On January 7, 1610, Italian astronomer Galileo Galilei wrote a letter describing his discovery of three moons orbiting Jupiter with a recently upgraded telescope. He discovered a fourth moon within the next two weeks, and the four are called the Galilean Moons. They are named Io, Callisto, Europa, and Ganymede.
Sources:
http://en.wikipedia.org/wiki/Galilean_moon
http://en.wikipedia.org/wiki/Galileo_Galilei
