1. The Phases of Venus: Galileo observed that Venus went through phases just like the Moon, from crescent to full and back again. This was a major blow to the prevailing geocentric model of the universe, which held that all celestial bodies revolved around Earth. The phases of Venus could only be explained if Venus orbited the Sun, as Copernicus had proposed.
2. The Moons of Jupiter: Galileo discovered four moons orbiting Jupiter, which he named the "Medicean Stars" in honor of his patron, Cosimo de' Medici. This discovery further challenged the geocentric model, as it showed that not everything revolved around Earth. It also suggested that other planets, like Jupiter, might have their own satellite systems.
3. The Milky Way is Composed of Stars: Through his telescope, Galileo was able to see that the Milky Way, which appears as a hazy band across the night sky, is actually composed of countless individual stars. This discovery challenged the prevailing view that the Milky Way was a nebulous cloud.
These discoveries, along with his other observations and writings, helped to solidify the heliocentric model of the solar system and revolutionized our understanding of the universe.
