Spotting the shadows of Jupiter's Galilean moons

On the evening of Jan. 7, 1610, Galileo Galilei turned his newly constructed telescope on the planet Jupiter and was amazed to observe three tiny stars in a neat row alongside it, two on the left and one on the right.

Galileo didn't realize it at first, but these "stars" were in fact moons orbiting the planet. If his telescope had better resolution, he would have seen four moons, because the moons Io and Europa were so close together that they looked like one in his telescope.

Over the next eight nights, he re-observed Jupiter and quickly realized that these were not background stars, but were four moons in regular orbits, mimicking in miniature the motions of the planets around the sun. In the below facsimile of Galileo's notes, his observation on Jan. 7 is top and center, Jan. 8 below to the left, Jan. 10 and 11 to the right, followed by Jan. 12 and 13, when he observed the fourth moon for the first time. Jan. 14 was cloudy, but he was able to make another observation on Jan. 15.

On March 13, 1610, 406 years ago this week, he published a book describing this and other discoveries called "Sidereus Nuncius," or "The Starry Messenger."

But Galileo wasn't ready to rest on his laurels. Jupiter was passing behind the sun, in conjunction, on June 25. As soon as it emerged in the morning sky, he undertook a systematic series of observations in order to accurately determine the orbits and periods of the four moons. His Jupiter log resumes on July 25 and continues nearly every clear night for four months. At that point, his Jesuit friends at the Collegio Romano took over the observations through April 1611. They used the diameter of the planet Jupiter as a "measuring stick" to calculate the distances of the moons from the planet.

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