JUPITER'S FAMILY OF MOONS. Lift up your eyes on high, and behold who hath created these things. ISAIAH xl. 26. We have seen that Jupiter, although the chief planet of the solar system, is probably not the abode of any living creatures, and is certainly unfitted to be the abode of such creatures as we are familiar with. Yet his enormous bulk and mass, the noble sweep of his orbit, the importance, beauty, and symmetry of the system over which he bears sway, all suggest the idea that he was not created in vain. Nor can we readily conceive any purpose he can fulfil save that of supplying or helping to supply the wants of living creatures. In fact, it is in this way that we view all the celestial bodies. We are not contented when studying the sun, for example, with the mere consideration of the wonderful processes taking place upon his surface and around him; but we enquire how these processes are related to his power of supplying our wants, and the wants of all that live upon the earth, by means of the light and heat which he emits. We study our moon in the same spirit; we see that whether she be herself inhabited or not, she was not created in vain-she rules our tides, she gives us an important though intermitting supply of light by night, she serves as a measure of time, she helps to guide the seaman over the trackless waves of ocean, and she subserves our wants in a variety of other ways. And it is the same method of viewing the celestial bodies which has led nearly all men to believe in the existence of multitudes of other worlds than ours. Now, when we apply these considerations to Jupiter and his system, we find in his moon-family an increase to the difficulty which has already engaged our attention. For what are those four orbs intended? If Jupiter is not inhabited, they serve none of the requirements which our own moon fulfils. If Jupiter is inhabited, the moons still seem to be of little use. For we can see that Jupiter has a very dense and vapour-laden atmosphere, and it is altogether improbable that any of the moons can be seen from his real surface, supposing he has a surface, which is very far from being a certainty. But even if we suppose that his moons can be seen, they can supply very little light during the planet's night. A different opinion has long been entertained, owing to the details of the matter being left unconsidered. Sir David Brewster, for instance, in his pleasant little work called 'More Worlds than One,' has dwelt upon the moons of Jupiter as obviously fulfilling the important purpose of compensating the planet for the small amount of light received from the sun; and he speaks of the wonderful beauty of the scene presented by Jupiter's moons when all visible at once. But if we con sider the actual circumstances under which these moons are placed, we shall find that they cannot be even so effective as our own single moon in supplying light to their primary planet, while we know well that the light of ten such moons as ours would be but a poor compensation for the loss of twenty-four parts (out of twenty-five) of the sun's light and heat. These moons lie at such distances from Jupiter, that while the nearest looks considerably larger than our moon,' all the others look far less. The farthest, indeed, must show a disc little more than a quarter of our moon's in diameter, and about a fourteenth of the moon's disc in apparent size. But of course all these moons together cover a considerably larger part of the sky (when they are all seen at once) than ever is covered by our moon. It might seem, then, that they must give much more light. But then it must be remembered that they are themselves only illuminated by the same small sun which shines in the Jovian skies. Supposing them to be constituted like our own moon, the apparent brightness of their discs must be about a twenty-fifth part only of that of hers. When due account is taken of this circumstance, it is found that the full-moon brightness of At least when nearly overhead; but owing to the enormous size of Jupiter, there is a great difference in the apparent size of his nearest moon when high above the horizon and when low down. When overhead this moon is at its nearest, and shows a disc exceeding our moon's by more than a fifth part in diameter, and nearly half as large again in apparent size. But when near the horizon this moon is very little larger than our own. the four moons of Jupiter amounts only in all to a six teenth part of the brightness of our full moon. And even this is not all. The four moons never can be all full together, though they can be all above the horizon at the same time. The innermost, which of course looks the largest, is always eclipsed by the vast shadow of Jupiter, when directly opposite the sun; so that this moon is never seen full. The same applies to the second moon, which is, however, eclipsed on a much shorter part of its course. It appears, then, that the moons of Jupiter are utterly unfit to compensate for the defect of sunlight. But before we dismiss the Jovian family as useless moons, after already dismissing Jupiter as a useless world (speaking always of his adaptation to the wants of creatures living upon him), let us enquire whether we may not, by reversing the functions of planet and planet-family, obtain an explanation of both. Why should not the moons of Jupiter be inhabited, instead of Jupiter himself, and Jupiter be appointed to compensate them (not they him) for the smallness of the direct supply of solar light and heat? Here we must not be staggered by the great superiority of Jupiter in bulk and mass. We must remember That great Or bright infers not excellence: the earth, Whose virtue in itself works no effect, But in the fruitful earth; there first received, Jupiter's relation to his family of four moons does indeed resemble in a somewhat marked manner the relation of our sun to the four worlds-Mercury, Venus, the Earth, and Mars-which travel nearest to the central luminary, and are most peculiarly to be regarded as the sun's family. Jupiter surpasses each of his moons in bulk and mass in a degree corresponding to that in which the sun surpasses the four small planets just named (not equally, but the disproportion is of the same order). The third of his moons is the largest, just as this earth, the third of the sun's inner family of four planets, is the largest of that family. The other three moons are about equal together in mass to the largest, just as Mercury, Venus, and Mars are about equal together, in mass, to the earth, and the distances at which the moons travel are proportioned to each other somewhat like those observed in the case of the four small planets. We know that the distances of Mercury, Venus, and the Earth, and Mars from the sun are fairly represented by the numbers 4, 7, 10, and 16; those of Jupiter's family of moons are fairly represented by the numbers 4, 61, 10, and 18, which, under the circumstances, indicates a sufficiently close resemblance. In particular I would invite the reader to notice how complete is the contrast between the positions which the moons of Jupiter bear as compared with that |