaic model, why was it impossible to see Venus fully illuminated according to geometric constraints?

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