GALILEO GALILEI, Sidereus nuncius, Venice, 1610
Biblioteca Nazionale Centrale di Firenze, Pal. 1200/23
Facsimile

Galileo describes the discovery of Jupiter's satellites, starting from that night of 7 January 1610, when he saw beside the planet three little stars arranged in a straight line. The fourth satellite appeared through his telescope only on the night of 13 January.