The discovery of Jupiter’s moons was arguably Galileo Galilei’s most noteworthy discovery. In 1610, Galileo conducted his first observation of Jupiter through his homemade telescope. In the midst of observing, he noticed three small bodies of light that looked like stars positioned relatively close to Jupiter. He did not pay much attention to it till the very next day when he saw the same stars on the opposite side of Jupiter. Approximately 1 week after his first observation, a fourth star appeared.
This sparked his interest and he proceeded to observed these phenomenons for the next two weeks. Night after night, he sketched the changing positions of the stars. After looking at his sketches at the end of the week, he realized that the stars never strayed too far away from Jupiter. In fact, their movement seemed to occur with respect to Jupiter’s position. Thus, he concluded that the bodies of light were not stars as he first thought but moons instead. According to Galileo, the moons were orbiting around Jupiter which proposed the possibility of planets specifically Earth doing the exact same thing around the sun.
This was an extremely controversial discovery as it went against the geocentric belief of the Church.