How well did Galileo observe Jupiter's moons? MAR 13 2013
In the pages of Sidereus Nuncius, Galileo described the four large moons of Jupiter in a series of 64 sketches which looked a lot like ASCII art in the text:
Using an online tool for computing the positions of Jupiter's moons, Ernie Wright compared Galileo's sketches to the moons' actual motions.
Click through for an animated GIF of all the comparisons. Not bad for the telescopic state of the art in 1610. For a taste of how celestial objects actually appeared when viewed through Galileo's telescope, check out this video starting around 7:30. (thx, john)