Galileo Galilei is credited with discovering four of Jupiter's moons. He made this and many other discoveries in 1610 after building his first telescope.
When Galileo Galilei was a student, his father sent him off to the University of Pisa in the hopes his son would study medicine. However, Galilei had other plans and studied math instead. After graduating from the university, he stayed on as a math professor and made several important discoveries in physics. Galilei wrote "Principles of Inertia" which is similar to Newton's First Law.
Once he became interested in astronomy, he built his first telescope in 1609, which helped him discover Jupiter's largest moons. He also discovered many new stars, the phases of Venus, and sunspots. By looking at Venus's phases and sunspots, he came to the conclusion that the sun, like the Earth rotates and that all the planets orbit around the sun. This discovery came at a time when most people believed the Earth was the center of the galaxy and that all the planets revolved around it.
Galilei confirmed the studies of other famous scientists like Copernicus, and although he was chastised by the church for his findings, the church allowed him to continue with his work. Eventually, the church had enough, put him on trial for heresy and tried to force him to tell the world his findings were wrong. He refused and was first imprisoned and then later confined to his home.