Caption: The phases of Venus in the Ptolemaic system (left side) and in the Copernican system (right side).

Features:

1. If one assumes that Venus shines by reflected sunlight---which is true---then Venus' is predicted to have planetary phases in both the Ptolemaic system and the Copernican system. But the phases are predicted to be different in the two cases as illustrated in the diagram.

2. In the Ptolemaic system, Venus orbits on an epicycle in front of the Sun. Both Venus and Sun then orbit synchronously on their respective deferents.

   As the diagram shows, given our assumption, Venus should NOT show a full set of phases in the Ptolemaic system.

3. In the Copernican system, Venus orbits the Sun and should show a full set of phases.

   The full and nearly full phases of Venus are difficult to observe, but it can be done by projecting the telescope image on a screen rather than using direct visual astronomy.

4. Galileo (1564--1642) was the first to observe (in 1610) and report (in the Letters on Sunspots 1613) that Venus had phases (and so did shine by reflected sunlight) and that there are a full set of phases (see Wikipedia: Phases of Venus: History).

   Note that Galileo did NOT, in fact, report on the phases of Venus in Sidereus Nuncius (1610, in English The Star Messenger). However, in the telescoping of history, it's much the same as if he did.

5. Galileo's observations showed the Ptolemaic system was wrong about the phases of Venus, and so was falsified insofar as Venus was concerned.

   But the observations were consistent with the Copernican system. The observations do NOT prove the Copernican system, but they are consistent with it.

   But the observations are also consistent with Tychonic system. So fraidy cats could always take refuge in the Tychonic system.

6. That Venus has a full set of phases is also inconsistent with Aristotelian cosmology which either has Venus always closer or farther than the Sun depending on how exactly you want to arrange the compounded celestial spheres on which Venus and the Sun are carried.

7. One can also add that since Venus did shine by reflected sunlight, Venus was in that sense like the Earth.

   This Earth-likeness of Venus was evidence---but not by itself proving evidence---that the Earth was a planet as the Copernican system predicted. Probably, Galileo and some of contemporaries did note this evidence.

   It has to be added that Venus unlike the Moon showed no sign of mountains and other geology.

   Galileo and his contemporaries may that thought this was probably just due to the inadequacy of their telescopes. Their telescopes were inadequate, but, in fact, Venus is totally shrouded in a bland white-yellow Venusian atmosphere, and so the Venusian geology hidden in the visible band. The Venusian geology has been revealed in modern times principally by the Soviet Venera landers (1970--1982) and the NASA Magellan orbiter (1989--1994) which used radar mapping.