During Galileo's time, people believe that the universe is geocentric, even though there was no concrete proof to back this up. When Galileo discovered the phases of Venus and other heavenly bodies, he knew that his discovery could not fit the mold of the geocentric theory. His discoveries contradicted the geocentric theory. With evidence and actual observation (and not just hypothesizing), Galileo was convinced that geocentrism is wrong and the real center of the solar system is the Sun.
