God's glory in the heavens/The Structure Of Saturn's Rings

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Saturn's Rings.

XIII.
THE STRUCTURE OF SATURN'S RINGS

Galileo pointed to the system of Jupiter, as exhibiting the solar system in miniature, but, had he had adequate optical power, he could have pointed more triumphantly to that of Saturn. On the first discovery of the rings, they were regarded as a breach of uniformity in the solar system, but all subsequent discovery has tended to fill up the gap, and to shew that there is a sameness of pattern throughout the whole fabric. As in a Gothic cathedral, we do not find the pervading style merely in the spire and the great western entrance, but in every niche and window, however small; so, in the architecture of the heavens, we find one general idea, though with much diversity, in every part of the solar system.

Recent research has detected a striking resemblance between the solar and Saturnian systems. The discovery of the analogy was a long time retarded by the notion, that the rings must necessarily be solid. The idea of solidity is irresistibly forced upon the mind, when you look upon the rings, as sharply defined as the horizon circle round a school globe. And if a first impression be strong, very slender arguments are sufficient to confirm it. For example. Sir John Herschel concludes that they must be solid and opaque, because they cast a shadow upon the ball of the planet, and receive one in turn. But it is quite possible that they should cast a shadow, and, yet be not solid. A cloud, for example, casts and receives a shadow, though it is not opaque. A cloud, too, may assume such a solid aspect, that you cannot distinguish it from some snow-capped mountain.

The idea of solidity so strongly took possession of the minds of astronomers, that the most strained hypotheses were resorted to in order to account for the stability of the rings. How was the ball to be kept precisely in the middle of the solid ring? For, though originally adjusted with the utmost precision, the slightest disturbance would bring the ball in contact with the rings, just as an egg-shell floating in the middle of a tea-cup is in a state of unstable equilibrium from capillary attraction, so that the slightest disturbance of its central position brings it in contact with the sides of the cup. The satellites exercise such a disturbing force, so that if the movement once began, it would necessarily continue till the contact was effected; and, if the rings once touched the planet, they would be in a state of stable equilibrium, so that they would never rise from it. It was not a satisfactory reply that the rings rotated in the same time that a satellite, at the same distance, would. A satellite, making the appropriate number of revolutions in a given time, is in no danger of falling upon the primary, but the case is totally different with a ring. Should a comet come into collision with a satellite, and throw it a little out of its course, no further harm would result. The disturbance would not be progressive. The orbit might be altered, but then it would continue permanent in that altered course. If a comet, on the other hand, struck a ring so as to bring one side nearer the ball than the other, the motion would be progressive. It would be like that of a stone moved from the top of a hill, which does not stop till it reaches the bottom. The ring, once pushed by a satellite, virtually starts down the hill till it reaches the body of the planet, and it can no more rise from its prostrate position, than a stone can of itself move up hill. To meet this difficulty, Laplace resorted to an ingenious hypothesis. Let us suppose that the rigid solid ring is not of uniform density or thickness. Let us, for example, imagine that a portion of it is made of lead, while the rest is lighter than cork, and its gravitation insignificant in comparison; it is plain that such a system would approximate to that of a simple satellite, and so far be stable. The lead would move as a satellite, and the light portion, by supposition, is not sufficiently powerful to alter the conditions. If we attach a sovereign to a light ring of paper and throw both into the air, the curve described will be that due to the sovereign and not that due to the paper; simply because the gold so far preponderates, that the effect of the paper may be regarded as nil. This is, however, a wholly gratuitous supposition of Laplace. Observation does not confirm any notable difference in thickness or density. No doubt, when the edge of the ring is presented to us, some bright points remain when the rest disappears; but this appearance, first ascribed to lofty mountains, is not due to any fixed features of the ring; for the bright points are stationary, while the ring rotates. These points of light are believed to be due to the reflection of the inner edges of the rings presented to the sun. They do not appear to move, for the same reason that the sun's image does not move on with the flowing river from which it is reflected.

The next great difficulty in the rigid theory is, that the rate of rotation in different parts of the breadth of the ring does not conform to the conditions of stability. The inner edge of the ring should rotate much more rapidly than the outer—as the combined breadth of the two bright rings is 28,000 miles. They should rotate as satellites would do at corresponding distances. The nearer a satellite is to the body of a planet, the more rapidly must it revolve, in order that it may not fall upon the surface. If you swing a stone at the end of a string round your head, you must, in order to keep it up, increase the number of revolutions as you shorten the string; and, for a similar reason, the nearer a satellite is, the greater must be the number of its revolutions in a given time. It has been surmised, that there is a small satellite or large meteoric stone revolving round the earth at about the distance of 5000 miles from its surface; if this be so, it must, in order that it may not fall to the earth, swing round seven times a day. The moon is about twenty-six times further off, and, to maintain its position, it has only to swing round once in twenty-eight days. Let us suppose that the moon and the little satellite are yoked together by being made parts of a rigid ring whose breadth extends from the one to the other. It is plain that the tendency will be to rend the system in pieces. The one is a slow horse, obstinately keeping back the vehicle 3 the other is a fiery steed, that will break his harness rather than be retarded. The pressure of the difficulty is somewhat lightened by the fact, that the bright ring is divided into two halves, so that the inner half can go at a more rapid pace without being retarded by the outer one. Still, notwithstanding this relief, the difficulty is great; for the outer bright ring is 11,000 miles broad, while the inner is 17,000. We must also keep in view the fragile structure of these rings. If we take the whole system of rings, it may be represented by a strip of tissue paper, a foot long, and an inch broad. This strip, in the form of a ring, is to maintain a state of equilibrium; and it is obvious that its stability is not consistent with powerful internal forces, tending to rend it asunder, as there must be, if the substance is solid, and the rings broad. We can conceive that internal rupture would be avoided by reducing the breadth of the rings; and observation has given indications that the rings are very numerous. A structure, something like the coil of a mainspring of a watch, and a pretty broad division in the outer bright ring, have been detected. This would, in one way, relieve the difficulty of rigidity; but, in another, would increase it. A system of close, narrow rings, packed the one within the other, each with its own period of rotation, would be subject to constant collisions among its constituent parts. Besides all this, the system, as a whole, would be in a state of unstable equilibrium in reference to the body of the planet.

These difficulties, along with new observations, led to the idea that the rings were all fluid. The concentric structure seemed to change; divisions, seen plainly at one time, were not seen at another; and this was so frequently noticed, that the inference seemed natural, that there was a real fluctuation in the divisions of the fluid system. The idea of fluidity was certainly an advance, as it allowed each portion of the system to assume its appropriate period of rotation. A slow ring was not yoked to a fast one. The idea of fluidity has, however, given way to one which more naturally explains the phenomena, and brings this part of the solar system into harmony with other parts. The theory in question is, that the rings are composed of meteoric matter, each particle moving as an independent planet in its own orbit. It is not at all necessary to suppose that the particles are very minute. There may be meteorites of considerable dimensions, but, at the distance of the earth, they can appear only as a continuous mass. The rings of Saturn would thus correspond to the zones of meteoric bodies and asteroids encircling the sun.

A remarkable confirmation of this theory is afforded by the history of the rings. Saturn, above all the other planets, is full of interest, in this respect, that it furnishes proof of great organic changes. It is in this point of view, also, that it furnishes a link between the comets and the planets. The planet occupies the one pole of permanency, while the comet represents the opposite, of mutability; but Saturn occupies a middle position, exhibiting great organic changes, the progress of which is slow, compared to that of the comet. The gradual expansion of the rings towards the body of the planet, has been observed. The rings may be regarded as the iris of the eye; and as the iris contracts the pupil, so the rings close in upon the planet. This closing in is not effected by the contraction of the outer circumference of the system of rings, but only by the increase of the breadth of the rings. The strip of tissue paper, to which we have compared the system, is becoming broader, and probably thinner, but the outer edge remains of the same length. There is a process of stretching-out going on, but it is not the same for the whole breadth of the ring. The parts nearest the planet expand more than those more remote. This result is due to the researches of M. Otto Strüve, who has examined the measurements of the rings for two hundred years back, and by comparing them with those taken at the present day, he has been able to compute the rate of growth. He .calculates that, in about one hundred and twenty years, the inner edge of the system of rings will reach the planet. This conclusion has been questioned by Mr Main of the Royal Observatory, Greenwich, but the weight of evidence is on the side of the distinguished foreign astronomer.

The most remarkable evidence of growth is the dusky, transparent ring, discovered by Mr Bond, in 1850. There is evidence, indeed, that it was seen before, but its nature was not suspected. It was seen crossing the ball of the planet, but it was taken for a permanent belt of the planet. The question arises. How should it be so distinctly seen and recognised by several independent observers all about the same time? Mr Dawes in England, and Father Secchi at Kome, discovered its nature independently, and that with far inferior telescopes than those which, at a previous period, could not detect it. This leads to the surmise, that, although not developed for the first time, it now received such an accession of brightness as no longer to be mistaken. This is confirmed by the detection of actual growth since its discovery; and M. Striive is quite clear that it has widened, and crept nearer the ball.

This new ring confirms the supposition that the system is composed of meteoric matter. The matter is so rare, that we see through it, as we would see through a cloud of dust or a shower of hail. The meteoric bodies may be of considerable size, but they are not so close as to intercept the light of the planet. The phenomena do not agree with the hypothesis, that the ring is gaseous or fluid. M. Strüve has also detected a division in the dark ring, so that it is of a compound character like the bright rings. It is not improbable that, with increased power, or in consequence of organic changes, rings still nearer the planet may yet be discovered.

From the above consideration it is extremely probable, that the bright rings do not differ in nature from the obscure ring. The difference is likely only one of compression. The individual bodies in the bright rings may be so closely compacted, that we cannot, at this distance, see through the interstices.

Is any corroboration of this view furnished by the phenomena of growth? The increase of the breadth of the rings would naturally result from the collision of the innumerable bodies moving in a contracted zone, and with orbits crossing each other. The effect of such collisions would be to arrest the motion of the bodies coming in contact, and the result of this arrested motion would be to bring them nearer the planet. The tendency of the meteoric stones that fall upon the surface of the earth, must be to check its orbitual motion, when they come in a contrary direction; and just in proportion as its speed is slackened, must it narrow its orbit. The mass of such bodies is so small, that no sensible approach to the sun can be looked for; but if the mass of the earth was smaller, the effect would be very perceptible. In this way may we conceive that the rings are drawn out. The smaller bodies would be piecipitated most rapidly, and the transparency of the obscure would be accounted for, by supposing it composed of the finer particles. The precipitate is, however, constantly receiving new accessions, so that it is always becoming more conspicuous, and its recent detection may be due to its increasing density. The middle portion of the system we would expect to be the most dense; and we do find the inner bright ring to be the brightest. The outermost we would expect to be fainter, as from it the lower strata are supplied with additional matter, while it has, itself, no superior source from which to draw. We, accordingly, find, that while the inner bright ring shines like burnished silver, the outer is faint like tarnished silver. This hypothesis of the collision and union of meteoric bodies, receives countenance from the phenomena of the asteroids. We have seen that some of the minor planets have been discovered in regions which were, shortly before, diligently searched, though in vain; so that the surmise is probable, that they have become conspicuous from the union of two bodies, each of which would be invisible when separate. This concentration of matter would necessarily take place to the largest extent in the middle of the system of rings, where we actually find the greatest lustre.

The extreme thinness of the rings can also be accounted for in the same manner. If, at one time, the rings were thicker, but with a concentration in the middle of the thickness, the tendency would be to condense still further the outlying bodies. In crossing the stratum of meteoric matter, they would be caught in its meshes, and fixed in its plane.

It is plain, from the above observations, that, in one sense, the system of Saturn is not permanent, and a natural inquiry is suggested—Is the system of Saturn, in this respect, a type of the solar system? We have seen that the solar system is absolutely stable, but only on the supposition that the planetary bodies experience no resistance in passing through space. But have we ground for believing that there is no obstruction? So far from this, there is the most conclusive evidence that there is such obstruction. The effect upon the planets has not been detected, but the comets have proclaimed the fact, that there is an element of decay in the system. Encke's comet is narrowing its circle at every revolution, and is going down, as it were, a spiral staircase to its ultimate destiny—absorption in the sun. The comet is of a consistency so light, that it is exceedingly sensitive to the retarding influence, just as the light balloon at once feels the slightest breath of air. But the question is only one of degree. The comet has a rapid and gaining spiral, like the moth circling round the fatal light; but the densest planet has also to pursue a downward course, though incalculable ages may elapse before the close spiral shall reach the central furnace. It may be objected that, if the stamp of decay is upon the solar system, the argument from its stability falls to the ground, and all the resources of the most profound analysis have been thrown away in proving this stability. The permanency of the adjustments of the solar system has been appealed to as the most convincing proof of the Divine Intelligence, and are we, by admitting the element of decay, to surrender the stronghold of Astro-theology?

To this the answer is, that there may be the most exquisite provision for the stability of a system which is, notwithstanding, perishable. Take, for example, the human body. How marvellous is its structure, though it is destined to endure only for the period of human life! The marvel of the adjustments would not at all be enhanced, though the length of man's life was doubled. Threescore and ten years were allotted to man's spirit on earth, and a tenement is so adjusted as to last for that period. The Divine wisdom is not measured by the length of the period of permanence, but by the exquisite balancing of forces, so that the system will endure for the allotted period, whether it be long or short. The ephemeron that dances in the sunshine for its brief hour of life, displays as much wisdom in its structure as the eagle, that continues its gyrations for a long century. In human mechanism, the mere element of time is not one that usually enhances the skill of the contrivance. Time-keepers are sometimes made to go for a whole year; yet this is considered of so little consequence, that the finest astronomical clocks are not made to go longer than eight days. The whole skill of the clockmaker is devoted to the object of giving stability to the going of the clock for the period, whether long or short, during which it is to go. To gain this object, he contrives a pendulum by which the disturbances of the temperature may be compensated. He cannot prevent the expansion of metals, but he can so balance, by the expansion of different metals, the disturbances produced, that the result is invariable motion. This is precisely what is done in the solar system. We see disturbance so nicely compensated, that for a required period, the system may be regarded as absolutely stable. But what is this period of stability in the case of the planets? If we regard the planets as destined for the abode of living beings, then this stability is to be viewed in reference to the period allotted for their existence. The stability of the organisation of the human body, is calculated for the life of the individual man. This globe is the abode of the human race, as the body is the abode of the individual man; and the conditions of stability are related, teleologically, to the terrestrial life of the race. From the analogy furnished by geological science, the probability is, that man has his allotted: period, like the inferior forms of life that have already passed away. The records of geology furnish indubitable proof, that the conditions of life have varied greatly on the face of the earth; so that when one form of animal life has become extinct, the creative act of God has called into existence new forms, suited to the altered conditions. In proving, then, the stability of the system, we have only to shew, as far as the argument of design is concerned, that the conditions of life are permanent during the epoch of life for which the system is adjusted. Man was called into existence by God when this abode was fully prepared for his comfort; and the fabric is kept up till his destiny on earth is fulfilled.

It may be said, that while this argument may apply to orbs on which life exists, it is inapplicable to systems like Saturn's rings, where we may suppose life to be impossible. The question of stability is to be dealt with per se, and if we find that the system of the rings is not stable, must we not abandon it as a proof of Divine wisdom? Formerly we could point to these arches, nicely poised in space, as the most wonderful proof of mechanical adjustment; but, now that we see them going through the process of decay, we can no longer avail ourselves of this argument. The answer is, that we have still proof of the most wonderful provisions for permanence, but permanence for only one phase of a mighty evolution. The only difference is, that in former views of the stability of the system, we considered the mechanism as a fixed, rigid, isolated thing, whereas we are now forced to regard every part of the universe as in a state of change or progress. Formerly the heavens were regarded as a frozen river; but the light of science has shewn that there is an onward movement, as in the glacier, though it be almost imperceptible. The material world is passing through cycles of decay and restoration, like living organisms on the face of the earth. Our notions of stability formerly arrested this process, but we now discover that the stability is only during one phase in the evolution. There is an organic stability in the seed, the flower, and the fruit of a plant, but it is a stability consistent with development; and one form merges into another. We lose nothing, in a theological point of view, by the recognition of a limit to the stability of the solar and other subordinate systems. The idea of transition only points to a higher argument. The human spirit has long shrunk from the idea of organic changes in the constitution and relations- of the heavenly bodies, just as it shrank from the reception of the Copernican theory of the solar system; but the admission of the former will, as emphatically, redound to the glory of God as that of the latter. The universe regarded as a rhythm and process, instead of an unalterable piece of architecture, only declares more loudly the wisdom of Him who guides and controls it with His ever-present power. The idea of Sir Isaac Newton, that the material universe may be regarded as the sensorium of Deity, savours somewhat of Pantheism; but it was meant by the illustrious philosopher to be in perfect consonance with the full recognition of the Divine personality; and the conception was designed to bring us closer in thought to the living God, who is in all and through all. The conception of the universe as a ceaseless flow, a grand rhythmical evolution, in like manner tends to produce a livelier and more direct recognition of Him who is ever moving around us, and whose uttered thoughts are the successive steps of this evolution. Theology has confined itself too much to the wisdom displayed in the adjustment and proportions of an unalterable fabric; but a still higher wisdom is displayed in the divine melody of the grand cycles of change. The music, even more than the architecture of the heavens, declares the glory of God.

The system of Saturn's rings is transitory; but why should this prevent us from recognising their beauty and symmetry, as well as the skill with which they are poised in space? We do not refuse to recognise the beauty and skill of a dissolving view, though it gradually fades away upon the screen, and merges into another. We do not fail to acknowledge the beauty and symmetry of the crystals of ice and arborescent patterns on the pane of glass, though they vanish at the first touch of the sunbeam, to give place to other forms of beauty. The mere transition does not at all affect the beauty, symmetry, and skill of the fleeting forms that arrest our notice.

The transition of the rings of Saturn may only be a step to the accomplishment of some higher form of stability. The individual cell in the living organism dissolves, but only that the structure, of which it is an element, may be strengthened and developed. It lays down its own life that the organism, as a whole, may he sustained; and the individual organism, in turn, passes away, after contrihuting to the continuation of the species. In like manner, the dissolution of Saturn's rings, and even that of the solar system, may be only steps of a process by which the stability of some higher and more comprehensive system is secured. Although we cannot trace far the footprints of the Creator, backwards and onwards, still there is no difficulty in recognising those that mark the active progression of the plans of Divine wisdom and power.

While the tendency of astronomical science is to establish progressive phases in the evolution of the material world, it only brings out in more prominent relief the truth, that the introduction of life is a fact in itself unique. All attempts to detect a physical law uniting the successive steps have signally failed. There is, undoubtedly, an order and system by which successive forms of life have been introduced, but all discovery points to the conclusion, that the law is of a higher order than the physical laws of nature. It is simply the order in which a personal God exercises His creative energy.

No doubt, the order of the succession of various forms of life is correlated to the successive stages of material adaptation, but the bond is not of cause and effect. The successive acts of the farmer in ploughing, sowing, and reaping, are correlated to the order in which the seasons return, but the order of seasons is not the physical cause of the acts of the larmer. The two orders of succession, though correlated, are totally different. In the one case, a material law is the bond of union between the successive steps; in the other, volition effects the synthesis. The like relation subsists between the secondary laws by which successive stages of adaptation are produced, and the order by which successive forms of life are introduced. All research has failed to detect any physical cause, by which the one order is only the result of the other. Both orders flow from the same Divine source, but the lines of causation are totally distinct.