Which satellite did Galileo miss?

Above one sees an image of Jupiter with, Io, one of the four Galilean moons, in transit across the planet. When Galileo first pointed his telescope at Jupiter, on 7th January 1610, he only saw 3 of its four largest moons. See his Letter to the Prince of Venice for the image and the translation of the lower half. Which one of the four major satellites was missing? This can now be easily established using a program called Stellarium which allows you to simulate the solar system as if you had your own planetarium. If you can't wait to download this code and find out the answer to this intriguing mystery for yourself, you should read this paper for the surprising conclusion.