The Galilean moons are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto, discovered by Galileo Galilei in 1610. These moons are significant for several reasons. Scientifically, they are some of the most fascinating celestial bodies in the solar system, with Io's volcanic activity, Europa's potential subsurface ocean, Ganymede's magnetic field, and Callisto's ancient, heavily cratered surface. Their discovery was also pivotal in changing our understanding of the universe, providing clear evidence that not everything orbited the Earth, which was a major support for the Copernican model of heliocentrism over the geocentric model.