How do Venusian phases prove heliocentrism?

Question

Copernicus demonstrated that we live in a heliocentric solar system by showing that that Venus has phases like Luna does, and that this could only be the case if Venus orbits Sol. Why is this so? Of course, I don't doubt that our solar system is heliocentric and that Copper Knickers knew what he was talking about, but why couldn't Venus display lunar-like phases if it orbited our planet? After all, Luna does.

Answer 1

I think it was Galileo who demonstrated that Venus showed the whole range of phases, not Copernicus. Galileo had a telescope and these were not invented when Copernicus put forwards the heliocentric model of the Solar System. Copernicus simply predicted that Venus would show phases.

What Wikipedia says is this:

"From September 1610 Galileo observed that Venus exhibited a full set of phases similar to that of the Moon. The heliocentric model of the solar system developed by Copernicus predicted that all phases would be visible since the orbit of Venus around the Sun would cause its illuminated hemisphere to face the Earth when it was on the opposite side of the Sun and to face away from the Earth when it was on the Earth-side of the Sun.

In contrast, the geocentric model of Ptolemy predicted that only crescent and new phases would be seen, since Venus was thought to remain between the Sun and Earth during its orbit around the Earth.

Galileo's observations of the phases of Venus proved that it orbited the Sun and lent support to (but did not prove) the heliocentric model."

To which my comment is: it demonstrated that the geocentric model did not describe what went on in the solar system, as did Galleo's discovery of the four largest moons of Jupiter. earlier that same year. It damaged the Ptolemaic model's credibilty such that it could no longer be taken seriously as a scientific theory.

Answer 2

Ptolemy's geocentric model had The Moon closest to Earth, followed by Mercury, Venus, the Sun and then all the other planets farther away than that. That model does indeed cause Venus to have phases like the Moon. The first source shows a diagram. An alternate theory developed by Tycho had all the planets going around the Sun but the Sun going around the Earth. That model also explains the phases of Venus, and both these models were developed before Galileo ever turned his telescope on Venus. That is probably why Galileo was not completely successful in his efforts to use the phases of Venus to support the heliocentric model of Copernicus.

Answer 3

I think the point is that Venusian phases were further proof that the geocentric model did not fit the observable facts, It may be that Tycho Brahe offered an alternatuive explanation of the phases by putting forward a modified Ptolemaic model and it may be that a number of other hybrid models retaining some of the geocentric model's features offered other alternative explanations of the phases, But the point is that Galileo's published dialogue was concerning the two CHIEF world systems ie the pure gecebntric model and the pure heliocentric model, and as between those two main alternatives, the Venusian phases and the four moons of Jupiter, taken together, swung the argument decisively towards the heliocentric model.

Answer 4

Because when it is "near" the sun, as seen from Earth, it is fully illuminated, which means it is on the far side of the sun. If it were this side of the sun, orbiting Earth, we would see the dark side at those times.