A full Venus always occurs when it is on the opposite side of the Sun as viewed from Earth. (However, we cannot see the full Venus, because it is always very close to the Sun in the sky at that time.) Galileo used this fact as evidence for the Sun-centered view of the solar system: The fact that Venus goes through all the phases must mean it goes all the way around the Sun. In contrast, in the Ptolemaic model, Venus only varies between new and crescent phases.
