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.
Io, the satellite closest to Jupiter (named after the maiden who was transformed into a cow and chased to the end of the world), is a very swiftly moving satellite, traveling completely around its orbit in 1 day, 18 hours and 28 minutes. This means that it must move at the tremendous speed of 320 miles a minute! Our moon takes 27½ days or nearly a month to travel around the earth in an orbit of almost the same size, but the attractive force of our earth is so much less than the attractive force of Jupiter that our moon does not have to travel so fast in order to keep from being pulled down on its surface.
Europa's period of revolution is 3 days, 13 hours and 14 minutes; Ganymede's, 7 days, 3 hours and 43 minutes and Callisto's 16 days, 16 hours and 32 minutes.
In 1892 Professor Barnard discovered through the Lick telescope a moon closer to Jupiter than Io. This little moon is only ¹⁄₆₀₀th part as bright as the other satellites and is only 30 miles across.
In 1905 two other satellites were discovered by photography, still another in 1908 and another in 1914, making a retinue of nine satellites in all. These last two, which are very remote from the planet, were found to be revolving in the opposite direction from the other seven, which is from west to east, the direction that generally prevails throughout the heavens. Other exceptions to this rule are the outermost moon of the planet Saturn, the moon of Neptune and all four moons of Uranus. The ninth moon of Jupiter is about 15,400,000 miles from the planet and takes nearly 3 years to complete a revolution. Only 11 hours and 57 minutes for Barnard's satellite and nearly 3 years for the most distant one, is quite a contrast in periods. Although Jupiter's 5-hour night is rich in moons, he is not nearly as well supplied with moonlight as is the earth.
It has already been mentioned how Roemer, a Danish astronomer of the 17th century, discovered the finite velocity of light through the disappearance and reappearance of Jupiter's satellites within his shadow. This was later found true through direct experiment.
At least a peep at this mammoth and colorful world, with its bands and spots and swiftly moving satellites, should be the privilege of every man, woman and child.