This article by Keith Gleason first appeared SBO's Astro-Info column for February, 2010 in celebration of the International Year of Astronomy
Just over four hundred years ago - on January 7th, 1610 - the Italian astronomer and mathemetician Galileo Galilei made a discovery that shook the world and changed mankind's view of our place in the universe. On that evening, about an hour after sunset, Galileo pointed his home-built telescope towards Jupiter, and saw a peculiar sight: two tiny "stars" to the east of Jupiter, and one to the west, all arranged in a tight straight line along the ecliptic path with Jupiter itself. Galileo made a sketch of this unusual "chance" alignment of celestial objects (top drawing in the illustration to the right), and then went about making other observations.
The next evening, almost on a whim, Galileo decided to check Jupiter again just to verify that the three "fixed stars" lay to the east of Jupiter, since he knew that the planet was moving westward against the background stars in its (well known, but not understood) retrograde motion. Sure enough, there were the three stars again ... but on the west side of Jupiter, not its east! The only explanation was that those stars weren't "fixed" at all, but moved with Jupiter and indeed, seemed to move around it like our own Moon moves around Earh. This was the beginning of the end of the Aristotelian geocentric universe.
The next night was cloudy, but for the remaining four days of the week (and indeed, for many months thereafter) Galileo continued to make nightly observations and sketches. On the 10th and 11th, he only saw two stars, on the 12th observed three again, and finally, on January 13th, 1610, he identified all four of what we now refer to as the "Galilean Moons": Io, Europa, Ganymede, and Callisto.
Galileo quickly published the results of his six months of astronomical observations from late 1609 to early 1610 in one of the most famous (and very readable in its English translation!) scientific publications of all time: Sidereus Nucius, or Sidereal Messenger. In it he referred to Jupiter's newly discovered moons as the "Medicean Stars", in hopes of currying the favor of his Medici family patron.
But which moons did Galileo observe on these nights, and why did the numbers change? With modern computer planetarium programs, it's easy to "travel" back in time and space to Padua, Italy, some 400 years ago, and see the skies as Galileo saw them.
Below is our reconstruction of that first week. We've plotted and labelled the actual positions of the Gallilan moons with respect to Jupiter, at around the time that we believe he must have made each observation, and overlaid Galileo's own drawings on top.
January 7th Galileo actually saw all four moons, but was unable to resolve Io and Europa, which were very close together at the time; he saw them as a single "star". While Galileo had built himself the world's finest optical telescope of the time, by today's standards it would hardly qualify as a cheap department-store toy telescope.
January 8th Again all four moons were visible, but Galileo was expecting to see only three, so he missed the fourth - Callisto - far to the east of Jupiter away from the three that amazed him because of their movement to the west.
January 9th Cloudy, no observations. We can imagine Galileo's likely frustration and anxiety when, just as he is beginning to suspect the geocentric earth-shattering nature and importance of what he has just witnessed, and he has to put his investigations on hold for another twenty-four hours!
January 10th Two nights later, Europa and Ganymede were close together and unresolved, while Io was passing in front of the giant planet; this time, Callisto was one of the "two" that were observed.
January 11th Ganymede and Callisto were seen east of Jupiter, while Io and Europa were hidden behind and on the farside of the planet.
January 12th This night Callisto was the "invisible noon", lying almost directly in front of Jove. We note that Galileo's observations are becoming more acute as his realization and appreciation for what he was observing became more apparent. Earlier in the week, it seems unlikely that he would have been able to discern Io so close to the planet; indeed, he shows it as being dimmer, likely due to the contrast with brilliant Jupiter immediately adjacent.
January 13th Finally, Galileo simultaneously observes all four of the players that he's been watched all week. He now is starting to note finer details as well, such as the slight displacement of Io northward of the rest of the line of objects. Over the next few months, Galileo would track all four, determine which was which from night to night, and ultimately derive the orbital sizes and periods of revolution for each.