Which Italian astronomer discovered the four largest moons of Jupiter?

Galileo Galilei discovered the four largest moons of Jupiter, now called the Galilean moons, in 1610.

Using an improved telescope, Galileo observed the objects in late 1609 or early 1610 and recognized that they were orbiting Jupiter rather than Earth. The four moons are Io, Europa, Ganymede, and Callisto. Their existence provided striking evidence that not every celestial body revolved around Earth, challenging traditional geocentric astronomy.

Galileo first referred to them as the Medicean Stars, honoring the powerful Medici family in Florence. Their modern names come from mythological figures associated with Jupiter and were popularized by the German astronomer Simon Marius, who claimed an independent observation around the same time. Galileo’s observations were published in Sidereus Nuncius in March 1610.

Ganymede is the largest moon in the Solar System, while Io is famous for its intense volcanic activity. The moons remained Jupiter’s only known satellites until Amalthea was discovered in 1892. Galileo’s discovery is therefore distinct from later observations by Kepler, Copernicus, or Giordano Bruno.