We all know Galileo Galilei championed the heliocentric model of the universe that Copernicus first proposed, but his reasoning behind that belief is less well known. Galileo was not the first to invent a telescope, but he was in Florence where the best glass in the world was made at the time. When he turned his telescope to the sky, he revealed many aspects of the Solar System that were inconsistent with the geocentric, Aristotelian model of the universe that most people believed at the time. The geocentric Solar System included perfect spheres, a fixed earth, and a single center of motion: Earth.
One of the best known discoveries of Galileo was his discovery of Jupiter’s four large moons, which are now known as the Galilean Moons. This discovery helped convince Galileo that the solar system was heliocentric not geocentric because Jupiter was another center of motion if it had moons, which destroys the idea that Earth is the only center of motion in the universe. Galileo also discovered the rough edge of the moon by looking at it through a telescope, which destroyed another main point of the previously held geocentric model of the Solar System: the perfect spheres in space.
The final piece of the puzzle for Galileo was the phases of Venus that he discovered with his telescope. For him, conclusively proving that Venus orbits the Sun also proved that the Earth and every other planet did the same. These discoveries prompted his long struggle with getting Copernicus’s idea for a heliocentric universe accepted by the world.
Despite what common knowledge tells us, Galileo’s long struggle was the story of devout Catholic who believed that science and religion should be kept in separate spheres. His difficulties with the separation of science and religion are difficulties we still struggle with today, especially when it comes to decisions of public policy with respect to education and regulations.