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. CHAPTER XIV MARS AND SATURN THE RED PLANET OF MARS, THE GOD OF WAR _Diameter--4230 miles_ THE orbit of the little planet Mars, 141,000,000 miles from the sun, encompasses the orbits of Earth, Venus and Mercury, and forms the boundary line between the four inner planets and the planetoids or minor planets. Beyond the planetoids are the four great planets, Jupiter, Saturn, Uranus and Neptune.