"On January 7, 1610 Jupiter could be seen with the canon (the telescope) flanked by three stationary stars". This is the first notation made by Galileo of one of his most extraordinary discoveries. Three days later, he made this correction: "It appears from these observations that there are three moving stars around Jupiter, something that has never been observed by anyone else". On January 13 of that same year, Galileo understood that there were four and not three of these "moving stars". He gave them the name of Medicea Sidera (Medicean Stars) in honor of Cosimo II de' Medici. This marked the birth, for man, of the first of Jupiter's moons. From that time, with the development of telescope technology, the number of these moons has gradually increased, up to the discovery of the thirteenth and fourteenth moon. This last moon, found slightly prior to Voyager's arrival at Jupiter, was then confirmed by the two probes. A fifteenth moon, temporarily called 179 J-1, seems to have been discovered by the Voyager probes very close to the planet between the two Medicean satellites. These Medicean stars, which rotate in very close orbits around the gas giant, are called Io, Europa, Ganymede and Callisto. They are probably the only ones, except for those in more distant orbits, Lysitea and Leda, which were formed at the same time as the planet. The other eight smaller moons, which rotate at a great distance from Jupiter, might instead be asteroids captured by its intense gravitational field. And they might not be the only ones to have experienced the powerful "grip" of the giant of the solar system. What is certain is that this large group of satellites makes Jupiter even more special as a planet: in reality, it seems to be more and more like a small independent planetary system. Though some of the moons have diameters of only a few dozen kilometers, which make them resemble to mere pebbles, the four Medicean satellites have truly large dimensions, ranging from the Moon up to the size of Mercury.
"On January 7, 1610 Jupiter could be seen with the canon (the telescope) flanked by three stationary stars". This is the first notation made by Galileo of one of his most extraordinary discoveries. Three days later, he made this correction: "It appears from these observations that there are three moving stars around Jupiter, something that has never been observed by anyone else". On January 13 of that same year, Galileo understood that there were four and not three of these "moving stars". He gave them the name of Medicea Sidera (Medicean Stars) in honor of Cosimo II de' Medici. This marked the birth, for man, of the first of Jupiter's moons. From that time, with the development of telescope technology, the number of these moons has gradually increased, up to the discovery of the thirteenth and fourteenth moon. This last moon, found slightly prior to Voyager's arrival at Jupiter, was then confirmed by the two probes. A fifteenth moon, temporarily called 179 J-1, seems to have been discovered by the Voyager probes very close to the planet between the two Medicean satellites. These Medicean stars, which rotate in very close orbits around the gas giant, are called Io, Europa, Ganymede and Callisto. They are probably the only ones, except for those in more distant orbits, Lysitea and Leda, which were formed at the same time as the planet. The other eight smaller moons, which rotate at a great distance from Jupiter, might instead be asteroids captured by its intense gravitational field. And they might not be the only ones to have experienced the powerful "grip" of the giant of the solar system. What is certain is that this large group of satellites makes Jupiter even more special as a planet: in reality, it seems to be more and more like a small independent planetary system. Though some of the moons have diameters of only a few dozen kilometers, which make them resemble to mere pebbles, the four Medicean satellites have truly large dimensions, ranging from the Moon up to the size of Mercury.
