Galileo Discovers Jupiter's Moons and Venus's Phases

In early 1610, Italian astronomer Galileo Galilei observed four moons orbiting Jupiter using his telescope, first noting three on January 7 and identifying the fourth by January 15. Beginning in September 1610, he observed that Venus exhibited phases similar to the Moon. Galileo published the Jupiter findings in March 1610 in 'Sidereus Nuncius' and the Venus observations in his 1613 'Letters on Sunspots.' These discoveries provided evidence for the Copernican heliocentric model, as Jupiter's moons demonstrated celestial bodies could orbit objects other than Earth, and Venus's phases indicated it orbited the Sun rather than Earth.