Do you know why the observation of a full set of phases for Venus backs the Copernican system?

Answer 1

It reinforces the idea that both the Earth and Venus revolve around the sun, and that Venus is in an inferior position. If Venus and the Sun both revolved around the earth (as per the geocentric model), then Venus would never be occulted by the sun.

Answer 2

In order for Venus to show the phases it does it MUST be going around the Sun. The phases in themselves don't prove it, after all, the Moon shows a full set of phases and it goes around the Earth, but the location of the planet when the phases occur does. Starting with 'new' Venus, its cresent phase gets wider as it appears to move further from the sun in the sky, just as with our Moon. But at the same time the apparent size of the planet diminishes, so it must be getting further away from us. Then, when a bit past the 'half' phase, Venus starts moving back towards the sun in the sky, but continues to move onward in its phase cycle towards full, and it continues to get smaller. This cannot happen if Venus is between Earth and the Sun, therefore Venus must be beyond the Sun as it approaches its full phase. It appears full because it is more or less in a line with us and the Sun, but we are looking at it from behind the light source, hence we see the full lit side.

This would be easier with diagrams, but I hope that made sense!