science · 07 January 1610 · 415 years ago
Galileo Galilei first observes the four large Galilean moons of Jupiter: Ganymede, Callisto, Europa and Io, although he is unable to distinguish the latter two until the following day. In the same year he publishes his first observations by telescope in a short treatise entitled Sidereus Nuncius ("Sidereal Messenger").
OnThisDay.net