Part 14

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 10½ 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 8½-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.

FOOTNOTES:

Who assigned to him, as his representative metal, lead--a metal "heavy, dull, and slow," as Don Armado puts it, in "Love's Labour's Lost."

Attention has lately been called, by the astronomers of the Washington Observatory, to the fact that the statement usually made in our books of astronomy, that Sir W. Herschel's latest determination of Saturn's rotation period was 10h. 29m., is incorrect. His only determination of the period gave 10h. 16m. 44s. for the Saturnian day.

XII.

FANCIED FIGURES AMONG THE STARS.

I THINK that every thoughtful student of the stars must have wondered how the figures of the various objects now pictured in our star-maps came to be imagined in the heavens themselves. It is a convenient answer to inquiries of the sort to say that it became necessary at an early stage in the progress of astronomy to have some means of identifying and naming star-groups, and that the arrangement into constellations was as suitable as any other that could have been desired. But it seems to me altogether unlikely that, in the infancy of a science, a mere arbitrary arrangement, such as this explanation supposes, should have been adopted. If we try to imagine the position of the first observers of the stars, what they wanted, and what they were likely to do,--and this a priori method of dealing with such questions is, I believe, the only safe one,--we perceive that the division of the stars into sets named after animals and other objects, without any real resemblance to suggest such nomenclature, is as unlikely a course as could possibly be conceived. Beyond all question, I think, the first watchers of the skies (they can scarcely be called astronomers) would have taken advantage of imagined similarity, more or less close, between each remarkable group of stars and some known object, to identify the group, and to obtain a name by which to speak of it.

Yet it must be admitted that, as the constellations are at present arranged and figured, it is very difficult, in the great majority of cases, to imagine the least resemblance between a constellation and the object from which it derives its name. This is not only true of the modern constellations, the preposterous pneumatic machines, printing presses, microscopes, and so forth, with which Hevelius and his successors foolishly crowded the heavens. Even the oldest of the old constellations of Ptolemy, nay, some even of those which are found among all nations, present, according to their present configuration, scarce any resemblance to their antitypes. For instance, it is well known that the Great Bear was recognised by many nations besides the Greeks and those, whoever they may have been, from whom the Greeks derived the constellation. We learn that when America was discovered the Iroquois Indians called this constellation Okouari, or the Bear. So the inhabitants of Northern Asia, the Phoenicians, the Persians, and others, called this constellation the Bear. The Egyptians, not knowing the bear, called the constellation the Hippopotamus, an animal resembling the bear in several respects, as in its heavy body, short inconspicuous tail, small head, and short ears. Yet the constellation, as at present figured, is certainly not in the remotest degree like a bear. Apart from the enormous tail given in the pictures to the bear (almost tailless in reality), it is impossible for the liveliest imagination to recognise a bear as the constellation is at present formed. Flammarion says that, "even if we take in the smaller stars that stand in the feet and head, no ingenuity can make it in this or any other way resemble a bear," adding the absurd explanation given by Aristotle, "that the name is derived from the fact that of all human animals the bear was thought to be the only one that dared to venture into the frozen regions of the north, and tempt their solitude and cold." As though the shepherds and tillers of the soil, who first gave names to the stars, were likely to consider such far-fetched reasons, even if they had known either the habits of the polar bears or had considered the relation of the northern star-groups to the polar regions of the earth.

Now the question whether any real resemblance attracted the attention of the earlier observers in such cases as this is by no means without interest. If such a resemblance formerly existed, and does not now exist, it would follow that quite a considerable proportion of the stars have changed in brightness. Considering that each star is a sun, the centre, most probably, of a system like that which circles around our own sun, such a conclusion would be very startling indeed. It would have a special interest for ourselves, somewhat in the same way that the news that many railway accidents occur has an interest for those who travel much by rail. If accidents frequently happen to those other suns, in such sort that they either lose or gain greatly in brightness, an accident of one or other kind might well happen to our own sun, in which case the inhabitants of this earth would perish. For many of the stars, by our supposition, would have changed so much as either to lose their character as the defining stars of a constellation or by accession of brightness to acquire that character which in old times they had not possessed. Now, assuredly, a change of brightness competent to affect our sun's character (as viewed from any remote star system) in equal degree, would be destructive to the inhabitants of the earth. None at least of the higher races of animals or plants could bear the intense cold resulting from a change of the former kind, or the intense heat resulting from a change of the latter kind. Yet, if the constellations were once named because of their imagined resemblance to various objects, and if no such resemblance can now be even imagined, a change of one or other kind in the condition of our sun must be regarded as probable,--much in the same way that a regular traveller by train on any line must be regarded as exposed to danger, if accidents are known to be continually happening on that line.

What I now propose to do is to inquire whether we may not find the true figures and proportions of the ancient constellations in another way--viz., not by looking for them among the constellations as at present bounded and figured in our star-maps, but by searching the heavens themselves for them. This general method of search occurred to me very long ago while I was preparing various star-atlases, but the special mode of illustration here adopted occurred to me lately, while preparing for young astronomers in the United States a series of monthly maps showing the skies towards the north, south, east, and west, at different times of the night all the year round, and in various latitudes within the limits of the States. When I was in America I noticed, as I travelled about over a tolerably wide range of latitude, that the varying attitudes assumed by several of the constellations suggested features of resemblance to different objects. In constructing maps, simple in appearance, but based in reality on careful calculations, this characteristic came out more clearly. Adopting a particular way of presenting the connection between the various stars of a constellation, I often found the figure suggested which had actually been associated with the group of stars thus connected. Lastly, the idea of extending this method to other cases naturally occurred to me, and some of the results are presented in the present essay.

The method of delineation referred to is simply that of connecting the stars of a group by lines, ad libitum, that is, not merely introducing so many lines as will connect all the stars into a single set, but where necessary to complete the delineation of the imagined figure, adding other lines connecting pairs of stars belonging to the group, yet not so many that every pair of stars is connected by a line. The lines, again, need not be straight. On the contrary, where a group of stars forms a stream, the natural way of joining them is by lines so curved as to follow the serpentine course thus suggested. And in other cases a slight curvature of the lines joining pairs of stars will seem permissible, because corresponding to a configuration suggested by the stars themselves.

It is easily seen that in some of the simplest cases, the figure associated with a constellation is at once suggested by this method of delineation. For instance, take the case of the Northern Crown.

In this constellation we have a group which, while consisting of only a few stars, yet suggests very naturally the idea of a coronet of gems, as shown in fig. 32. The same is true also, though perhaps in less degree, of the Dolphin, as shown in fig. 33. It is noteworthy, by the way, that this constellation can hardly have been invented by landsmen. For though in our own time when the pictures of sea-creatures are accurately drawn, so that persons who have never been to sea may have a correct idea of the figure of such creatures, in old times it was exceedingly unlikely that any but sailors would have such familiar knowledge of the dolphin as to be reminded of that creature by a group of stars.

A much more complex constellation than either of those just mentioned--the Scorpion,--is even better represented by lineation, as shown in fig. 34. It is not, however, with cases so remarkable as these that the difficulty suggested at the outset is really connected. The instances of really remarkable resemblance are so few that they must be regarded as altogether exceptional. The best proof that the Scorpion is unmistakably pictured by the stars is to be found in the fact that the modern map-makers have not in this case departed much from the older delineations. No one, in fact, who knows what a Scorpion is like, could have any doubt as to the configuration of the body, at least, of the celestial Scorpion. So that though such a case illustrates well the way in which the method of delineation I have suggested may be made to picture the object seen by the ancient observers in the heavens, it does not afford any answer to the difficulty indicated by those who assert that the Great Bear, the Lion, the Ship, and other of the old constellation figures, have no real existence among the stars.