Galileo and Aristotle
- Until 1610, when Galileo built his first telescope at age 46, he focused mainly on physics, not astronomy. He soon made discoveries which shook the foundations of the Aristotelian cosmos. He saw mountains, valleys and other features indicating change on the moon. He observed the motion of four of Jupiter's moons, now referred to as the Galilean moons. No longer could scientists say that heavenly bodies revolve exclusively around the earth. He also observed the phases of Venus, the only explanation of which is that Venus moves around the sun and not the earth.