Become Galileo’s assistant
Galileo was really confused about the geocentric system. He was strongly convinced that sun is in the center of our planetary system, so he was looking for undoubtedly evidences. Then he got the idea to target his telescope to Venus. According to the geocentric system (Ptolemy’s Model) it would not be possible to view all Venus in all phases, but only as a meniscus. But, when he pointed his telescope at Venus he saw all phases from full phase to new Venus. This was an ultimate evidence of the heliocentric system.