Autograph notes on the satellites of Jupiter, 14–25 January 1611
On this scrap of paper (an unfolded envelope), Galileo recorded the positions of four satellites of Jupiter over a period of several nights. He had observed the moons with the aid of his newly constructed telescope and published his findings in his revolutionary book The Starry Messenger (1610). He then worked to define more precisely the periods of the orbits of the Jovian moons, setting up his telescope night after night and making notes such as these. In a radical departure from his university training, Galileo insisted that scientific theory be grounded in observation and physical evidence rather than reliance on ancient authority.