XI THE RINGED PLANET SATURN.

Very different from the ruddy planet which approached so closely to him in November, 1877, is Saturn, the ringed world, the most wonderful of all the planets if the complexity of the system attending on him is considered, and in size inferior only to the giant Jupiter.

It will have been noticed, perhaps, by those who are familiar with the aspect of the planets, that the contrast between Mars and Saturn during their late approach to us was not only greater than usual, but greater than was to be expected even when account was taken of the unusual lustre of Mars. I have often wondered whether the ancient astronomers were ever perplexed by the varying lustre of Saturn. They recognised the fact that Mars has an orbit of great eccentricity (see the picture of the orbits of Mars, Venus, etc., at page 156); and there was nothing in the varying lustre of Mars which could not be perfectly well explained by his known variations of distance, whether the Ptolemaic or the Copernican system were accepted. But with Saturn the case is different. His distance at successive returns to our midnight skies is subject to moderate changes only. Yet his brilliancy varies in a remarkable manner. We now know that those changes are due to the opening and closing of that marvellous system of rings which renders this planet the most beautiful of all the objects of telescopic observation which the heavens present to us. When the edge of the rings is turned towards the earth, we see only the most delicate thread of light on either side of the planet's disc. But when the rings are opened out to their full extent they reflect towards us as much light as we receive from the disc. At such times the planet presents a much more brilliant appearance than when the ring is turned nearly edgewise; in fact to the naked eye he seems very nearly twice as bright. Now at present the rings are turned nearly edgewise towards the earth. In July and August, 1869, the planet presented in the telescope the appearance presented in fig. 30, where it will be seen that the shorter axis of the oval into which any one of the ring-outlines is thrown is nearly equal to half the larger axis. Since then the rings have been slowly closing up; and at present the rings are so little open that the corresponding shorter axis, if it could be directly seen, would appear to be about one-sixteenth only of the larger axis. The rings were turned exactly edgewise towards the sun at two in the afternoon, on St. Valentine's day, 1878, according to calculations which I made in 1864, and published in a table under the head "Passages of the Rings plane through the Sun between the years 1600 and 2000," in my treatise entitled "Saturn and its System." The Nautical Almanac for 1878, indeed makes the passage of the rings plane through the sun occur somewhat earlier, stating that at noon on February 14 the sun's centre would pass south of the ring's horizon by about one-fifth of its apparent diameter (as seen by us). But my own calculation took into account certain small details which, in matters of this sort, the Nautical Almanac computers neglect. After all, it mattered very little to terrestrial observers whether the sun's light passed from the northern to the southern side of the rings a few hours earlier or later: the moment when it passed could not possibly be observed, even if it had occurred during the night hours. In the present instance it occurred at midday, and unfortunately none of the interesting phenomena presented in powerful telescopes when the rings are turned edgewise to the sun or earth could be observed, for they occurred when Saturn and the sun were nearly in the same part of the heavens, and when the planet therefore was utterly lost in the splendour of the solar rays.
Fig. 30.—The planet Saturn in July and August, 1869.

But now let us briefly consider what is known or may be surmised respecting the noble planet which was so far outshone in November, 1877, by the comparatively minute orb of Mars.

Saturn travels at a distance from the sun exceeding rather more than nine and a half times that of our own earth. The second figure of orbits (see page 157) shows the wide span of his orbit compared with the earth's, and yet it will be seen that the orbits of Uranus and Neptune, planets unknown to the ancients, are so wide that the path of Saturn becomes in turn small by comparison.

Saturn has a globe about 70,000 miles in diameter, where it bulges out at the equator; but he is somewhat flattened at his poles, so that his polar diameter is about 7000 miles less than the equatorial diameter. In volume he exceeds our earth about 700 times; but in mass only about ninety times: for his mean density is but about 13/100 of the earth's. In fact, if we could imagine an ocean of water wide enough and deep enough for the planets to be all set in it, Saturn would float with about one-fourth of his bulk out of water,—always supposing that no change took place in his density directly after he was immersed. Saturn, indeed, would float highest of all the planets, or rather all of them would sink except Saturn and Neptune, and Saturn would float higher than Neptune. Uranus would just sink. Jupiter is half as heavy again as he should be to float. All the terrestrial planets, Mercury, Venus, the Earth, and Mars, would go to the bottom at once.

It is almost impossible to regard any feature of Saturn as better deserving to be considered first than his ring system. Yet for the sake of preserving a due sequence of ideas we must first consider his globe.

We find ourselves at once in presence of difficulties like those we encountered when we considered the planet Jupiter. How is it that the mighty mass of a planet like Saturn, constructed, we have every reason to believe, of materials resembling those which constitute our earth, has so failed to gather in its substance that the mean density is much less than that of the earth's globe? It must be remembered that gravity prevails throughout the frame of Saturn as throughout our earth's frame. Every particle of that enormous globe is drawn towards the centre with a force almost exactly the same as would be exerted by the attraction of the entire mass of that portion of the planet which lies nearer to the centre than the particle, if this mass were collected at the centre. But this is not all. It is not merely the attraction exerted on each particle of Saturn's mass which has to be considered, but the entire weight of all the superincumbent matter. The distinction between attraction and weight, by the way, is very commonly overlooked in considering the planets' interiors. I think it was Sir David Brewster who argued that as attraction can easily be shown to diminish downwards towards the centre, it is possible to conceive that the interior of a planet may be hollow. The error is readily perceived, if we take a familiar instance where the attraction is the same yet the effect of pressure very much greater. (Without voyaging to the centre of the earth, which is troublesome, and certainly not a familiar experience, we cannot reach places where the attraction of gravity is greatly less than at the surface.) Take a massive arch of brickwork: the bricks near the top are subject to the same attraction as those belonging to the foundation; but the pressure to which the foundation bricks are exposed is very much greater than that affecting the upper bricks. So again with a deep sea: the particles at the bottom of such a sea are subject to no greater attraction than the particles near the top; but we know that a strong hollow case of metal which near the top of such a sea would be scarcely pressed at all, and would suffer no change of shape, will be crushed perfectly flat under the tremendous pressure to which it will be exposed when sunk to the bottom.

There is, in fact, no escape from the conclusion that the interior portions of a planet like Saturn or Jupiter, nay, even of a body like our earth or the moon, must be subject to tremendous pressure, a pressure exceeding many hundred-fold the greatest which we can obtain experimentally, and that under that enormous pressure the density of the materials composing those central parts must be increased. How is it then that Saturn is of much smaller density than the earth? I can imagine no other explanation at once so natural and so complete as this, that an intense heat pervading the entire frame of the planet enables it to resist the tremendous pressure due to mere weight. The planet's mass is expanded by the heat; large portions which at ordinary temperatures would be solid are liquified or even vaporised; matters which are liquid on our earth are vaporised; and, in fine, the planet assumes (as seen from our distant station) the appearance of being very much larger than it really is. We measure not the true globe, which, for aught that is known, may be exceedingly dense, but the dimensions of cloud-layers floating high in the planet's atmosphere.

In describing Jupiter, I considered the changes which have been noticed in that planet's outline, and observed that it is impossible adequately to explain the evidence, without assuming that the changes of outline are real. The outline is not that of a solid globe, however, but of cloud-layers surrounding such a globe, and probably at a great distance from its surface.
Fig. 31.—Saturn's square-shouldered aspect.

In Saturn's case we have very singular evidence to the same purpose. It was observed by Sir W. Herschel in April, 1805, that Saturn occasionally appears distorted, as though bulging out in the latitudes midway between the pole and the equator of the planet. Fig. 31 represents the appearance of the planet so far as shape is concerned, but the ring was not, when Sir W. Herschel observed it, so narrow as it is shown in fig. 31. In fact the ring had been turned edgewise to the earth two years before; and when Herschel noticed the abnormal appearance of Saturn, the rings had begun to open out, though their outermost outline was still far within the regions of the planet which seemed to project as shown. Fig. 31 in fact represents Saturn as seen by Schr?ter in 1803, when, as he said, the planet did not seem to present a truly spheroidal figure. Herschel tested his observations by using several telescopes of different dimensions,—ten, seven, twenty, and forty feet in length. In 1818, when the rings were scarcely visible, Kitchener saw the planet as shown in fig. 31, or "square-shouldered," as some have called it. On one occasion the present Astronomer-Royal saw the planet of that figure. In January, 1855, Coolidge, using the fine refractor of the Cambridge U.S. Observatory, noticed that the equatorial diameter was not the greatest; on the 9th the planet seemed of its usual shape; but on December 6, Coolidge writes, "I cannot persuade myself that it is an optical illusion which makes the maximum diameter of the ball intersect the limb half-way between the northern edge of the equatorial belt, and the inner ellipse of the inner bright ring." This was at a time when the rings were nearly at their greatest opening; so that, including Schr?ter's observation, we have Saturn out of shape when his ring has presented every shape between that shown in fig. 30 and that shown in fig. 31. Again, in the report of the Greenwich Observatory for 1860-61, when the ring was nearly closed, it is stated that "Saturn has sometimes appeared to assume the square-shouldered aspect." Lastly, the eminent observers, G. Bond and G. P. Bond, father and son, have seen Saturn abnormally shaped, flattened unduly in the north polar regions in 1848, when the ring was turned edgewise towards us, and unsymmetrical in varying ways in 1855-57, when the ring was most widely opened.

Yet the planet's outline is usually a perfect oval, and has been shown to be so by careful measurements effected in some instances by the same observers, who, making equally careful measurements, have found the planet to be distorted.

Does it not seem abundantly clear that the great cloud-layers which float in the atmosphere of Saturn have a widely varying range in height, and that therefore as we see and measure the outline of the cloud-layers, we see and measure in effect a planet which is variable in figure? This seems so natural and complete an explanation of the observed peculiarities that it appears idle on the one hand to reject the evidence of some among the most skilful observers who have ever lived; or, on the other, to imagine that the solid frame of the planet has undergone changes so tremendous as would be involved by the observed variations of outline if they really signified that a solid planet had changed in shape.

The mighty globe of Saturn turns upon its axis nearly as quickly as Jupiter. It will be remembered that the Jovian day lasts only 9? of our hours, and as the diameter of Jupiter is about ten times the earth's, the equatorial parts of the giant planet travel some twenty-six or twenty-seven times as fast as those of our own earth, which move (rotationally) at the rate of more than a thousand miles an hour. Saturn's equatorial parts do not move quite so fast,—in fact, in this respect, Jupiter comes first of all the members of the solar system, including the sun himself. Saturn's equatorial circuit being almost nine times the earth's, while his day is little more than five-twelfths of the earth's, it follows that his equatorial parts move twelve-fifths of nine times, or nearly twenty-two times faster than the earth's. Their actual rotational rate is rather more than 22,000 miles an hour, or 367 miles a minute, or more than six miles a second. This is a wonderful rate of motion. It always seems to me one of the most striking results of modern astronomical research that we have to recognise in bodies like that dull looking star,—the heavy slow-moving Saturn, as the ancients called him,[14]—motions of such tremendous swiftness. The planet is not only rushing bodily along through space with a velocity of nearly six miles per second, but his equatorial parts are being carried round with a velocity somewhat exceeding six miles per second. (The coincidence must be regarded as accidental, but it has this curious effect, that the equatorial parts of Saturn near the middle of the disc we see are actually almost at rest with respect to the sun, being carried forward with the planet at the rate of about five miles and nine-tenths per second, and backward round the planet at the rate of about six miles and one-tenth per second. In fact there are always two points on the disc which are almost exactly at rest with respect to the sun, viz., those two points north and south of the equator where the rotational velocity is about five miles and nine-tenths per second, the velocity of Saturn in his orbit.)[15]

But let us turn from the contemplation of Saturn's globe, interesting though it undoubtedly is, to study those marvellous objects, the Saturnian rings.

The history of their discovery is interesting, but must not here detain us long. Briefly, it runs as follows:—

Galileo, in July, 1610, observing the planet Saturn with a telescope not powerful enough to show the rings, imagined at first that Saturn had two companion planets, one on either side of him, as though helping the planet along upon his road. (From a table relating to the rings, in my treatise on "Saturn and its System," the aspect of the ring, at the time of any such observation, can at once be inferred. In the present case, for example, it will be seen from the table that the rings were closing up as the time of their disappearance, December 28, 1612, drew near.) A year and a half later, Galileo looked again at Saturn, and lo! the companion planets were gone. He was perplexed beyond measure. "What is to be said concerning so strange a metamorphosis?" he asked. "Are the two lesser stars consumed after the manner of the solar spots? Have they vanished or suddenly fled? Has Saturn, perhaps, devoured his own children? Or were the appearances indeed an illusion or fraud, with which the glasses have so long deceived me, as well as many others to whom I have shown them? Now, perhaps, is the time come to revive the well-nigh withered hopes of those who, guided by more profound contemplations, have discovered the fallacy of the new observations, and demonstrated the utter impossibility of their existence. I do not know what to say in a case so surprising, so unlooked for, and so novel. The shortness of the time, the unexpected nature of the event, the weakness of my understanding, and the fear of being mistaken, have greatly confounded me."

Hevelius was similarly perplexed by the constantly varying appearance of the planet. "Saturn," he informed his contemporaries, "presents five various figures to the observer, to wit—first, the mono-spherical; secondly, the tri-spherical; thirdly, the spherico-ansated; fourthly, the elliptico-ansated; fifthly, and finally, the spherico-cuspidated;" of which we can only say, like Mr. Gilbert's Ferdinando, that "we know it's very clever; but we do not understand it."

It was not till 1659 that Huyghens, using a telescope forty yards long, was able to make out the real meaning of the appendages which had so perplexed Galileo and Hevelius. He announced to the world, in an anagram, his discovery that Saturn is girdled about by a flat ring nowhere touching the planet.

Huyghens also discovered the largest of Saturn's moons. He looked for no more, having the idea that, since six planets and six moons were now known, no more moons existed.

In 1663 the Brothers Ball discovered that the rings are divided into two, or, at any rate, that a broad black stripe, such as is shown in fig. 30, separates the outer portion of the ring from the inner. Two years later these observers saw the stripe on the northern side of the rings, when the rings had so shifted in position that observers saw their southern side. Dominic Cassini recognised a corresponding stripe on the southern side. This was regarded as proving that there is a real division between the rings. The width of the gap thus separating the outside of the inner ring from the inside of the outer cannot be less than 1,600 miles.

Cassini also detected another Saturnian moon in October, 1671, and, later, he discovered three others, making five Saturnian moons in all.

Sir W. Herschel observed the rings with great care. He confirmed the discovery of the great division between the rings; but rejected the idea which was beginning to be entertained in his time, that there are many divisions. He found reasons for suspecting, but never actually proved, that the outer ring turns round in about 10? hours.

He also detected two small moons close to the outer ring. One other moon, detected independently by Bond at the Harvard Observatory, Cambridge, U.S., and by Lassell in this country in 1848, completes the set of eight moons now known to revolve around the planet Saturn. We need not here say much more about these moons, saving, perhaps, to note that the span of the entire Saturnian system of moons amounts to about 4,400,000 miles, nearly double that of the Jovian system. This is the largest system of satellites known to us. It is wonderful to reflect, when we look at the dull, slow-moving Saturn, that not only is the planet itself 700 times larger than the earth, not only is it girdled about by a ring system having a span exceeding more than 20 times the diameter of this earth on which we live, but that the entire span of the system over which that distant planet rules exceeds more than eighteen-fold the distance separating our earth from the moon.

Return we now, however, to the consideration of the Saturnian ring-system.

In 1850 a singular discovery was made. It was found by Bond, in America, and, a few days later, independently, by Dawes, in England, that inside the inner bright ring there is a dark ring almost as wide as the outer bright ring. One of the strangest circumstances about this inner ring is that where it crosses Saturn's disc the outline of the planet can be distinctly traced through the dark ring, which is thus, in a sense, a semi-transparent body. I say "in a sense," because it does not follow that it really consists of semi-transparent matter any more than it follows from our being able to see through a gauze veil that the individual threads forming the gauze are made of a semi-transparent material.

On examining recorded observations of the planet evidence was found that this dark ring is not, as was at first supposed, a recent formation. Where it crosses Saturn it had been mistaken in former times for a dark belt.

It had always been supposed that the rings are solid, or at any rate continuous bodies. The younger Cassini, indeed, ventured to express doubts on the subject, but with this solitary exception, no suspicion had ever existed among astronomers that the rings are otherwise than continuous, until the discovery of the dark ring.

When the singular fact was discovered that the body of the planet can be seen through the slate-coloured ring, the solidity of this ring, at any rate, began naturally to be questioned. The idea was suggested that this formation may be fluid. Mathematicians applied rigorous processes of investigation to the question whether a fluid ring can possibly exist in such a position. The inquiry led to a re-examination of the whole subject of the ring-system and its stability. Mathematicians took up the question where Laplace had left it more than half a century before. He had decided that solid rings might, under certain conditions, revolve around a planet without being broken. But his inquiry had not been carried to a conclusion. Now, when the work was completed, it was found that the requisite conditions are certainly not fulfilled by the Saturnian ring-system. The rings should be situated eccentrically, and heavier at one side than the opposite. In fact they should have a perceptible "bias." They exhibit, on the contrary, the most perfect symmetry of figure—this symmetry, indeed, constitutes the great charm of Saturn's telescopic appearance; and although, occasionally, the ball has not seemed to be quite in the middle of the ring-system, the displacement has never approached that which theory requires.

The conclusion to which mathematicians arrived was accordingly the following:—

The rings may be held to be formed of a multitude of tiny satellites, travelling nearly in one plane, each pursuing its own course around Saturn, according to the laws of satellite motion, though of course disturbed by the attraction of its fellow-satellites.

We owe this theory principally to the labours of Professor J. Clerk Maxwell, who gained the Adams Prize offered by the University of Cambridge for the best mathematical essay upon the conditions under which a ring-system such as Saturn's can exist. But Professor Pierce, of America, had (somewhat earlier) supplied a complete refutation of the idea that the rings are solid and continuous bodies.

When the rings are fully open, as in fig. 30, the Saturnian system affords as charming an object for telescopic observation as the astronomer can desire. The rings are then exhibited in their full beauty. The divisions, the dark ring, and the strange shading of the middle ring, can be well seen in a telescope of adequate power. The telescopic view is still more interesting when (as in fig. 30) the planet throws a well-marked shadow upon the rings.

But perhaps the most beautiful of all the features which Saturn presents to the telescopist is the strange variety of colour to be observed upon his surface, and upon that of the rings. Mr. Browning, the eminent optician, thus describes the colours which the planet presents in his 12-inch reflector:—

"The colours I have used," he says, referring to a painting of the planet, "were—for the rings, yellow-ochre (shaded with the same) and sepia; for the globe, yellow ochre and brown madder, orange and purple, shaded with sepia. The great division in the rings is coloured sepia" (not black as commonly described). "The pole and the narrow belts situated near it on the globe are pale cobalt blue." "These tints," he adds, "are the nearest I could find to those seen on the planet; but there is a muddiness about all terrestrial colours when compared with the colours of the objects seen in the heavens. These colours could not be represented in all their brilliancy and purity, unless we could dip our pencil in a rainbow, and transfer the prismatic tints to our paper."

I can corroborate these remarks from observations made upon the planet with an 8?-inch reflector. It is, indeed, a circumstance worthy of note, that the colours of the planets are much more strikingly exhibited by reflecting telescopes than by refractors, insomuch that, while Sir W. Herschel and Messrs. De la Rue and Lassell, making use of the former class of instruments, have all recorded the marked impression which the colours of Saturn and Jupiter have made upon them, we find that few corresponding observations have been made by observers who have been armed with even the most perfect specimens of the refracting telescope.

It must be noticed, however, that the colours of Saturn and his ring-system can only be seen in the most favourable observing weather.