Germany. Astronomer Simon Marius first observed Jupiter’s four Medicean moons, a few weeks before Galileo Galilei, and gave them their current names: Io, Europa, Ganymede, and Callisto. Simon Marius, however, began tracking these observations on December 29, 1609, according to the Julian calendar used in Germany, which corresponds to January 8, according to Galileo Galilei’s Gregorian calendar, the day after Galileo’s first letter mentioning the aforementioned moons of Jupiter.
