Two other important discoveries were made by Galileo by means of the telescope. On January 7, 1610, in one of his first observations, he managed to view Jupiter, the largest planet, through the telescope. Very close to the planet he observed three small bodies. At first Galileo thought these objects were distant stars never observed before. With the aid of the telescope he succeeded in seeing many other stars which cannot be seen with the naked eye. If these were indeed stars, they must be very far away and should be fixed in their relative positions, and only Jupiter should move in relation to them. However, on the following day, when Galileo looked at them through his telescope, he noted that the distances between them had changed. Following repeated observations of these objects, it became evident that they circled Jupiter periodically, and that they were not luminous but only reflected light like the other planets. Galileo quickly arrived at the conclusion that these planets (at this stage he had already recorded four of them) were in fact moons of Jupiter. Just as our moon circles the earth, so they circle Jupiter. He called them the Medici planets after the ruling family of Tuscany, whose philosopher and mathematician he had become.

After Galileo discovered Jupiter's moons, they were also observed by a German astronomer named Marius. The four moons, sometimes called the Galilean moons, are only part of Jupiter's complement of moons, which we now know numbers twelve.

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