Galileo on the Phases of Venus
Galileo went on to develop another argument for the impossibility of the Ptolemaic model to account for a telescope observation. In observing Venus, he noted that the variations in size and brightness were actually phases — crescent and full Venus.Only in a heliocentric system would this be geometrically possible.
Ptolemaic on Left — Copernican on Right. The full phase of Venus can only occur in heliocentric system.
Galileo’s diagram, as first published in The Assayer (1623)