How Do We Know That Venus Orbits The Sun?
Venus exhibits phases like the Moon. It is a lot further away, so it looks a lot smaller than the Moon. The phases are not visible to the naked eye, except to a very few sharp-eyed people; and were unknown until the advent of the telescope about 400 years ago.
You cannot see the phases very well in 10 × 50 binoculars, but my 6 in. f/5 Newtonian telescope shows them very nicely. At least it did in late winter 2006. The crescent phase was unmistakable. But when it first appeared in the evening sky around January 2007, I was very puzzled because I had seen the phases so easily the previous winter just before sunrise. I could not see any phases. The darn thing was just a bright blob even at maximum magnification.
I told my wife, not very confidently, that it must be much further away than a year ago. Was I right?