SATURN

At nearly double the distance of Jupiter from the sun circles the second largest planet of our system, unique, so far as human knowledge goes, in the character of its appendages. The orbit of Saturn has a mean radius of 886,000,000 miles, but owing to its eccentricity, his distance may be diminished to 841,000,000 or increased to 931,000,000. This large variation may not play so important a part in his economy as might have been supposed, owing to the fact that the sun heat received by him is not much more than ¹/100th of that received by the earth. The planet occupies twenty-nine and a half years in travelling round its immense orbit. Barnard's measures with the Lick telescope give for the polar diameter 69,770, and for the equatorial 76,470 miles. Saturn's polar compression is accordingly very great, amounting to about ¹/12th. Generally speaking, however, it is not so obvious in the telescopic view as the smaller compression of Jupiter, being masked by the proximity of the rings.

Saturn is the least dense of all the planets; in fact, this enormous globe, nine times the diameter of the earth, would float in water. This fact of extremely low density at once suggests a state of matters similar to that already seen to exist, in all likelihood, in the case of Jupiter; and all the evidence goes to support the view that Saturn, along with the other three large exterior planets, is in the condition of a semi-sun.

The globe presents, on the whole, similar characteristics to those already noticed as prevailing on Jupiter, but, as was to be expected, in a condition enfeebled by the much greater distance across which they are viewed and the smaller scale on which they are exhibited. It is generally girdled by one or two tropical belts of a grey-green tone; the equatorial region is yellow, and sometimes, like the corresponding region of Jupiter, bears light spots upon it and a narrow equatorial band of a dusky tone; the polar regions are of a cold ashy or leaden colour. Professor Barnard's fine drawing (Plate XXIV.) gives an admirable representation of these features as seen with the 36-inch Lick telescope. Altogether, whether from greater distance or from intrinsic deficiency, the colouring of Saturn is by no means so vivid or so interesting as that of his larger neighbour.

The period of rotation was, till within the last few years, thought to be definitely and satisfactorily ascertained. Sir William Herschel fixed it, from his observations, at ten hours sixteen minutes. Professor Asaph Hall, from observations of a white spot near the equator, reduced this period to ten hours fourteen minutes twenty-four seconds. Stanley Williams and Denning, in 1891, reached results differing only by about two seconds from that of Hall; but the former, discussing observations of 1893, arrived at the conclusion that there were variations of rotation presented in different latitudes and longitudes of the planet's surface, the longest period being ten hours fifteen minutes, and the shortest ten hours twelve minutes forty-five seconds. Subsequently Keeler obtained, by spectroscopic methods, a result exactly agreeing with that of Hall. It appeared, therefore, that fairly satisfactory agreement had been reached on a mean period of ten hours fourteen minutes twenty-four seconds.

In 1903, however, a number of bright spots appeared in a middle north latitude which, when observed by Barnard, Comas SolÀ, Denning, and other observers, gave a period remarkably longer than that deduced from spots in lower latitudes—namely, about ten hours thirty-eight minutes. Accordingly, it follows that the surface of Saturn's equatorial regions rotates much more rapidly than that of the regions further north—a state of affairs which presents an obvious likeness to that prevailing on Jupiter. But in the case of Saturn the equatorial current must move relatively to the rest of the surface at the enormous rate of from 800 to 900 miles an hour, a speed between three and four times greater than that of the corresponding current on Jupiter!

The resemblance between the two great planets is thus very marked indeed. Great size, coupled with small density; very rapid rotation, with its accompaniment of large polar compression; and, even more markedly in the case of the more distant planet than in that of Jupiter, a variety of rotation periods for different markings, which indicates that these features have been thrown up from different strata of the planet's substance—such points of likeness are too significant to be ignored. It is not at all likely that Saturn has any solidity to speak of, any more than Jupiter; the probabilities all point in the direction of a comparatively small nucleus of somewhat greater solidity than the rest, surrounded by an immense condensation shell, where the products of various eruptions are represented.

Were this all that can be said about Saturn, the planet would scarcely be more than a reduced and somewhat less interesting edition of Jupiter. As it is, he possesses characteristics which make him Jupiter's rival in point of interest, and, as a mere telescopic picture, perhaps even his superior. When Galileo turned his telescope upon Saturn, he was presented with what seemed an insoluble enigma. It appeared to him that, instead of being a single globe, the planet consisted of three globes in contact with one another; and this supposed fact he intimated to Kepler in an anagram, which, when rearranged, read: 'Altissimum planetam tergeminum observavi'—'I have observed the most distant planet to be threefold.' Under better conditions of observation, he remarked subsequently, the planet appeared like an olive, as it still does with low powers. This was sufficiently puzzling, but worse was to follow. After an interval, on observing Saturn again, he found that the appearances which had so perplexed him had altogether disappeared; the globe was single, like those of the other planets. In his letter to Welser, dated December 4, 1612, the great astronomer describes his bewilderment, and his fear lest, after all, it should turn out that his adversaries had been right, and that his discoveries had been mere illusions.

Then followed a period when observers could only command optical power sufficient to show the puzzling nature of the planet's appendages, without revealing their true form. It appeared that Saturn had 'ansÆ,' or handles, on either side of him, between which and his body the sky could be seen; and many uncouth figures are still preserved which eloquently testify to the bewilderment of those who drew them, though some of them are wonderfully accurate representations of the planet's appearance when seen with insufficient means. The bewilderment was sometimes veiled, in amusing cuttle-fish fashion, under an inky cloud of sesquipedalian words. Thus Hevelius describes the aspects of Saturn in the following blasting flight of projectiles: 'The mono-spherical, the tri-spherical, the spherico-ansated, the elliptico-ansated, and the spherico-cuspidated,' which is very beautiful no doubt, but scarcely so simple as one could wish a popular explanation to be.

In the year 1659, however, Huygens, who had been observing Saturn with a telescope of 2? inches aperture and 23 feet focal length, bearing a magnifying power of 100, arrived at the correct solution of the mystery, which he announced to, or rather concealed from, the world in a barbarous jumble of letters, which, when properly arranged, read 'annulo cingitur, tenui, plano, nusquam cohaerente, ad eclipticam inclinato'—'he (Saturn) is surrounded by a thin flat ring, nowhere touching (him, and) inclined to the ecliptic.' Huygens also discovered the first and largest of Saturn's satellites, Titan. His discoveries were followed by those of Cassini, who in 1676 announced his observation of that division in the ring which now goes by his name. From Cassini's time onwards to the middle of the nineteenth century, nothing was observed to alter to any great extent the conception of the Saturnian system which had been reached; though certain observations were made, which, though viewed with some suspicion, seemed to indicate that there were more divisions in the ring than that which Cassini had discovered, and that the system was thus a multiple one. In particular a marking on the outer ring was detected by Encke, and named after him, though generally seen, if seen at all, rather as a faint shading than as a definite division. (It is not shown in Barnard's drawing, Plate XXIV.). But in 1850 came the last great addition to our knowledge of the ring system, W. C. Bond in America, and Dawes in England making independently the discovery of the faint third ring, known as the Crape Ring, which lies between the inner bright ring and the globe.

The extraordinary appendages thus gradually revealed present a constantly varying aspect according to the seasons of the long Saturnian year. At Saturn's equinoxes they disappear, being turned edgewise; then, reappearing, they gradually broaden until at the solstice, 7? years later, they are seen at their widest expansion; while from this point they narrow again to the following equinox, and repeat the same process with the opposite side of the ring illuminated, the whole set of changes being gone through in 29 years 167·2 days. Barnard's measures give for the outer diameter of the outer ring 172,310 miles; while the clear interval between the inner margin of the Crape Ring and the ball is about 5,800 miles, and the width of the great division in the ring-system (Cassini's) 2,270 miles. In sharp contrast to these enormous figures is the fact that the rings have no measurable thickness at all, and can only be estimated at not more than 50 miles. They disappear absolutely when seen edgewise; even the great Lick telescope lost them altogether for three days in October, 1891.*

The answer to the question of what may be the constitution of these remarkable features may now be given with a moderate approach to certainty. It has been shown successively that the rings could not be solid, or liquid, and in 1857 Clerk-Maxwell demonstrated that the only possible constitution for such a body is that of an infinite number of small satellites. The rings of Saturn thus presumably consist of myriads of tiny moonlets, each pursuing its own individual orbit in its individual period, and all drawn to their present form of aggregation by the attraction of Saturn's bulging equator. The appearances presented by the rings are explicable on this theory, and on no other. Thus the brightness of the two rings A and B would arise from the closer grouping of the satellites within these zones; while the semi-darkness of the Crape Ring arises from the sparser sprinkling of the moonlets, which allows the dark sky to be seen between them. Cassini's division corresponds to a zone which has been deprived of satellites; and as it has been shown that this vacant zone occupies a position where a revolving body would be subject to disturbance from four of Saturn's satellites, the force which cleared this gap in the ring is obvious. It has been urged as an objection to the satellite theory that while the thin spreading of the moonlets would account for the comparative darkness of the Crape Ring when seen against the sky, it by no means accounts for the fact that this ring is seen as a dark stripe upon the body of the planet. Seeliger's explanation of this is both satisfactory and obvious, when once suggested—namely, that the darkness of the Crape Ring against the planet is due to the fact that what we see is not the actual transits of the satellites themselves, but the perpetual flitting of their shadows across the ball. The final and conclusive argument in favour of this theory of the constitution of the rings was supplied by the late Professor Keeler by means of the spectroscope. It is evident that if the rings were solid, the speed of rotation should increase from their inner to their outer margin—i.e., the outer margin must move faster, in miles per second, than the inner does. If, on the contrary, the rings are composed of a great number of satellites, the relation will be exactly reversed, and, owing to the superior attractive force exercised upon them by the planet through their greater nearness to him, the inner satellites will revolve faster than the outer ones. Now, this point is capable of settlement by spectroscopic methods involving the application of the well-known Doppler's principle, that the speed of a body's motion produces definite and regular effects upon the pitch of the light emitted or reflected by it. The measurements were of extreme delicacy, but the result was to give a rate of motion of 12½ miles per second for the inner edge of ring B, and of 10 miles for the outer edge of A, thus affording unmistakable confirmation of the satellite theory of the rings. Keeler's results have since been confirmed by Campbell and others; and it may be regarded as a demonstrated fact that the rings, as already stated, consist of a vast number of small satellites.

It has been maintained that the ball of Saturn is eccentrically placed within the ring, and further, that this eccentricity is essential to the stability of the system; while the suggestion has also been made that the ring-system is undergoing progressive change, and that the interval between it and the ball is lessening. It has to be noticed, however, that the best measures, those of Barnard, indicate that the ball is symmetrically placed within the rings; and the suggestion of a diminishing interval between the ring-system and the ball receives no countenance from comparison of the measures which have been made at different times.

There can be no question that of all objects presented to observation in the solar system, there is not one, which, for mere beauty and symmetry can be for a moment compared with Saturn, even though, as already indicated, Mars and Jupiter present features of more lasting interest. To quote Proctor's words: 'The golden disc, faintly striped with silver-tinted belts; the circling rings, with their various shades of brilliancy and colour; and the perfect symmetry of the system as it sweeps across the dark background of the field of view, combine to form a picture as charming as it is sublime and impressive.' Fortunately the main features of this beautiful picture are within the reach of very humble instruments. Webb states that when the ring system was at its greatest breadth he has seen it with a power of about twenty on only 1?-inch aperture. A beginner cannot expect to do so much with such small means; but at all events a 2-inch telescope with powers of from 50 to 100 will reveal the main outlines of the ring very well indeed, and, with careful attention will show the shadow of the ring upon the ball, and that of the ball upon the ring. When we come to the question of the division in the ring, we are on somewhat more doubtful ground. Proctor affirms that 'the division in the ring (Cassini's) can be seen in a good 2-inch aperture in favourable weather.' One would have felt inclined to say that the weather would require to be very favourable indeed, were it not that Proctor's statement is corroborated by Denning, who remarks that 'With a 2-inch refractor, power about ninety, not only are the rings splendidly visible, but Cassini's division is readily glimpsed, as well as the narrow dark belt on the body of the planet.' The student may, however, be warned against expecting that such statements will apply to his own individual efforts. There are comparatively few observers whose eyes have had such systematic training as to qualify them for work like this, and those who begin by expecting to see all that skilled observers see with an instrument of the same power are only laying up for themselves stores of disappointment. Mr. Mee's frank confession may be commended to the notice of those who hope to see at the first glance all that old students have learned to see by years of hard work. 'The first time I saw Saturn through a large telescope, a fine 12-inch reflector, I confess I could not see the division (Cassini's), though the view of the planet was one of exquisite beauty and long to be remembered, and notwithstanding the fact that the much fainter division of Encke was at the moment visible to the owner of the instrument!' It is extremely unlikely that the beginner will see the division with anything much less than 3 inches, and even with that aperture he will not see it until the rings are well opened. The writer's experience is that it is not by any means so readily seen as is sometimes supposed. Three inches will show it under good conditions; with 3? it can be steadily held, even when the rings are only moderately open (steady holding is a very different thing from 'glimpsing'), but even with larger apertures the division becomes by no means a simple object as the rings close up (Fig. 26). In fact, there is nothing better fitted to fill the modern observer's mind with a most wholesome respect for the memory of a man like Cassini, than the thought that with his most imperfect appliances this great observer detected the division, a much more difficult feat than the mere seeing it when its existence and position are already known, and discovered also four of the Saturnian satellites. As for the minor divisions in the ring, if they are divisions, they are out of the question altogether for small apertures, and are often invisible even to skilled observers using the finest telescopes. Barnard's drawing (Plate XXIV.), as already noted, shows no trace of Encke's division; but nine months later the same observer saw it faintly in both ansÆ of the ring. The conclusion from this and many similar observations seems to be that the marking is variable, as may very well be, from the constitution of the ring. The Crape Ring is beyond any instrument of less than four inches, and even with such an aperture requires favourable circumstances.

With regard to a great number of very remarkable details which of late years have been seen and drawn by various observers, it may be remarked that the student need not be unduly disappointed should his small instrument fail, as it almost certainly will, to show these. This is a defect which his telescope shares with an instrument of such respectable size and undoubted optical quality as the Lick 36-inch. Writing in January, 1895, concerning the beautiful drawing which accompanies this chapter, Professor Barnard somewhat caustically observes: 'The black and white spots lately seen upon Saturn by various little telescopes were totally beyond the reach of the 36-inch—as well as of the 12-inch—under either good or bad conditions of seeing.... The inner edge (of the Crape Ring) was a uniform curve; the serrated or saw-toothed appearance of its inner edge which had previously been seen with some small telescopes was also beyond the reach of the 36-inch.' Such remarks should be consoling to those who find themselves and their instruments unequal to the remarkable feats which are sometimes accomplished, or recorded.

So far as one's personal experience goes, Saturn is generally the most easily defined of all the planets. Of late years he has been very badly placed for observers in the Northern Hemisphere, and this has considerably interfered with definition. But when well placed the planet presents a sharpness and steadiness of outline which render him capable of bearing higher magnifying powers than Jupiter, and even than Mars, though a curious rippling movement will often be noticed passing along the rings. It can scarcely be said, however, that there is much work for small instruments upon Saturn—the seeing of imaginary details being excluded. Accordingly, in spite of the undoubted beauty of the ringed planet, Jupiter will on the whole be found to be an object of more permanent interest. Yet, viewed merely as a spectacle, and as an example of extraordinary grace and symmetry, Saturn must always command attention. The sight of his wonderful system can hardly fail to excite speculation as to its destiny; and the question of the permanence of the rings is one that is almost thrust upon the spectator. With regard to this matter it may be noted that, according to Professor G. H. Darwin, the rings represent merely a passing stage in the evolution of the Saturnian system. At present they are within the limit proved by Roche, in 1848, to be that within which no secondary body of reasonable size could exist; and thus the discrete character of their constituents is maintained by the strains of unequal attraction. Professor Darwin believes that in time the inner particles of the ring will be drawn inwards, and will eventually fall upon the planet's surface, while the outer ones will disperse outwards to a point beyond Roche's limit, where they may eventually coalesce into a satellite or satellites—a poor compensation for the loss of appendages so brilliant and unique as the rings.

Saturn's train of satellites is the most numerous and remarkable in our system. As already mentioned, Huygens, the discoverer of the true form of the ring, discovered also the first and brightest satellite, Titan, which is a body somewhat larger than our own moon, having a diameter of 2,720 miles. A few years later came Cassini's discoveries of four other satellites, beginning in 1671 and ending in 1684. For more than 100 years discovery paused there, and it was not until August and September, 1789, that Sir William Herschel added the sixth and seventh to our knowledge of the Saturnian system.

In 1848 Bond in America and Lassell in England made independently the discovery of the eighth satellite—another of the coincidences which marked the progress of research upon Saturn, and in both of which Bond was concerned. Then followed another pause of fifty years broken by the discovery, in 1898, by Professor Pickering, of a ninth, whose existence was not completely confirmed till 1904. The motion of this satellite has proved to be retrograde, unlike that of the earlier discovered members of the family, so that its discovery has introduced us to a new and abnormal feature of the Saturnian system. The discoverer of Phoebe, as the ninth satellite has been named, has followed up his success by the discovery of a tenth member of Saturn's retinue, known provisionally as Themis. Accordingly the system, as at present known, consists of a triple ring and ten satellites. The last discovered moons are very small bodies, the diameter of Phoebe, for instance, being estimated at 150 miles; while its distance from Saturn is 8,000,000 miles. From the surface of the planet Phoebe would appear like a star of fifth or sixth magnitude; to observers on our own earth its magnitude is fifteenth or sixteenth. The ten satellites have been named as follows: 1, Titan, discovered by Huygens; 2, Japetus; 3, Rhea; 4, Dione; 5, Tethys, all discovered by Cassini; 6, Enceladus; and 7, Mimas, Sir William Herschel; 8, Hyperion, Bond and Lassell; 9, Phoebe; and 10, Themis, W. H. Pickering. Titan, the largest satellite, has been found to be considerably denser than Saturn himself.

The most of these little moons are, of course, beyond the power of small glasses; but a 2-inch will show Titan perfectly well. Japetus also is not a difficult object, but is much easier at his western than at his eastern elongation, a fact which probably points to a surface of unequal reflective power. Rhea, Dione, and Tethys are much more difficult. Kitchiner states that a friend of his saw them with 2⁷/10-inch aperture, the planet being hidden; but probably his friend had been amusing himself at the quaint old gentleman's expense. Noble concludes that with a first-class 3-inch and under favourable circumstances four, or as a bare possibility even five, satellites may be seen; and I have repeatedly seen all the five with 3?-inches. The only particular advantages of seeing them are the test which they afford of the instrument used, and the accompanying practice of the eye in picking up minute points of light. There is a considerable interest in watching the gradual disappearance of the brilliant disc of Saturn behind the edge of the field, or of the thick wire which may be placed in the eye-piece to hide the planet, and then catching the sudden flash up of the tiny dots of light which were previously lost in the glare of the larger body. For purposes of identification, recourse must be had to the 'Companion to the Observatory,' which prints lists of the elongations of the various satellites and a diagram of their orbits which renders it an easy matter to identify any particular satellite seen. Transits are, with the exception of that of Titan, beyond the powers of such instruments as we are contemplating. The shadow of Titan has, however, been seen in transit with a telescope of only 2?-inch aperture.

* The plane of the rings passes through the earth on April 13, and through the sun on July 27, 1907, at which periods it is probable that the rings will altogether disappear.