What observation regarding Venus provided a geometric constraint supporting the Copernican view?

Answer

Venus went through a full cycle of phases, from thin crescent to nearly full.

Observing that Venus displayed a full sequence of phases, like the Moon, is geometrically possible only if Venus orbits the Sun, sometimes positioning itself between the Earth and the Sun.

What observation regarding Venus provided a geometric constraint supporting the Copernican view?
GalileoobservationMooncelestial bodyProof