Popular Science Monthly/Volume 4/November 1873/The Ringed Planet
| THE RINGED PLANET. |
DURING the months of September, October, and November, Mars and Saturn are companions as evening-stars. It will not be difficult to recognize them, though the ruddy glories of Mars have been greatly reduced since July and August, when he shared with Jupiter the dominion over the western skies after sunset. The dull-yellow lustre of Saturn differs markedly from the red but more star-like light of Mars; and, as the two planets draw near to each other late in November (making their nearest approach on the 20th), it will be interesting to observe the contrast between the red and yellow planets of the solar system. Striking, however, as this contrast will be found to be, it is insignificant compared with the real contrast which exists between the two planets. Mars is the least but one of the primary members of the solar family, and, although he pursues a course outside the earth's, he is unlike all the other superior planets in being unaccompanied by any moon; his small orb, also, appears to have but a shallow atmospheric envelope, while in physical constitution he apparently occupies a position between the earth and the moon. Saturn, on the other hand, is inferior only to Jupiter in dimensions and mass, while he is superior to Jupiter not only in the astronomical sense that he travels on a wider orbit, but in the extent and importance of the scheme over which he bears sway; his orb, moreover, like that of Jupiter, appears to be the scene of marvelous processes of change, implying a condition altogether unlike that of the earth on which we live.
We propose to give a brief sketch of what has been ascertained respecting this wonderful planet, the most beautiful telescopic object in the whole heavens, and the one which throws the clearest light upon the nature of the solar system, and particularly of those giant planets which circle outside the zone of asteroids.
We would at the outset impress upon the reader the necessity of raising his thoughts above those feeble conceptions respecting Saturn and his system which are suggested by the ordinary pictures of the planet. When we see Saturn presented as a ball within a ring, or more carefully pictured as a striped globe within a system of rings, we are apt to regard the ideas suggested by such drawings as affording a true estimate of the planet's nature. In fact, many believe that the planet and its rings are really like what is presented in these pictures. It should be understood that what has been actually seen of Saturn by telescopic means cannot, in the nature of things, afford any true picture of the planet and its ring system. The picture must be filled in, not by the imagination, but by the aid of reason; and then, though much will still remain unknown, we shall have at least a far juster conception of the glories of the ringed world than when we simply contemplate drawings which show how the planet looks under telescopic scrutiny. This will at once appear when we consider that Saturn never lies at a less distance than 732,000,000 miles from the earth. With the most powerful telescope we see him no better (taking atmospheric effects into account) than we should if this distance were reduced to about a million miles. It is manifest that at this enormous distance all save the general features of his globe and of his rings must be indistinguishable. Where we seem to see a smooth, solid globe striped with belts, there may be an orb no part of which is solid,
Fig. 1.
Telescopic Aspect of Saturn, and Size compared with the Earth.
girt round by masses of matter lying many miles above its seeming surface. Where we seem to see solid, flat rings, neatly divided one from the other either by dark spaces or by difference of tint, there may be no continuous rings at all; the apparent spaces may be no real gaps; the difference of tint may imply no difference of material. On these and other points, the known facts afford important evidence, and, by reasoning upon them, we are carried far beyond the results directly conveyed to us by telescopic researches.
Saturn is distinguished, in the first place, by the enormous range of his orbit, not merely in distance from the sun, but in the distances which separate it from the orbits of his neighbor planets. His mean distance from the sun is about 872,000,000 miles, his actual range of distance lying between 921,000,000 and 823,000,000. These figures are imposing, but they are, in fact, meaningless save by comparison with other distances of the same class. Let it be noticed, then, that Saturn's mean distance from the sun exceeds the earth's more than nine and a half times. Now, Jupiter's distance exceeds the earth's rather more than five times (five and a fifth is very nearly the true proportion); so that between Jupiter's path and Saturn's there lies everywhere a span fully equal to four times the earth's distance from the sun. So much for Saturn's nearest neighbor on that side. But on the farthest side lies Uranus, more than nineteen times as far away from the sun as our earth is; so that between the paths of Saturn and Uranus there lies everywhere a span equal to Saturn's own distance from the sun. Now, all this is not intended as a mere display of wonderful distances. So far as mere dimensions are concerned, these arrays of figures are more imposing than impressive. But, so soon as we take into account the circumstance that a planet is in some sense ruler over the spaces through which its course carries it, those spaces being by no means tenantless, we see that, cæteris paribus, the dignity of a planet is enhanced by the extent of the space separating its orbit from the orbits of its neighbors on either side. Now, the space between the paths of Saturn and Jupiter exceeds the space inclosed by the earth's orbit no less than 63 times, while the space between the paths of Saturn and Uranus exceeds the space inclosed by the earth's orbit 270 times! Assuming (as we seem compelled to do by continually-growing evidence) that Saturn and his system were formed by the gathering in of matter from the region over which Saturn now bears sway, we cannot wonder that the planet is a giant, and his system wonderful in extent and complexity of structure. It is true that Jupiter on one side, and Uranus on the other, share Saturn's rule over the vast space, 330 times the whole space circled round by the earth, which lies between the orbits of his neighbor planets. But Saturn's rule is almost supreme over the greater part of that enormous space. Combining the vastness of the space with its position—not so near to the sun that solar influence can greatly interfere with Saturn's, nor so far away as to approach the relatively-barren outskirts of the solar system—we seem to find a sufficient explanation of Saturn's high position in the scheme of the planets as respects volume and mass, and his foremost position as respects the complexity of the system over which he bears sway.
Briefly, then, to indicate his proportions, and the dimensions of his system:
Saturn has a globe considerably flattened, his equatorial diameter being about 70,000 miles, while his polar axis is nearly 7,000 miles shorter. Thus in volume he exceeds the earth nearly 700 times, and all the four terrestrial planets––Mercury, Venus, the Earth, and Mars—taken together, more than 336 times. In mass he does not exceed the earth and these other smaller planets so enormously, because his density (regarding him as a whole) is much less than the earth's. In fact, his density is less than that of any other known body (comets, of course, excepted) in the solar system. The reader is doubtless aware that the sun's mean density is almost exactly one-fourth of the earth's; Jupiter's is almost exactly the same as the sun's; but Saturn's is little more than half the sun's, being represented by the number 13 only, where 100 represents the earth's. Thus, instead of exceeding the earth nearly 700 times in mass, as he would if he were of the same density, he exceeds her but about 90 times. But this disproportion must still be regarded as enormous, especially when it is added that the combined mass of the four terrestrial planets amounts to little more than the forty-fourth part of Saturn's mass. The combined mass of Uranus and Neptune, though these are members of the family of major planets, falls short of one-third of Saturn's mass; yet, by comparison with Jupiter, whose mass exceeds his more than threefold, Saturn appears almost dwarfed. And it may be noted as a striking circumstance—one that is not sufficiently recognized in our astronomical treatises—that, while Jupiter's mass exceeds the combined mass of all the other planets (including Saturn) about two and a half times, Saturn exceeds all the remaining planets in mass about two and three-quarter times. So unequally is the material of the planetary system distributed.
Fig. 2.
Saturn and his Moons.
The mighty globe of Saturn rotates on its axis in about nine hours and a half, the most rapid rotation in the solar system so far as is yet known.
But what shall we say to indicate adequately the dimensions of that enormous ring-system which circles around Saturn? Here we have no unit of comparison, and scarcely any mode of presenting the facts except the mere statement of numerical relations. Thus the full span of the rings, measured across the centre of the planet, amounts to 167,000 miles; the full breadth of the ring-system amounts to 35,600 miles. But these numbers convey only imperfect ideas. Perhaps the best way of indicating the enormous extent of the ring-system is to mention that circumnavigation of the world by a ship sailing from England to New Zealand by the Cape of Good Hope, and from New Zealand to England by Cape Horn, would have to be repeated 21 times to give a distance equaling the outer circumference of the ring-system. The same double journey amounts in distance to but about two-thirds the breadth of the ring-system.
As to the scale on which Saturn's system of satellites is constructed, we shall merely remark that the span of the outermost satellite's orbit exceeds nearly twofold the complete span of the Jovian system of satellites, and exceeds the span of our moon's orbit nearly tenfold.
And now let us consider what is the probable nature of the vast orb which travels girt round always by its mighty ring-system—at so enormous a distance from the sun that his disk has but the ninetieth part of the size of the solar disk we see. Have we in Saturn, as has been so long the ordinary teaching of astronomy, a world like our own, though larger—the abode of millions on millions of living creatures—or must we adopt a totally different view of the planet, regarding it as differing as much from our earth as our earth differs from the moon, or as Saturn and Jupiter differ from the sun?
We must confess that, if we set on one side altogether the ideas received from books on astronomy, endeavoring to view these questions independently of all preconceived opinions, it appears antecedently improbable that Saturn or Jupiter can resemble the earth either in attributes or purpose. We conceive that, if a being capable of traversing at will the interstellar spaces were to approach the neighborhood of our solar system, and to form his opinion respecting it from what he had observed in other parts of the sidereal universe, he would regard Jupiter and Saturn, the brother giants of our system, as resembling rather those companion orbs which are seen in the case of certain unequal double stars, than small dependent worlds like our earth and Venus. There are, perhaps, no instances known to our telescopists in which the disparity of light, as distinguished from real magnitude, is quite so great as that which exists in the case of the sun and the two chief planets of the solar system.[1] But we see in the heaven of the fixed stars all orders of disproportion between double stars, from the closest approach to equality down to such extreme inequality, that, while the larger star of the pair is one of the leading brilliants of the heavens, the smaller can only just be discerned with the largest telescopes yet made, used on the darkest and clearest nights. We have no reason to believe that the series stops just where our power of tracing it ceases; on the contrary, since the series is continuous as far as it goes, and since our own solar system is constituted as if it belonged to the series prolonged far beyond the limits which telescopic scrutiny has reached, we have reason for believing that such is indeed the interpretation of the observed facts. In other words, we may not unreasonably regard our solar system as a multiple system, a double star at certain ranges of distance, whence only the sun and Jupiter could be seen; a triple star at distances whence Saturn could be seen; and a quintuple star where Uranus and Neptune would come into view. To show what excellent reason exists for regarding Mercury, Venus, the Earth, and Mars, as not to be included in this view, it is only necessary to remark that not one of these planets could be seen until the limits of the solar system had been crossed. To eyesight such as ours, not one of the four terrestrial planets could be seen from Saturn, and still less, of course, from Uranus or Neptune. It would be as unreasonable to hold the ring of asteroids, or even the myriads of systems of meteorolites and aërolites, to be bodies resembling the earth and her fellow-terrestrial planets, as it is to hold these terrestrial planets to be bodies resembling Jupiter and his fellow-giants.
In all characteristics yet recognized by astronomers, Jupiter and Saturn differ most markedly from the earth and her fellow-planets. In bulk and mass they belong manifestly to a different order of created things; in density they differ more from the earth than the sun does; they rotate much more swiftly on their axes; they receive much less light and heat from the sun; the lengths of their year exceed the length of the earth's year as remarkably as their day falls short of hers; the atmospheric envelope of each is divided into variable belts, utterly unlike any thing existing in the earth's atmosphere; and, lastly, each is the centre of an important subsidiary scheme of bodies quite unlike the moon (the only secondary planet in the terrestrial family) as respects their relations to the primary around which they travel.
Notwithstanding all these circumstances in evidence of utter dissimilarity, and the fact that not one circumstance in the condition of the major planets suggests resemblance to the terrestrial planets, astronomy continues to treat of the planets of the solar system as though they formed a single family. It would appear as though the teachings of the astronomers who lived before the telescope was invented had so strong an inherent vitality, that more than two centuries and a half of discoveries adverse to those teachings are powerless to dispossess them of their authority. For no other reason can be suggested, as it appears to me, for the complete disregard with which the most striking characteristics of the major planets have been treated by modern astronomers.
If we consider one feature alone of those which have been just mentioned—the small mean density of the giant planets—we have at once the strongest possible evidence to show that the condition of these bodies must be unlike that of the earth. Of course, if we assume that Saturn's substance (to limit our attention to this planet) is composed of materials altogether unlike any which exist on earth, a way out of our difficulty is found, though not an easy one. In that case, however, we are only substituting one form of complete dissimilarity for another. And all the results of spectroscopic analysis, as applied to the celestial bodies, tend to show the improbability that such differences of elementary constitution exist—we will not say in the solar system only, but in the sidereal universe itself. If, however, we admit that Saturn is in the main constituted of elements such as we are familiar with, we find it extremely difficult, or rather it is absolutely impossible, to suppose that the condition of his substance is like that of the earth's. There are certain unmistakable facts to be accounted for. There is the mighty mass of Saturn, exceeding that of the earth ninety-fold. That mass is endued with gravitating energy, precisely in the same way as the earth's mass. There must be from the surface toward the centre a continually increasing pressure. This pressure is calculable,[2] and enormously exceeds the internal pressures existing within the earth's interior. There is no possibility of cavities, as Brewster and others have opined; for there is no known material, not the strongest known to us, iron, or platinum, or adamant, which could resist the pressures produced by Saturn's internal gravitation. Steel would be as yielding as water under these pressures. There must be compression with its consequent increase of density, such compression exceeding many million-fold the greatest with which terrestrial experimenters have dealt. That, with these enormous forces at work, the actual density of Saturn as a whole should be far less than that of water is utterly inexplicable, unless Saturn's condition be regarded as altogether unlike that of the earth. We see in the sun an orb which, notwithstanding its enormous mass, has a mean density much less than the earth's, and little greater than that of water; but we have no difficulty in understanding this circumstance, because we see that the sun is in a state of intense heat, and we know that this heat produces effects antagonistic, as it were, to those produced by the attraction of his mass as a whole upon every portion of his substance. But, if we make no similar assumption in Saturn's case, we find his small density inexplicable.
Another circumstance associated with the question of Saturn's density introduces new difficulties of the most perplexing nature if it be regarded according to the ordinary view, while it seems not only explicable, but manifestly to be expected, on the theory that Saturn's whole orb is in an intensely heated condition. Saturn certainly has an atmosphere of considerable depth. The belts which surround his globe are evidently produced by clouds in his atmosphere, though what the nature of these clouds may be is not as yet known. The brighter belts are the cloud-belts, while the darker either show his real surface, or, far more probably, belong simply to lower cloud-layers. These belts are variable in appearance and position, sometimes changing with great rapidity. Their real extent is enormous, exceeding the whole surface of our earth, even in the case of the narrowest belts yet seen. No one who has viewed them through telescopes of great power can refuse to adopt the conclusion that the atmosphere in which these great cloud-zones are suspended must be of great depth, certainly far deeper than our atmosphere. But such an atmosphere, subjected to the attractions of Saturn's mass, would be enormously compressed underneath those manifestly thick cloud-layers. A very moderate assumption as to the depth of the atmosphere would lead to the conclusion that at its base it must be denser than water—that is, denser than Saturn himself. No gas could exist as gas at this density. Apart from this, we are here arriving at the very theory which the ordinary view of Saturn teaches us to avoid—viz., the theory that he is utterly unlike our earth in physical condition. We may much more conveniently arrive at the same general conclusion, while avoiding other difficulties, by simply adopting the same explanation in this case which serves to account also for the small density of Saturn's mass—viz., the theory that Saturn's globe is in a state of intense heat.
But now let it be noticed how perfectly this view of Saturn's condition accords with the theories which are beginning to be established respecting the genesis of the solar system. Whether we regard the planets as formed from the condensation of enormous nebulous masses, or whether we assume that they were produced by the gathering together of matter originally traveling in dense meteoric flights around the central aggregation whence the sun was one day to be formed, we see that the larger the planet the greater must have been its original heat. The heat generated during the condensation of a nebulous mass must depend upon the magnitude of the mass, since in fact the accepted theory of heat teaches us that the original heat of a globe so formed is measurable by the actual difference in dimensions between the globe and its parent cloud-mass, and of course the larger the cloud-mass the greater this difference would necessarily be. It is equally certain that the heat generated by the gathering-in of meteoric matter would be so much the greater according as the quantity of matter gathered and gathering was greater; for the heat is produced by the downfall of such matter on the globe it helps to form, and the greater the mass of that globe the greater is its attracting might, the greater the velocity it generates in the falling meteors, and therefore the greater the heat produced when they are brought to rest.
Saturn, then, would originally be much hotter than our earth, on any theory of the evolution of our solar system—and there are few astronomers who doubt that the solar system was wrought by processes of evolution to its present condition. But not only would Saturn be much hotter than the earth, but, owing to his enormous size, he would part with his heat at a much slower rate. On both accounts we should infer that at this present time Saturn is much hotter than the earth—in other words, since our earth still retains no inconsiderable proportion of its original heat, Saturn may be assumed to be in a state of intense heat. What his actual heat may be is not so easily determined. We shall presently show reasons for believing that an inferior limit, below which his heat does not lie, is indicated by the fact that he still possesses inherent luminosity. On the other hand, a superior limit is indicated by the fact that his inherent luminosity is not great, and that, in all probability, the thicker cloud-zones of Saturn prevent the passage of the greater part of his light.[3]
We should infer, then, that Saturn in some respects resembles the sun, though of course the very same reasoning which teaches us to believe that Saturn is very much hotter than the earth, leads us also to the conclusion that it is not nearly so hot as the sun. Now, thus viewing Saturn, we should be led to expect, apart from all telescopic evidence to that effect, that he would resemble the sun in certain general features. For instance, we might expect that he would have spot-zones, while his equatorial zone would be free from spots; or, if it were thought that so close a resemblance was not to be looked for, then we might still expect that his equatorial zone, like the sun's, would be distinguished from the rest of his surface by some well-marked peculiarity. This is the case. The equatorial zone of Saturn is distinguished by a peculiar brightness from the rest of his surface, insomuch that the late Prof. Nichol was led to regard this zone as the scene of a constant precipitation of meteoric matter from the inside of the ring-system.
Now, there is one important peculiarity which distinguishes the equatorial bright zone of Saturn from that of Jupiter. Jupiter's axis is almost square to the level of the path in which he travels around the sun; so that his equatorial zone lies nearly in that level, and is therefore directly illuminated by the sun. The aspect of Jupiter in fact, as seen from the sun, is always that which our earth presents in spring and autumn. But Saturn has an axis very considerably sloped to the level of the path in which he travels. It is more sloped even than our earth's axis. So that in the course of his long year of 10,759 days (29½ of our years) Saturn's globe presents toward the sun all the varying aspects which our earth presents, only with a somewhat greater range of variation. At one time he is placed as our earth is in spring, and then his equatorial belt, as seen from the sun, appears to lie straight across the middle of his disk. Rather more than seven years later he is posed as our earth is posed at midsummer, his northern pole is bowed toward the sun, and his equator is seen as a half-oval, curving far south of the middle point of his disk. He passes on from this position, and in seven more years he is placed as our earth is in autumn, with his equator again lying straight across his disk. Then, seven years or so later, he presents the aspect of our earth at midwinter, his equator curved into a half-oval passing far to the north of the middle point of his disk. And, finally, at the end of yet seven years more (or, more exactly, of one complete Saturnian year from the commencement of these changes), he is again as at first. Now, it seems manifest that, if the great cloud-zone which surrounds Saturn, appearing as a nearly white ring, were due to solar action, it would fluctuate in position as these changes proceeded. The very length of the Saturnian year should insure the occurrence of such fluctuations. We have only to inquire what takes place on our own earth, where, though we have nothing comparable with the belt systems of Jupiter and Saturn, we have, nevertheless, over ocean-regions, a sun-raised tropical cloud-band in the middle of the day. This cloud-band follows the sun, being equatorial in spring, passing far north of the equator, even to the very limit of the torrid zone, in summer, returning to the equator in autumn, passing to the southern limit of the torrid zone in winter, and returning again to the equator in spring. In fact, this cloud-band as seen from the sun would always cross the middle of the earth's face as a straight line in spring and autumn, and as considerably more than a half-oval, agreeing in position with the tropics of Cancer and Capricorn, at midsummer and midwinter. But nothing of the sort happens in Saturn's case. His equatorial white ring is really equatorial at all times, instead of being drawn to his tropics at his midsummer and midwinter seasons. This, in our opinion, is decisive of the origin of this great band. If it were sun-raised, it would obey the sun; but, being raised by Saturnian action, its position is solely determined by Saturn's rotation, and it therefore remains constantly equatorial.
But next a very strange and, at a first view, incredible circumstance has to be considered in immediate connection with the relations we have been dealing with.
It sounds startling to suggest that Saturn probably changes at times in size and shape. Yet the evidence in favor of the suggestion is very weighty. It may briefly be presented as follows:
In April, 1805, Sir William Herschel, who had hitherto always seen the planet of an oval figure, found that it presented a strangely distorted appearance. It was flattened as usual at the poles, but also at the equator; accordingly, it had a quadrangular or oblong figure (with rounded corners, of course), its longest diameters being the two which (crossing each other in the middle of the disk) passed from north latitude 43° on Saturn to the same southerly latitude. Or we may otherwise describe the appearances presented, by saying that Saturn seemed swollen in both the temperate zones. Herschel found that the same appearance was presented, no matter what telescope he employed, and he tried many, some seven feet, some ten, one twenty, and one forty feet in length. With these telescopes Jupiter presented his ordinary oval aspect. But Herschel is not the only astronomer by whom such appearances have been noticed. On August 5, 1805, Schröter found that Saturn's figure was distorted. Dr. Kitchener says that in the autumn of 1818 he found Saturn to have the figure described by Herschel. The present Astronomer Royal has seen Saturn similarly distorted, and on another occasion flattened in the temperate zones. In January, 1855, Coolidge, with the splendid refractor of the Cambridge (U. S.) Observatory noticed a swollen appearance in Saturnian latitude 20°; yet on the 9th the planet had resumed its usual aspect. In the report of the Greenwich Observatory for 1860-'61, it is stated that "Saturn has sometimes appeared to exhibit the square-shouldered aspect." The two Bonds, of America, surpassed by few in observing skill, have seen Saturn square-shouldered and have noticed variations of shape. It seems impossible to reject such testimony as this. Nor can it be disposed of by showing that ordinarily Saturn presents a perfectly elliptical figure. It is the essential point of the circumstances we are considering, that they are unusual.
Now, we do not pretend to explain how such changes of shape are brought about. But we would invite special attention to the circumstance that if these changes be admitted as having occasionally occurred (and we do not see how they can be called in question), then the result is only startling in connection with that theory of Saturn's condition which we are here opposing. If Saturn be a globe resembling our earth, then sinkings and upheavals, such as these appearances indicate, must be regarded as involving amazing and most stupendous throes—as in fact absolutely incredible, no matter what evidence may be found in their favor. But, so soon as we regard Saturn's whole globe as in a state of intense heat, and his belt-system as indicating the continual action of forces of enormous activity, we no longer find any difficulty in understanding the possibility of changes such as Sir W. Herschel, Sir G. Airy, the Bonds, and others of like observing skill, have seen with some of the finest reflecting and refracting telescopes ever constructed by man. Nay, we may even go further, and find in solar phenomena certain reasons for believing that Saturn's globe would be subjected to precisely such changes. It appears to have been rendered extremely probable by Secchi and others, that our sun's globe varies in dimensions under the varying influences to which he is subjected. At the height of the spot-period the sun seems to be reduced in diameter, while his colored sierra is deeper, and the red prominences are larger than usual, the reverse holding at the time when the sun has no spots or few. Of course this is not understood as implying a real change in the quantity of solar matter, but only as indicating the varying level at which the solar cloud-envelope lies. We may safely assume that these changes, which correspond to the great spot-period, affect chiefly the spot-zones which lie in the parts of the sun's globe corresponding to our temperate zones; but, for the same reasons that the sun's globe is perfectly spherical so far as measurements can be depended upon, namely, because of its relatively slow rotation—such differences would be too slight to be measurable. Regarding Saturn, then, as we have already been compelled to do for other reasons, as resembling the sun so far that he is in an intensely heated condition, we see grounds for believing that his temperate zones would be exposed to variations of level (cloud-level), which at times might be very considerable, and thus discernible from our earth. For, owing to his rapid rotation on his axis, all such effects would be relatively greater than on a slowly rotating orb like the sun; and in fact we recognize this distinction in the great compression of Saturn's globe. Moreover, if we regard the waxing and waning of the solar spots as associated with the motions of the members of the sun's family, we can well understand that the members of Saturn's family, which lie so much nearer to him compared with his own dimensions, should produce more remarkable effects.[4] But, whether this be so or not, it is certain that, whereas there is nothing inexplicable or even very surprising in supposing that Saturnian cloud-layers, resulting from the action of intense Saturnian heat, alter greatly at times in level, the observations we have described become altogether inexplicable, and cannot, in fact, be rejected, if we adopt the theory that Saturn resembles the earth on which we live.
It may be asked whether Jupiter, to which planet the same reasoning may be applied, has ever shown signs of similar changes. To this it may first be replied, that we should not expect Jupiter to be affected to the same degree, simply because the chief disturbing causes—his satellites and the sun—are always nearly in the same level, owing to the peculiarity in Jupiter's pose to which attention has already been directed. But, secondly, such briefly-lasting changes as we might expect to detect have occasionally been suspected by observers of considerable skill; and among others by the well-known Schröter, of Lilienthal. Such changes have consisted, for the most part, merely in a slight flattening of a part of Jupiter's outline. But on one occasion a very remarkable phenomenon, only (but very readily) explicable in this way, was witnessed by three practised observers—Admiral Smyth, Prof. Pearson, and Sir T. Maclear—at three different stations. Admiral Smyth thus describes what he saw: "On Thursday, June 26, 1828, the evening being extremely fine, I was watching the second satellite of Jupiter as it gradually approached to transit Jupiter's disk. It appeared in contact at about half-past ten, and for some minutes remained on the edge of the disk, presenting an appearance not unlike that of the lunar mountains coming into view during the moon's first quarter, until it finally disappeared on the body of the planet. At least twelve or thirteen minutes must have elapsed, when, accidentally turning to Jupiter again, to my astonishment I perceived the same satellite outside the disk! It remained distinctly visible for at least four minutes, and then suddenly vanished!" For our own part, we can conceive of no possible explanation of this remarkable phenomenon, unless it be admitted that the change was in the apparent outline of Jupiter. Of course, to suppose that even a cloud-layer rose or fell, in a few minutes, several thousand miles (about 8,000, if the stated times be correct), is as inadmissible as to suppose the solid crust of a globe to undergo so vast a change of level; but nothing of this sensational description is required. All that would be necessary would be, that an upper cloud-layer should for a few minutes be dissipated into vapor, either by warm currents, or more probably by a temporary increase of the heat supplied by Jupiter's fiery globe within the cloud-envelopes, and that a few minutes later the clouds should form again by the condensation of the vaporized matter. The changes in the aspect of the Jovian belts are often sufficiently rapid to indicate the operation of precisely such processes.
Associated with such phenomena as we have mentioned is the evidence we have as to the brightness of Saturn and Jupiter. If these planets were perfectly cloud-encompassed, we should expect them to shine much more brightly than earthy or rocky globes of equal size, similarly placed, and surrounded only with a tenuous atmosphere. In fact, we should expect the planets, if cloud-encompassed, to shine about four times as brightly as though they were constituted like our moon. They would in that case, however, be white planets, not only as seen by the naked eye, but when examined with the telescope. In point of fact, they shine, according to the very careful measurements of Zöllner, about as brightly as though they were perfectly cloud-enveloped; but they are neither of them found to be white under telescopic scrutiny. Bond, of America, says, indeed, that Jupiter shines fourteen times as brightly as he would if constituted like the moon; and though this is a surprising result, and would imply that some portion of Jupiter's light is certainly inherent, it is well to notice that it is confirmed by De La Rue's photographic researches; for he found that a photographic image of the moon can be taken in about two-thirds of the time required in Jupiter's case, whereas the moon should require but a twenty-fifth of the time required by Jupiter, if her reflecting power were equal to his, since Jupiter is five times as far away from the sun. It would follow from this that Jupiter shines nearly seventeen times as brightly as he would if he were constituted like the moon. Taking the lowest estimate, however, we find that both Saturn and Jupiter shine much more brightly than planets of equal size and similarly placed, but having a surface formed of any kind of earth or rock known to us. And, taking into account the well-marked colors of these planets, it follows as an almost demonstrated fact that each shines with no inconsiderable portion of inherent light.[5]
So soon as we view Saturn as a globe intensely heated, and the scene of forces of enormous energy, we are compelled to dismiss the idea that he is the abode of life. But, singularly enough, this conclusion, which was rejected by Brewster as rendering apparently unintelligible the existence of so large and massive an orb, girt about by a system so complex and beautiful, does in reality at once present, in an explicable aspect, not merely the vast bulk of Saturn himself, but the scheme over which he bears sway; for, as it seems to us, not the least of the objections against the theory that Saturn is an inhabited world, is found in the useless wealth of material exhibited, on that supposition, in his ring-system and family of satellites. It is very well to grow rapturous, as many besides Brewster and Chalmers have done, over the beauty of the Saturnian skies, illuminated by so many satellites and by the glorious rings; and it is very proper, no doubt, for those who so view Saturn's system, to dwell admiringly on the beneficence with which all this abundance of reflected light has been provided, to make up to the Saturnians for the small amount of light and heat which they receive from the sun. But, unfortunately for this way of viewing the matter, the satellites and rings do not by any means subserve the purposes thus ascribed to them. Even if all the satellites could be full together, they would not supply a sixteenth part of the light which we receive from our full moon; and they cannot even appear very beautiful when we consider that the apparent brightness of their surface can be but about one-ninetieth of the brightness of our moon's. As for the rings, so far from appearing to be contrived specially for the advantage of Saturnian beings, these rings, if Saturn were inhabited, would be the most mischievous and inconvenient appendages possible. They would give light during the summer nights, indeed, when light was little wanted, though even this service would be counteracted by the circumstance that at midnight the enormous shadow of the planet would hide the greater part of the rings. But it is in winter that the rings would act most inconveniently; for then, just at the season when the Saturnians would most require an additional supply of light and heat, the rings would cut off for extensive regions on Saturn the whole of the solar light and heat which would otherwise be received. Dr. Lardner was quite mistaken in supposing (after a cursory examination of the mathematical relations involved) that the eclipses so produced would be but partial. His object was excellent, since he sought to show that "the infinite skill of the Great Architect of the universe has not permitted that the stupendous annular appendage, the uses of which still remain undiscovered, should be the cause of such darkness and desolation to the inhabitants of the planet, and such an aggravation of the rigors of their fifteen years' winter," as would result from eclipses lasting many months or even years in succession. But we must not endeavor to strengthen faith in the wisdom of the Almighty by means of false mathematics. So soon as the subject is rigorously treated, we find that Sir John Herschel was quite right in his original statements on this subject. The present writer published, in 1865, a tabular statement of the length of time during which (according to rigid mathematical calculations) the eclipses produced by the rings last in different Saturnian latitudes. The following quotation from the work in which this table appeared will serve to show that the partial daily eclipses conceived by Lardner are very far from the truth, or rather are only a part, and a very small part, of the truth: "In latitude 40° (north or south), the eclipses begin when nearly three years have elapsed from the time of the autumnal equinox. The morning and evening eclipses continue for more than a year, gradually extending until the sun is eclipsed during the whole day. These total eclipses continue to the winter solstice, and for a corresponding period after the winter solstice; in all, for six years, 236 days, or 5,543 Saturnian days. This period is followed by more than a year of morning and evening eclipses. The total period during which eclipses of one kind or another take place is no less than eight years, 293 days. If we remember that latitude 40° on Saturn corresponds with the latitude of Madrid on our earth, it will be seen how largely the rings must influence the conditions of habitability of Saturn's globe, considered with reference to the wants of beings constituted like the inhabitants of our earth."[6] In the presence of such facts as these, we may follow Sir John Herschel in saying that "we should do wrong to judge of the fitness or unfitness of the arrangements described, from what we see around us, when perhaps the very combinations which convey to our minds only images of horror may be in reality theatres of the most striking and glorious displays of beneficent contrivance." But we do well to exercise our minds in inquiring how this may be; and, as it appears to us, the views which have been advocated in this essay at once afford an answer to this inquiry. We are taught to see in the Saturnian satellites a family of worlds dependent on him, in the same way that the members of the solar family are dependent on the sun. We see that, though the satellites can supply Saturn with very little light, he can supply them, whether by reflection or by inherent luminosity, with much. And, lastly, we see that the ring-system (which has been shown to consist of a multitude of small bodies, each traveling in its own course), while causing no inconvenience by eclipsing parts of Saturn, may not improbably serve highly-important purposes by maintaining an incessant downfall of meteoric matter upon his surface, and thus sustaining the Saturnian heat, in a manner not unlike that in which it is now generally believed that a portion at least of the sun's heat-supply is maintained by the fall of interplanetary meteors. In fine, we see in Saturn and his system a miniature, and a singularly truthful miniature, of the solar system. In one system, as in the other, there is a central orb, far surpassing all the members of the system in bulk and mass; in each system there are eight orbs circling around the central body; and, lastly, each system exhibits, close by the central orb, a multitude of discrete bodies—the zodiacal light in the solar system, and the scheme of rings in the Saturnian system—doubtless subserving important though as yet unexplained purposes in the economy of the systems to which they belong.—Cornhill Magazine.
- ↑ Even this is not certain. Jupiter, seen in full illumination from a stand-point so distant that both Jupiter and the sun might be regarded as equally distant from it, would appear to shine with rather more than the 3,000th part of the sun's light. This would correspond to the difference of apparent brightness between two stars of equal real magnitude and splendor, whereof one was about 54 times as far away as the other. There can be no doubt that the larger reflectors of the Herschels, Rosse, and Lassell, and the great refractors of Greenwich, Pulkowa, and Cambridge (United States), would bring the farther of two such stars into view if the nearer were of the first or second magnitude; and it is not at all unlikely that some of the exceedingly minute companions to bright stars, disclosed by these instruments, may be planets shining with reflected, not with inherent lustre.
- ↑ It is a misfortune for science that Newton never published the reasoning which led him to the conclusion that the earth's mean density is equal to between five and six times the density of water. This, as everyone knows, has been confirmed by several experimental methods; and, so far as appears, the problem is a purely experimental one. Newton, however, made no experiments; at least, none have been heard of as effected by him, and it is scarcely probable that he had any instruments of sufficient delicacy for a task so difficult. Prof. Grant ascribes Newton's conclusion to a happy intuition; yet it is very unlike Newton to make a guess on such a matter. It is more probable* that he guessed the elements of the problem than the result. He probably assumed that the earth's mass is composed of a substance like granite, and, adopting some law of compression for such a substance (based on experiment, perhaps), calculated thence the compression at different depths, and so obtained the mean destiny of the whole mass.
- ↑ To prevent misapprehension, it may be as well to remind the reader that the apparent continuity of Saturn's cloud-belts by no means implies that they are really continuous, and it is on a priori grounds highly improbable that they are so; openings in his cloud-zones two or three hundred miles in length and breadth would be quite undiscernible at Saturn's enormous distance.
- ↑ It must not be understood that in thus speaking we countenance the theory that either the planets produce the sun-spots, or the satellites of Saturn effect the remarkable changes we have been dealing with. The real causes of all solar phenomena must be sought in the sun's own globe; and Saturnian phenomena are in the main, we have little doubt, produced by Saturnian action. But even as our moon (probably) exerts an influence on the occurrence of earthquakes and volcanoes, not by her own attraction directly, but by affecting the balance between terrestrial forces, so it may well be that the planets indirectly affect the sun's condition, and that the Saturnian satellites even more effectually act upon Saturn. It would be extremely interesting to inquire whether any connection can be traced between the changes of the Saturnian belts and the motions of his satellites. Or the inquiry might be more readily and quite as effectually applied to Jupiter and his system.
- ↑ I might take, as equally convincing proof of the intensely heated condition of these giant planets, the fact that the shadows of the nearer satellites, which theoretically should be black, have sometimes been seen to be gray, and never appear to be much darker than the fourth satellite in transit. And, as sufficient proof of the great depth of Jupiter's atmosphere, I could take the fact that sometimes two shadows have been seen, both belonging to the same satellite. However, it would require more space than can here be spared to show the force of these facts. I remind the reader that whatever is proved respecting the condition of Jupiter, may be regarded as rendered probable of his brother giant, Saturn.
- ↑ As this passage has been quoted nearly verbatim, and without any sort of acknowledgment, in a compilation on "Elementary Astronomy," recently published, the present writer, that he may not be suspected of plagiarism, ventures to point out that it is not he who is the borrower.