Galileo Galilei's notes on the positions of Jupiter's four moons

These are Galileo Galilei's notes on the positions of Jupiter's four moons over a period of several nights in 1611.

Galileo was able to observe the moons thanks to his newly invented telescope. After publishing his findings, he worked to refine our understanding of the moons' periods by setting his telescope up night after night and recording his observations. At the time this method was pretty radical, because it was based on experimental observations rather than what the ancient authorities had written.