Today in History – January 7, 1610

On January 7, 1610, nearly four hundred years ago, Galileo wrote a letter containing the first mention of Jupiter’s moons. At the time, he only saw three of them, and he believed them to be fixed stars near Jupiter. He continued to observe these celestial orbs from January 8 through March 2. In these observations, he discovered a fourth body, and also observed that the four were not fixed stars, but rather were orbiting Jupiter. He attempted to call the objects the Medicea Sidera ("the Medician stars”) after the Medici family, whose patronage he was trying to secure.

The names that eventually prevailed were chosen by Simon Marius, who claimed to have discovered the moons at the same time as Galileo. He named them after lovers of the god Zeus (the Greek equivalent of Jupiter): Io, Europa, Ganymede and Callisto. Galileo was perturbed by this, and refused to use the names. He referred to them instead as Jupiter I, II, III and IV, creating a numbering scheme that is still in use today.

Though his name for the bodies didn’t stick, Galileo is credited with their discovery and the moons are still referred to as Galilean Moons.

A composite image showing the relative sizes of the four moons as compared to Jupiter. From the top: Io, Europa, Ganymede, and Callisto.