Besides, they never are full together, seen from any one part of Jupiter. The four inner moons are never to be seen “full” at all; for just when they might be so, they are eclipsed or shaded by Jupiter’s shadow. The fourth sometimes escapes this eclipse, from being so much farther away. A thought has been lately put forward, which may or may not have truth in it. What if--instead of Jupiter being a world, inhabited by animals and people, as is often supposed, with a small distant sun and five dim moons to give them light--what if Jupiter is himself in some sort a second sun to his moons, and what if those “moons” are really inhabited planets? It may be so. That is all we can say. The idea is not an impossible one. The so-called “moons” are certainly small. But they are by no means too small for such a purpose. Jupiter would in that case, with his five moons circling round him, enjoying his light and warmth, be a small picture of the sun, with his four inner planets and the asteroids circling round him, basking in a more lavish amount of the same. Picturing the moons as giving light to Jupiter, we find them seemingly dim and weak for such a purpose--though, of course, we may here make a grand mistake in supposing the eyesight of living creatures in Jupiter to be no better than our own eyesight. Even upon earth a cat can see plainly where a man has to grope his way in darkness. But by picturing the moons as inhabited, and Jupiter as giving out some measure of heat and light to make up for the lessened amount of light and heat received from the sun, the matter becomes more easy to our understanding. [Illustration: THE SYSTEM OF JUPITER.] The nearest moon has indeed a magnificent view of Jupiter as a huge bright disk in its sky, no less than three thousand times as large as our moon appears to us, shining brightly with reflected sunlight, and it may be glowing with a red light of his own in addition. Even the farthest off of the five sees him with a face sixty-five times the size of our moon. And the varying colors and stormy changes in the cloud-belts, viewed thus near at hand, must afford marvelously beautiful effects. Just as Mercury, Venus, Earth, and Mars travel round the sun, at different distances, nearly in the same plane, so Jupiter’s five moons travel round him, at different distances, nearly in the same plane. Jupiter’s moons are always to be seen in a line, not one high and another low, one near his pole and another near his equator. The moon nearest to Jupiter, discovered in September, 1892, by Professor Barnard, now of the Yerkes Observatory of the Chicago University, has a probable diameter of one hundred miles, and revolves round its primary in about twelve hours. The second satellite, named Io, is said to be over two thousand miles in diameter, travels round Jupiter in less than two of our days, and is eclipsed by Jupiter’s shadow once in every forty-two hours.