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
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