With a telescope magnifying thirty-three times which he had constructed himself, Galileo discovered the moons of Jupiter. Three of them were first seen on the 7th of January, 1610, and the fourth on the 13th of the same month. These moons, which are sometimes called the "Galilean quartet," were the first heavenly bodies which had not been known from time immemorial. Delighted beyond measure, Galileo sent his drawings and an account of his observations to his patron Cosmo de' Medici, Great Duke of Tuscany, in honor of whom he called the moons the "Medician Stars." Equally anxious to show the world, Galileo set his telescope on the tower of St. Mark where the advocates of the Copernican system hailed them with joyful satisfaction for here was a miniature of the sun and its planets hung up in the heavens to demonstrate the truth of the new doctrines. But the stubborn followers of Ptolemy, viewing them argued that such pretended discoveries were absurd, and that since we had seven openings in the head--two ears, two eyes, two nostrils and the mouth, there could be in the heavens but seven planets. He had therefore either bewitched them or the telescope, for which they threw him into prison. After the moons had finally been accepted as a fact, they were given the names of Io, Europa and Callisto, three lovely maidens of whom the great Olympian God was fond, and Ganymede, his cupbearer, although astronomers usually designate them as I, II, III (Ganymede) and IV, in order of their distance from Jupiter. Their orbits lie in the plane of Jupiter's equator and as that lies always in the plane of our vision, the satellites seem to swing back and forth in their journey around him, sometimes in one position and sometimes in another--sometimes all four on one side, sometimes two on one side and two on the other, or perhaps one of them will be hidden by the planet. They present a different aspect and relation to each other every successive evening and even in a few hours may be seen to change. Frequently these moons may be seen as bright spots against the surface of the planet or their shadows appear as conspicuous round black spots traveling along one of the colorful belts. Ganymede, satellite III, is the largest of all known moons in the solar system, having a diameter of 3560 miles, actually exceeding the planet Mercury in size. Satellite I exceeds our moon in diameter by a couple of hundred miles, satellites III and IV by over a thousand miles.