13 January
1610: On 13 January 1610, Galileo Galilei discovered Callisto, the fourth satellite of Jupiter.
Galileo originally called the Jupiter's moons the 'Medicean planets', after the Medici family and referred to the individual moons numerically as I, II, III and IV. Galileo's naming system was used for a couple of centuries.
It wouldn't be until the mid 1800s that the names of the Galilean moons, Io, Europa, Ganymede and Callisto were officially adopted, after it became apparent that naming moons by number would be very confusing as new additional moons were being discovered.
It is now known that Callisto is larger than the planet Mercury, and composed mostly of water and water ice with large quantities of ice exposed on the surface.
