Hello,<br /><br />I recently watched the Nova broadcast about Galileo's life. One point they made was that Galileo became convinced that the Earth moved about the Sun because he observed the planet Venus going through phases. Nova implied that Galileo understood this as conclusive proof of the Earth's motion. I don't understand this.<br /><br />After some research on the net, I found other sites that said that Venus progressing through phases was "evidence for the fact that Earth moves about the Sun" but that it wasn't by any means conclusive.<br /><br />What I don't understand is why seeing Venus progress through phases would be such an amazing breakthrough. After all, the Earth's Moon goes through phases, and it goes around the Earth, as was assumed did all the other planets.<br /><br />I would think that seeing Venus go through phases would be exactly what Galileo would have expected to see. <br /><br />Can anyone explain this to me?<br /><br />Thanks much!