turned his telescope to Venus, he saw that it went through phases like the Moon. In that one observation the geocentric solar system was shattered forever (okay, so Tycho
tried to save the geocentric system by putting all the planets orbiting the Sun, and the Sun orbiting the earth, but the Ptolemaic system was gone forever). Above are my images of the phases of Venus, you can clearly see the increase an size and crescentness of Venus as time goes on. This is what I love about science generally, is that ordinary people can test out the foundational concepts and observations of science themselves. When I look at the pictures I've taken, I see more than wonky webcam shots, I see a revolution in thinking. I had a big discussion with a member of the ABC Science Matters discussion list
about the role of "faith" in science, and the phases of Venus were an example I used of how you don't need to accept statements of Science on "faith", but can check them for yourselves. Admittedly, there are lots of things like particle physics it is hard for an amateur to test, but right now amateurs are imaging transiting exoplanets, so there is a lot you can do.
Speaking of Venus, via davep
is this stunning page showing shadows cast by Venus