In ptolemy's earth-centered model for the solar system, venus's phase is never full as viewed from earth because it always lies between earth and the sun. in reality, as galileo first recognized, venus is

Explanation:

Galileo observed that Venus presented phases (such as those of the moon) together with a variation in size; observations that are only compatible with the fact that Venus rotates around the Sun and not around Earth.  

This is because Venus presented its smaller size when it was in full phase and the largest size when it was in the new one, when it is between the Sun and the Earth.  

In other words: When Venus is in its full phase it is not possible to observe it from the Earth because always in this phase, Venus, the Sun and the Earth are in conjunction (or aligned).  This means the Sun is between Venus and Earth, in the orbit that both planets describe around this star.

Therefore, from the Earth, the Sun covers Venus.

This fact along with other discoveries were presented by Galileo to the Catholic Church (which supported the geocentric theory at that time) as a proof that completely refuted Ptolemy's geocentric system and affirmed Copernicus' heliocentric theory.

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