The Story of the Solar System :: CHAPTER X. SATURN.

CHAPTER X. SATURN.

Next beyond Jupiter, proceeding outwards from the Sun, we reach the planet Saturn, which beyond any doubt is the most beautiful and most interesting of all the planets. Nobody who has ever had a fairly good chance of seeing it can have the least doubt that this is the case. Briefly stated the three main features which constitute its claims are:—(1) Its belts, (2) its rings, (3) its satellites.

The belts of Saturn resemble generally those of Jupiter, but they are more faint and less changeable. Their physical cause, however, may be assumed to be the same. Taking the planet as a whole, it may be said that its ordinary colour is yellowish white, the belts inclining to grayish white; though the dark belts have often been thought to exhibit a greenish hue. Lassell considered that the south pole is generally darker than the north pole and more blue in tinge.

Fig. 15.—Saturn, Jan. 26, 1889 (Antoniadi).

There is one important particular in which the belts of Saturn differ from those of Jupiter. Jupiter’s belts are straight, whereas Saturn’s are sensibly curved. Supposing, as is probable, that Saturn’s belts are parallel to the planet’s equator, then we must assume that the plane of this equator makes a rather considerable angle with the ecliptic. Spots on Saturn are very rare. Whether Saturn has an atmosphere seems uncertain, or perhaps it may be said that one has not been proved to exist but may exist. The question of polar snow is also uncertain, but Sir W. Herschel thought he could trace changes of hue at the poles which might be due to the melting of snow.

It is usual to speak of the planet itself under the name of the “Ball” when it is not a question of referring to the whole Saturnian system collectively. In consequence of its distance from the Sun, Saturn undergoes no equivalent to a phase; or to be more exact, no phase can be detected, though theoretically when the planet is in quadrature the disc must undergo an infinitesimally small loss of light.

Though the point has now-a-days no scientific importance, it may perhaps be desirable just to make a brief allusion to Sir W. Herschel’s curious theory that Saturn was seen by him to be compressed not only at the poles but at the equator, so that it resembled a parallelogram with the corners rounded off. It is difficult to imagine what could have given rise to this strange idea, though, of course, Herschel’s good faith in advancing it cannot be called in question. I refer to it because it will be found mentioned in so many books on astronomy, often under the name of the “square-shouldered” figure of Saturn. As a theory it may be regarded as quite exploded in consequence of accurate measures by Bessel, Main and others having conclusively shown that the form of the ball does not depart from that of a regular spheroid.

In referring to Saturn generally, we speak of its ring in the singular number, but, in point of fact, there are several rings—three in particular. The principal bright ring is really double, and within the innermost bright ring there is a dusky one, perfect as a ring, but not luminous as the outer rings are. By way of distinguishing one ring from another, it is usual to adopt Struve’s nomenclature, whereby the outermost bright ring is called A, the inner bright ring B, and the dusky ring C.

A good engraving will convey more fully and more clearly an idea of what the Saturnian system consists of than the fullest verbal description will do. (See Frontispiece.)

To the earliest astronomers who possessed telescopes, Saturn proved a great puzzle, because it seemed to undergo changes of shape which were quite inexplicable on any principles then known. Galileo, when first he saw it, thought it presented an oval outline which might be due to a central planet having a smaller planet on each side of it, and accordingly he announced to his friend, Kepler, that the most distant planet was tergeminum or tri-form. But greater magnifying power led him to arrive at the conclusion that the planet was not a triple combination of spheres, but one body, either oblong or oval in outline. This conclusion, however, was soon found to be untenable, because the two (supposed) tributary bodies gradually decreased in size until they entirely disappeared. Galileo writing to his friend, Welser, in December 1612, thus expressed himself:—

“What is to be said concerning so strange a metamorphosis? 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 illusion or fraud, with which the glasses have so long deceived me, as well as many others to whom I have shewn 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.”

Fig. 16.—General view of the Phases of Saturn’s Rings.

Galileo seems to have become so out of heart in consequence of the difficulty of determining what these changes really meant, that he gave up altogether observing Saturn. In the course of time, but by very gradual steps, astronomers came to realise what the facts were. The next idea that was broached, was that the planet consisted of simply one central ball, and that the excrescences which Galileo had been puzzled by were merely handles as they were called, (ansÆ) projecting like the handles, say of a soup tureen, though why they should vary in size at stated intervals remained as great a mystery as ever. It was not until about 1656 that the true explanation was arrived at by a Dutchman, named Christopher Huygens. It was the fashion in those days for scientific men to intimate to the world discoveries which they had made by resort to mysterious anagrams, which served in some degree the purpose which in the present day is served by the law regulating copyright or patent rights. Accordingly Huygens published the following singular memorandum:—

aaaaaaa cccc d eeeee g h i iiiiii llll mm nnnnnnnnn oo oo pp q rr s ttttt uuuuu.

These letters arranged in their proper order furnish the following Latin sentence:—

Annulo cingitur, tenui, plano, nusquam cohaerente, ad eclipticam inclinato; which Latin sentence becomes in the English tongue:—

“[The planet] is surrounded by a slender flat ring inclined to the ecliptic, but which nowhere touches [the body of the planet.]”

Huygen’s discovery was not a mere piece of guesswork, for he spent several years carefully observing the alterations of form which Saturn underwent, before he came to the conclusion that it was only the existence of a ring surrounding the planet which would explain the various observed changes.

It was by way of guarding himself from being robbed of the fruits of his discovery whilst he was accumulating the necessary proof of its truth, that he buried his thoughts in the logogriph or anagram just quoted. Having arrived at the conclusion which he did, he thought himself sufficiently sure of his facts to predict that in July or August 1671, the planet would again appear round, the ring becoming invisible. This surmise proved practically correct, in so far, that in May 1671, or within 2 months of the time predicted by Huygens, Cassini saw the planet as a simple ball unaccompanied by any ring.

This is a convenient place at which to offer a brief explanation of the changes of appearance as regards the ball and rings which Saturn undergoes. These changes depend jointly on Saturn’s motion in its orbit round the Sun, and on the corresponding motion of the Earth in its orbit. Neither Saturn nor the Earth revolve round the Sun exactly in the ecliptic, and this want of coincidence results in the fact, that twice in the 29½ years occupied by Saturn in journeying round the Sun, the plane of its ring is seen edgeways by us on the Earth; whilst at two other periods intermediate but equi-distant the ring is seen opened out to the widest possible extent; that is, so far as we on the Earth can by any possibility have a chance of seeing it.

Fig. 17.—Phases of Saturn’s Rings at specified dates.

The appearances presented by the rings when undergoing the transformations to which they are subject, will be readily understood by an inspection of the annexed engravings. Fig. 17, indicates the actual appearances in the years specified, and these years may be considered as carried forward and brought up to date by substituting 1877 for 1848, 1885 for 1855, 1891 for 1862, and 1898 for 1869.

Adverting to fig. 16, it will suffice to remark that the two central phases of the rings, opened wide, are to be deemed co-related, or indeed identical in a geometrical sense (so to speak) the difference being that one of them is to be deemed to show the northern side of the ring (which is now in view and will continue in view till 1907) whilst the other represents the southern side, which was in view from 1877 till 1891. The foregoing is a brief statement of the general principle involved in the changes which take place, but the motions of the two planets introduce certain technical complications into the details which would be seen by an observer using a large telescope; with these, however, the ordinary reader will not care to concern himself, and need not do so.

A great deal might be said with respect to the rings treated descriptively. I will now mention a few matters of general interest. Huygens regarded the appendage to Saturn, whose existence he established, to be a single ring, but as far back as 1675, Cassini determined that Huygen’s single ring was really made up of two, one lying inside the other. Cassini in this conclusion outstepped not only all the observers of his own century, but those of the succeeding century, for Sir W. Herschel even 100 years after Cassini, was for a long time unable to satisfy himself, even with his superior telescopes, that the black streaks seen in the ring by Cassini, and regarded by him as indicative of a severance of the ring into two parts, really implied a severance. It is now, however, accepted as a fact that not only are the rings which are known as A and B absolutely distinct, but that A also is itself certainly duplex, that is, that it certainly consists of two independent rings. In addition to this many competent observers armed with powerful telescopes have obtained traces of other sub-divisions, both in A and B; and though there is some want of harmony in the details, as stated by the different observers, yet undoubtedly we must speak of Saturn’s rings collectively as forming a multiple system.

What the rings are is a highly debatable point, but the preponderating idea is that they are not what they appear to be, namely solid masses of matter, but are swarms of independent fragments of matter. Yet “fragment” is not the best word to use, because it implies that something has been broken up to make the fragments. Rather, perhaps, we should say with Professor Young, that the rings are “composed of a swarm of separate particles, each a little independent moon pursuing its own path around the planet. The idea was suggested long ago, by J. Cassini in 1715, and by Wright in 1750, but was lost sight of until Bond revived it in connection with his discovery of the dusky ring. Professor Benjamin Pierce soon afterwards demonstrated that the rings could not be continuous solids; and Clerk Maxwell finally showed that they can be neither solid nor liquid sheets, but that all the known conditions would be answered by supposing them to consist of a flock of separate and independent bodies, moving in orbits nearly circular, and in one plane—in fact, a swarm of meteors.”

The thickness of the rings seen edgeways has been variously estimated. Sir J. Herschel suggested 250 miles as an outside limit, which G. P. Bond reduced to 40 miles. It is generally considered, however, that 100 miles is probably not far from the truth. Young has pointed out that if a model of them were constructed on the scale of 1 inch to represent 10,000 miles, so that the outer ring of such a model would be nearly 17 inches in diameter, then the thickness of the ring would be represented by that of an ordinary sheet of writing paper.

Considered as a system, the rings are distinctly more luminous than the planet, and of the two bright rings, the inner one is brighter than the outer one; and the inner one is less bright at its inner edge than elsewhere. It is also to be noticed that when seen edgeways just about the time of the Saturnian equinoxes, when the Sun is shifting over from one side of the ring to the other, and the ring is dwindling down to a narrow streak, its edges (forming the ansÆ as they are termed) do not disappear and reappear at the same time, and are not always of the same apparent extent. One ansa, indeed, is sometimes visible without the other, and most commonly it is the Eastern one that is missing. To what causes these various peculiarities are due is unknown.

Many physical peculiarities have been either noticed or suspected with reference to the bright rings. For instance, on comparing one with another, some persons have thought that their surfaces are convex, and that they do not lie in the same plane. The existence of mountains on their surface has more than once been suspected. Again, it has been fancied that they are surrounded by an extensive atmosphere. It seems hardly likely that the rings would have an atmosphere and not the ball (or vice versÂ), and, therefore, no wonder that we have no observations which countenance the idea that the ball does really possess an atmosphere. This, indeed, seems to flow from Trouvelot’s observation, that the ball is less luminous at its circumference than at its centre.

The circumstances of ring C, otherwise called the “Dusky” or “Crape” ring are as curious historically, as they are mysterious physically. In 1838, Galle of Breslau, noticed what he thought to be a gradual shading off of the interior bright ring towards the ball. Though he published a statement of what he saw, the matter seems to have attracted little or no notice. In 1850, G. P. Bond in America perceived something luminous between the ring and the ball, and after repeated observations in concert with his father, came to the conclusion that the luminous appearance which he saw, was neither less nor more than an independent and imperfectly illuminated ring lying within the old rings and concentric with them. Before, however, tidings of Bond’s discovery reached England, but a few days after the discovery in point of actual date, Dawes suddenly noticed one evening as Bond had done, a luminous shading within the bright rings, which he was not long in finding out to be in reality a complete ring, except so far that a portion of it was of course hidden from view behind the ball. He, and O. Struve likewise, noticed that this new Dusky Ring was occasionally to be seen divided into two or more rings. The Dusky Ring is transparent, though this fact was not ascertained until 1852, or two years after Bond’s discovery of the ring.

The Dusky Ring is now recognised as a permanent feature of Saturn, but how far it used to be permanent, or how long it has been so, is a matter wrapped in doubt. Recorded observations by Picard in 1673, and by Hadley in 1723, made of course with telescopes infinitely less powerful than those of the present day, seem to suggest that both the observers named saw the Dusky Ring, without, however, being able to form a clear conception that it was a ring. It is strange that during the long period from 1723 to 1838, no one—not even Sir W. Herschel, with his various telescopes—should have obtained or at least have recorded any suspicion of its existence. There is, however, direct evidence that the Dusky Ring is wider and less faint than formerly. This was directly confirmed by Carpenter in 1863, who says he saw it “nearly as bright as the illuminated ring, so much so, that it might easily have been mistaken for a part of it.” In 1883, Davidson found a marked difference in the brilliancy of the two ends (ansÆ) of the ring.

In 1889 Barnard was fortunate enough to observe an eclipse of one of Saturn’s satellites by the ring, but the eclipse, that is the concealment of the satellite, was only effected when it passed behind the bright rings; the dusky ring did not obliterate it, and hence there was obtained a conclusive proof of the transparency of the dusky ring. Barnard further concluded from his observations that there was no separating space or division between the inner bright ring and the dusky ring, as has frequently been represented in drawings. This transparency of the Dusky Ring, as a matter of fact, is therefore undoubted; yet what are we to consider to be the meaning of an observation by Wray in 1861, that whilst looking at the dusky ring edgeways the impression was conveyed to his eye that that ring was very much thicker than the bright rings?

A very interesting question which has been much discussed has reference to the stability of the rings. It is generally agreed that the constituent particles of the rings must be in motion round the primary or their equilibrium could not be maintained: almost equally certain is it, and for the like reason, that the rings cannot be solid. Of actual change in the rings as regards their dimensions, we have no satisfactory proof, though authorities differ on the point, some thinking that the rings are expanding inwards, so that ultimately they will come into contact with the ball, whilst others consider no proof whatever of such change can be obtained from any of the observations yet made in the way of measurements.

We must now proceed to consider the satellites of Saturn. These are 8 in number, 7 of which move in orbits whose planes coincide nearly with the planet’s equator, whilst the remaining one is inclined about 12° thereto. One consequence of this coincidence in the planes of these satellites, which, it should be stated, are the 7 innermost, is that they are always visible to the inhabitants of both hemispheres when they are not actually undergoing eclipse in the shadow of Saturn. The satellites are of various sizes, and succeed one another in the following order, reckoning from the nearest, outwards:—Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion and Iapetus. Any good 2-inch telescope will show Titan; a 3-inch will sometimes show Iapetus; a 4-inch will show Iapetus well, together with Rhea and Dione, but hardly Tethys; all the others require large telescopes. If Saturn has any inhabitants at all constituted like ourselves, which is highly improbable, they will have a chance of seeing celestial phenomena of the greatest interest. What with the rings surrounding the planet and 8 moons in constant motion, there will be an endless succession of astronomical sights for them to study. The amount of light received from the Sun cannot be much—barely 1/100th what the earth receives. The ring and satellites will therefore be useful as supplementary sources of light; yet the satellites will not furnish much, for it has been calculated that the surface of the sky occupied by all the satellites put together would to a dweller on Saturn only amount to 6 times the area of the sky covered by our Moon; whilst the intrinsic brightness of all put together would be no more than ¹/₁₆th part of the light which we receive from our Moon.

The only physical fact worth noting here in connection with the satellites concerns Iapetus. Cassini two centuries ago with his indifferent telescopes thought he had ascertained that this satellite was subject to considerable variations of brilliancy. Sir W. Herschel confirmed Cassini as to this. He found that it was much less brilliant when traversing the eastern half of its orbit than at other times. Two conclusions have been drawn from this fact. One is that the satellite rotates once on its axis in the same time that it performs one revolution round its primary; and that there are portions of its surface which are almost entirely incapable of reflecting the rays of the Sun. This last named supposition may perhaps be well founded, but the former needs more proof than is as yet forthcoming. Iapetus on the whole may be said to shine as a star of the 9th magnitude. To this it may be added that Titan is of the 8th magnitude, but all the others much smaller.

Fig. 18.—Saturn with the shadow of Titan on it, March 11, 1892 (Terby).

Saturn revolves round the Sun in a little under 29½ years at a mean distance of 886 millions of miles. Its apparent diameter varies between 15 and 20; its true diameter may be put at 75,000 miles. The flattening of the poles, or “polar compression” as it is called, is greater than that of any other planet, but is usually less noticeable than in the case of Jupiter, because the ring is apt to distract the eye, except when near the edgeways phase. The compression may be taken at ¹/₉.

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