The sixth and seventh satellites are also very minute bodies, measuring probably less than one hundred miles in diameter. They circle about Jupiter at a distance nearly thirty times more remote than our moon is from us. They are about seven million miles from the planet, and probably not more than barely visible from it. It takes them two hundred and sixty-five days to make one revolution, which is more than five hundred times as long as the period of Jupiter’s nearest satellite. These two satellites are so nearly of one size and revolve so nearly in the same time and at the same distance from Jupiter that they are thought to have had a common origin. Just what their relation is has not yet been determined. The eighth satellite, discovered in January, 1908, is certainly no larger, and is perhaps still more tiny, than the sixth and the seventh, though it is a little brighter than either one of them. It is about three times farther away from Jupiter than the seventh satellite, and with eyes such as ours would not be visible from Jupiter. It shows to us as about a seventeenth-magnitude star, which is almost at the limit of our vision with even the largest telescope. It seems to revolve about Jupiter in a direction exactly opposite to that of the other satellites--a retrograde motion that appears in the solar system in only two or three other cases and has not yet been entirely accounted for. Jupiter’s satellites have played an important part in astronomical discoveries and investigations. It was through observation of their transits that it was discovered that light occupied time in passing through space. When Jupiter was near us in his orbit, the eclipses occurred too soon for their calculated time; when he was farther away, they occurred too late. It was found that these irregularities were due to the fact that light is not transmitted through space instantaneously, and further investigation showed that it travels at the rate of 186,400 miles a second. The eclipses of Jupiter’s moons are carefully computed and recorded in the _Nautical Almanac_, and it is through observations of them that chronometers are corrected at sea. Ganymede and Callisto have been found to keep always the same face toward the planet, as our moon keeps always the same face toward us; and it is thought that all of Jupiter’s satellites probably do this. The symbol of Jupiter is ♃, a hieroglyph for the eagle, which was the bird of Jove. XIV SATURN Among the four planets that we commonly see, the easiest, perhaps, to keep track of is Saturn. Its peculiar aspect is very distinctly marked. It appears as a large, pale, yellow star shining with a soft, misty light that sometimes barely escapes being dull. It is always as bright as a first-magnitude star, but not always as bright as Sirius, and never as brilliant as Mars, Jupiter, or Venus when they are at their brightest. The general effect of it is as a large rather than a brilliant star.