How many moons of Jupiter did Galileo discover?

Galileo discovered four moons of Jupiter: Io, Europa, Ganymede, and Callisto. These four satellites are collectively known as the Galilean moons because Galileo Galilei first observed them through a telescope in January 1610.

Galileo initially noticed points of light near Jupiter that changed position from night to night. He realized they were orbiting Jupiter rather than Earth, providing striking evidence that not every celestial body revolved around Earth. He described the observations in Sidereus Nuncius, published in March 1610.

The moons were later named Io, Europa, Ganymede, and Callisto after figures associated with Greek mythology. Ganymede is the largest moon in the Solar System, while Io is famous for its intense volcanic activity. A common mistake is saying Galileo discovered all of Jupiter’s moons; he discovered four, while later astronomers found many more.