SATURN.
The Position of Saturn in the System--Saturn one of the Three most
Interesting Objects in the Heavens--Compared with Jupiter--Saturn
to the Unaided Eye--Statistics relating to the Planet--Density of
Saturn--Lighter than Water--The Researches of Galileo--What he
found in Saturn--A Mysterious Object--The Discoveries made by
Huyghens half a Century later--How the Existence of the Ring was
Demonstrated--Invisibility of the Rings every Fifteen Years--The
Rotation of the Planet--The Celebrated Cypher--The
Explanation--Drawing of Saturn--The Dark Line--W. Herschel's
Researches--Is the Division in the Ring really a
Separation?--Possibility of Deciding the Question--The Ring in a
Critical Position--Are there other Divisions in the Ring?--The
Dusky Ring--Physical Nature of Saturn's Rings--Can they be
Solid?--Can they even be Slender Rings?--A Fluid?--True Nature of
the Rings--A Multitude of Small Satellites--Analogy of the Rings of
Saturn to the Group of Minor Planets--Problems Suggested by
Saturn--The Group of Satellites to Saturn--The Discoveries of
Additional Satellites--The Orbit of Saturn not the Frontier of our
System.
At a profound distance in space, which, on an average, is 886,000,000
miles, the planet Saturn performs its mighty revolution around the sun
in a period of twenty-nine and a half years. This gigantic orbit formed
the boundary to the planetary system, so far as it was known to the
ancients.
Although Saturn is not so great a body as Jupiter, yet it vastly exceeds
the earth in bulk and in mass, and is, indeed, much greater than any one
of the planets, Jupiter alone excepted. But while Saturn must yield the
palm to Jupiter so far as mere dimensions are concerned, yet it will be
generally admitted that even Jupiter, with all the retinue by which he
is attended, cannot compete in beauty with the marvellous system of
Saturn. To the present writer it has always seemed that Saturn is one of
the three most interesting celestial objects visible to observers in
northern latitudes. The other two will occupy our attention in future
chapters. They are the great nebula in Orion, and the star cluster in
Hercules.
So far as the globe of Saturn is concerned, we do not meet with any
features which give to the planet any exceptional interest. The globe is
less than that of Jupiter, and as the latter is also much nearer to us,
the apparent size of Saturn is in a twofold way much smaller than that
of Jupiter. It should also be noticed that, owing to the greater
distance of Saturn from the sun, its intrinsic brilliancy is less than
that of Jupiter. There are, no doubt, certain marks and bands often to
be seen on Saturn, but they are not nearly so striking nor so
characteristic as the ever-variable belts upon Jupiter. The telescopic
appearance of the globe of Saturn must also be ranked as greatly
inferior in interest to that of Mars. The delicacy of detail which we
can see on Mars when favourably placed has no parallel whatever in the
dim and distant Saturn. Nor has Saturn, regarded again merely as a
globe, anything like the interest of Venus. The great splendour of Venus
is altogether out of comparison with that of Saturn, while the brilliant
crescent of the evening star is infinitely more pleasing than any
telescopic view of the globe of Saturn. Yet even while we admit all this
to the fullest extent, it does not invalidate the claim of Saturn to be
one of the most supremely beautiful and interesting objects in the
heavens. This interest is not due to his globe; it is due to that
marvellous system of rings by which Saturn is surrounded--a system
wonderful from every point of view, and, so far as our knowledge goes,
without a parallel in the wide extent of the universe.
[Illustration: Fig. 64. Saturn. (July 2nd, 1894. 36-in. equatorial.)
(Prof. E.E. Barnard.)]
To the unaided eye Saturn usually appears like a star of the first
magnitude. Its light alone would hardly be sufficient to discriminate it
from many of the brighter fixed stars. Yet the ancients were acquainted
with Saturn, and they knew it as a planet. It was included with the
other four great planets--Mercury, Venus, Mars, and Jupiter--in the
group of wanderers, which were bound to no fixed points of the sky like
the stars. On account of the great distance of Saturn, its movements are
much slower than those of the other planets known to the ancients.
Twenty-nine years and a half are required for this distant object to
complete its circuit of the heavens; and, though this movement is slow
compared with the incessant changes of Venus, yet it is rapid enough to
attract the attention of any careful observer. In a single year Saturn
moves through a distance of about twelve degrees, a quantity
sufficiently large to be conspicuous to casual observation. Even in a
month, or sometimes in a week, the planet traverses an arc of the sky
which can be detected by anyone who will take the trouble to mark the
place of the planet with regard to the stars in its vicinity. Those who
are privileged to use accurate astronomical instruments can readily
detect the motion of Saturn in a few hours.
The average distance from the sun to Saturn is about 886 millions of
miles. The path of Saturn, as of every other planet, is really an
ellipse with the sun in one focus. In the case of Saturn the shape of
this ellipse is very appreciably different from a purely circular path.
Around this path Saturn moves with an average velocity of 5.96 miles per
second.
The mean diameter of the globe of Saturn is about 71,000 miles. Its
equatorial diameter is about 75,000 miles, and its polar diameter 67,000
miles--the ratio of these numbers being approximately that of 10 to 9.
It is thus obvious that Saturn departs from the truly spherical shape to
a very marked extent. The protuberance at its equator must, no doubt, be
attributed to the high velocity with which the planet is rotating. The
velocity of rotation of Saturn is more than double as fast as that of
the earth, though it is not quite so fast as that of Jupiter. Saturn
makes one complete rotation in about 10 hrs. 14 min. Mr. Stanley
Williams has, however, observed with great care a number of spots which
he has discovered, and he finds that some of these spots in about 27 deg.
north latitude indicate rotation in a period of 10 hrs. 14 mins. to 15
min., while equatorial spots require no more than 10 hrs. 12 min. to 13
min. There is, however, the peculiarity that spots in the same latitude,
but at different parts of the planet, rotate at rates which differ by a
minute or more, while the period found by various groups of spots seems
to change from year to year.
These facts prove that Saturn and the spots do not form a rigid system.
The lightness of this planet is such as to be wholly incompatible with
the supposition that its globe is constituted of solid materials at all
comparable with those of which the crust of our earth is composed. The
satellites, which surround Saturn and form a system only less
interesting than the renowned rings themselves, enable us to weigh the
planet in comparison with the sun, and hence to deduce its actual mass
relatively to the earth. The result is not a little remarkable. It
appears that the density of the earth is eight times as great as that of
Saturn. In fact, the density of the latter is less than that of water
itself, so that a mighty globe of water, equal in bulk to Saturn, would
actually weigh more. If we could conceive a vast ocean into which a
globe equal to Saturn in size and weight were cast, the great globe
would not sink like our earth or like any of the other planets; it would
float buoyantly at the surface with one-fourth of its bulk out of the
water.
We thus learn with high probability that what our telescopes show upon
Saturn is not a solid surface, but merely a vast envelope of clouds
surrounding a heated interior. It is impossible to resist the suggestion
that this planet, like Jupiter, has still retained its heat because its
mass is so large. We must, however, allude to a circumstance which
perhaps may seem somewhat inconsistent with the view here taken. We have
found that Jupiter and Saturn are, both of them, much less dense than
the earth. When we compare the two planets together, it appears that
Saturn is much less dense than Jupiter. In fact, every cubic mile of
Jupiter weighs nearly twice as much as each cubic mile of Saturn. This
would seem to point to the conclusion that Saturn is the more heated of
the two bodies. Yet, as Jupiter is the larger, it might more reasonably
have been expected to be hotter than the other planet. We do not attempt
to reconcile this discrepancy; in fact, in our ignorance as to the
material constitution of these bodies, it would be idle to discuss the
question.
Even if we allow for the lightness of Saturn, as compared bulk for bulk
with the earth, yet the volume of Saturn is so enormous that the planet
weighs more than ninety-five times as much as the earth. The adjoining
view represents the relative sizes of Saturn and the earth (Fig. 65).
[Illustration: Fig. 65.--Relative Sizes of Saturn and the Earth.]
As the unaided eye discloses none of those marvels by which Saturn is
surrounded, the interest which attaches to this planet may be said to
commence from the time when it began to be observed with the telescope.
The history must be briefly alluded to, for it was only by degrees that
the real nature of this complicated object was understood. When Galileo
completed his little refracting telescope, which, though it only
magnified thirty times, was yet an enormous addition to the powers of
unaided vision, he made with it his memorable review of the heavens. He
saw the spots on the sun and the mountains on the moon; he noticed the
crescent of Venus and the satellites of Jupiter. Stimulated and
encouraged by such brilliant discoveries, he naturally sought to examine
the other planets, and accordingly directed his telescope to Saturn.
Here, again, Galileo at once made a discovery. He saw that Saturn
presented a visible form like the other planets, but that it differed
from any other telescopic object, inasmuch as it appeared to him to be
composed of three bodies which always touched each other and always
maintained the same relative positions. These three bodies were in a
line--the central one was the largest, and the two others were east and
west of it. There was nothing he had hitherto seen in the heavens which
filled his mind with such astonishment, and which seemed so wholly
inexplicable.
In his endeavours to understand this mysterious object, Galileo
continued his observations during the year 1610, and, to his amazement,
he saw the two lesser bodies gradually become smaller and smaller,
until, in the course of the two following years, they had entirely
vanished, and the planet simply appeared with a round disc like Jupiter.
Here, again, was a new source of anxiety to Galileo. He had at that day
to contend against the advocates of the ancient system of astronomy, who
derided his discoveries and refused to accept his theories. He had
announced his observation of the composite nature of Saturn; he had now
to tell of the gradual decline and the ultimate extinction of these two
auxiliary globes, and he naturally feared that his opponents would seize
the opportunity of pronouncing that the whole of his observations were
illusory.[25] "What," he remarks, "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 and 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 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."
But Galileo was not mistaken. The objects were really there when he
first began to observe, they really did decline, and they really
disappeared; but this disappearance was only for a time--they again came
into view. They were then subjected to ceaseless examination, until
gradually their nature became unfolded. With increased telescopic power
it was found that the two bodies which Galileo had described as globes
on either side of Saturn were not really spherical--they were rather two
luminous crescents with the concavity of each turned towards the central
globe. It was also perceived that these objects underwent a remarkable
series of periodic changes. At the beginning of such a series the planet
was found with a truly circular disc. The appendages first appeared as
two arms extending directly outwards on each side of the planet; then
these arms gradually opened into two crescents, resembling handles to
the globe, and attained their maximum width after about seven or eight
years; then they began to contract, until after the lapse of about the
same time they vanished again.
The true nature of these objects was at length discovered by Huyghens in
1655, nearly half a century after Galileo had first detected their
appearance. He perceived the shadow thrown by the ring upon the globe,
and his explanation of the phenomena was obtained in a very
philosophical manner. He noticed that the earth, the sun, and the moon
rotated upon their axes, and he therefore regarded it as a general law
that each one of the bodies in the system rotates about an axis. It is
true, observations had not yet been made which actually showed that
Saturn was also rotating; but it would be highly, nay, indeed,
infinitely, improbable that any planet should be devoid of such
movement. All the analogies of the system pointed to the conclusion that
the velocity of rotation would be considerable. One satellite of Saturn
was already known to revolve in a period of sixteen days, being little
more than half our month. Huyghens assumed--and it was a most reasonable
assumption--that Saturn in all probability rotated rapidly on its axis.
It was also to be observed that if these remarkable appendages were
attached by an actual bodily connection to the planet they must rotate
with Saturn. If, however, the appendages were not actually attached it
would still be necessary that they should rotate if the analogy of
Saturn to other objects in the system were to be in any degree
preserved. We see satellites near Jupiter which revolve around him. We
see, nearer home, how the moon revolves around the earth. We see how all
the planetary system revolves around the sun. All these considerations
were present to Huyghens when he came to the conclusion that, whether
the curious appendages were actually attached to the planet or were
physically free from it, they must still be in rotation.
Provided with such reasonings, it soon became easy to conjecture the
true nature of the Saturnian system. We have seen how the appendages
declined to invisibility once every fifteen years, and then gradually
reappeared in the form, at first, of rectilinear arms projecting
outwards from the planet. The progressive development is a slow one, and
for weeks and months, night after night, the same appearance is
presented with but little change. But all this time both Saturn and the
mysterious objects around him are rotating. Whatever these may be, they
present the same appearance to the eye, notwithstanding their ceaseless
motion of rotation.
What must be the shape of an object which satisfies the conditions here
implied? It will obviously not suffice to regard the projections as two
spokes diverging from the planet. They would change from visibility to
invisibility in every rotation, and thus there would be ceaseless
alterations of the appearance instead of that slow and gradual change
which requires fifteen years for a complete period. There are, indeed,
other considerations which preclude the possibility of the objects being
anything of this character, for they are always of the same length as
compared with the diameter of the planet. A little reflection will show
that one supposition--and indeed only one--will meet all the facts of
the case. If there were a thin symmetrical ring rotating in its own
plane around the equator of Saturn, then the persistence of the object
from night to night would be accounted for. This at once removes the
greater part of the difficulty. For the rest, it was only necessary to
suppose that the ring was so thin that when turned actually edgewise to
the earth it became invisible, and then as the illuminated side of the
plane became turned more and more towards the earth the appendages to
the planet gradually increased. The handle-shaped appearance which the
object periodically assumed demonstrated that the ring could not be
attached to the globe.
At length Huyghens found that he had the clue to the great enigma which
had perplexed astronomers for the last fifty years. He saw that the ring
was an object of astonishing interest, unique at that time, as it is,
indeed, unique still. He felt, however, that he had hardly demonstrated
the matter with all the certainty which it merited, and which he thought
that by further attention he could secure. Yet he was loath to hazard
the loss of his discovery by an undue postponement of its announcement,
lest some other astronomer might intervene. How, then, was he to secure
his priority if the discovery should turn out correct, and at the same
time be enabled to perfect it at his leisure? He adopted the course,
usual at the time, of making his first announcement in cipher, and
accordingly, on March 5th, 1656, he published a tract, which contained
the following proposition:--
aaaaaaa ccccc d eeeee g h
iiiiiii llll mm nnnnnnnnn
oooo pp q rr s ttttt uuuuu
Perhaps some of those curious persons whose successors now devote so
much labour to double acrostics may have pondered on this renowned
cryptograph, and even attempted to decipher it. But even if such
attempts were made, we do not learn that they were successful. A few
years of further study were thus secured to Huyghens. He tested his
theory in every way that he could devise, and he found it verified in
every detail. He therefore thought that it was needless for him any
longer to conceal from the world his great discovery, and accordingly in
the year 1659--about three years after the appearance of his
cryptograph--he announced the interpretation of it. By restoring the
letters to their original arrangement the discovery was enunciated in
the following words:--"_Annulo cingitur_, _tenui_, _plano_, _nusquam
cohaerente_, _ad eclipticam inclinato_," which may be translated into the
statement:--"The planet is surrounded by a slender flat ring everywhere
distinct from its surface, and inclined to the elliptic."
Huyghens was not content with merely demonstrating how fully this
assumption explained all the observed phenomena. He submitted it to the
further and most delicate test which can be applied to any astronomical
theory. He attempted by its aid to make a prediction the fulfilment of
which would necessarily give his theory the seal of certainty. From his
calculations he saw that the planet would appear circular about July or
August in 1671. This anticipation was practically verified, for the ring
was seen to vanish in May of that year. No doubt, with our modern
calculations founded on long-continued and accurate observation, we are
now enabled to make forecasts as to the appearance or the disappearance
of Saturn's ring with far greater accuracy; but, remembering the early
stage in the history of the planet at which the prediction of Huyghens
was made, we must regard its fulfilment as quite sufficient, and as
confirming in a satisfactory manner the theory of Saturn and his ring.
The ring of Saturn having thus been thoroughly established as a fact in
celestial architecture, each generation of astronomers has laboured to
find out more and more of its marvellous features. In the frontispiece
(Plate I.) we have a view of the planet as seen at the Harvard College
Observatory, U.S.A., between July 28th and October 20th, 1872. It has
been drawn by the skilful astronomer and artist--Mr. L. Trouvelot--and
gives a faithful and beautiful representation of this unique object.
Fig. 64 is a drawing of the same object taken on July 2nd, 1894, by
Prof. E.E. Barnard, at the Lick Observatory.
The next great discovery in the Saturnian system after those of Huyghens
showed that the ring surrounding the planet was marked by a dark
concentric line, which divided it into two parts--the outer being
narrower than the inner. This line was first seen by J.D. Cassini, when
Saturn emerged from the rays of the sun in 1675. That this black line
is not merely a black mark on the ring, but that it is actually a
separation, was rendered very probable by the researches of Maraldi in
1715, followed many years later by those of Sir William Herschel, who,
with that thoroughness which was a marked characteristic of the man,
made a minute and scrupulous examination of Saturn. Night after night he
followed it for hours with his exquisite instruments, and considerably
added to our knowledge of the planet and his system.
Herschel devoted very particular attention to the examination of the
line dividing the ring. He saw that the colour of this line was not to
be distinguished from the colour of the space intermediate between the
globe and the ring. He observed it for ten years on the northern face of
the ring, and during that time it continued to present the same breadth
and colour and sharpness of outline. He was then fortunate enough to
observe the southern side of the ring. There again could the black line
be seen, corresponding both in appearance and in position with the dark
line as seen on the northern side. No doubt could remain as to the fact
that Saturn was girdled by two concentric rings equally thin, the outer
edge of one closely approaching to the inner edge of the other.
At the same time it is right to add that the only absolutely
indisputable proof of the division between the rings has not yet been
yielded by the telescope. The appearances noted by Herschel would be
consistent with the view that the black line was merely a part of the
ring extending through its thickness, and composed of materials very
much less capable of reflecting light than the rest of the ring. It is
still a matter of doubt how far it is ever possible actually to see
through the dark line. There is apparently only one satisfactory method
of accomplishing this. It would only occur in rare circumstances, and it
does not seem that the opportunity has as yet arisen. Suppose that in
the course of its motion through the heavens the path of Saturn happened
to cross directly between the earth and a fixed star. The telescopic
appearance of a star is merely a point of light much smaller than the
globes and rings of Saturn. If the ring passed in front of the star and
the black line on the ring came over the star, we should, if the black
line were really an opening, see the star shining through the narrow
aperture.
Up to the present, we believe, there has been no opportunity of
submitting the question of the duplex character of the ring to this
crucial test. Let us hope that as there are now so many telescopes in
use adequate to deal with the subject, there may, ere long, be
observations made which will decide the question. It can hardly be
expected that a very small star would be suitable. No doubt the
smallness of the star would render the observations more delicate and
precise if the star were visible; but we must remember that it will be
thrown into contrast with the bright rings of Saturn on each margin so
that unless the star were of considerable magnitude it would hardly
answer. It has, however, been recently observed that the globe of the
planet can be, in some degree, discerned through the dark line; this is
practically a demonstration of the fact that the line is at all events
partly transparent.
The outer ring is also divided into two by a line much fainter than that
just described. It requires a good telescope and a fine night, combined
with a favourable position of the planet, to render this line a
well-marked object. It is most easily seen at the extremities of the
ring most remote from the planet. To the present writer, who has
examined the planet with the twelve-inch refractor of the South
equatorial at Dunsink Observatory, this outer line appears as broad as
the well-known line; but it is unquestionably fainter, and has a more
shaded appearance. It certainly does not suggest the appearance of being
actually an opening in the ring, and it is often invisible for a long
time. It seems rather as if the ring were at this place thinner and less
substantial without being actually void of substance.
On these points it may be expected that much additional information will
be acquired when next the ring places itself in such a position that its
plane, if produced, would pass between the earth and the sun. Such
occasions are but rare, and even when they do occur it may happen that
the planet will not be well placed for observation. The next really
good opportunity will not be till 1907. In this case the sunlight
illuminates one side of the ring, while it is the other side of the ring
that is presented towards the earth. Powerful telescopes are necessary
to deal with the planet under such circumstances; but it may be
reasonably hoped that the questions relating to the division of the
ring, as well as to many other matters, will then receive some further
elucidation.
Occasionally, other divisions of the ring, both inner and outer, have
been recorded. It may, at all events, be stated that no such divisions
can be regarded as permanent features. Yet their existence has been so
frequently enunciated by skilful observers that it is impossible to
doubt that they have been sometimes seen.
It was about 200 years after Huyghens had first explained the true
theory of Saturn that another very important discovery was effected. It
had, up to the year 1850, been always supposed that the two rings,
divided by the well-known black line, comprised the entire ring system
surrounding the planet. In the year just mentioned, Professor Bond, the
distinguished astronomer of Cambridge, Mass., startled the astronomical
world by the announcement of his discovery of a third ring surrounding
Saturn. As so often happens in such cases, the same object was
discovered independently by another--an English astronomer named Dawes.
This third ring lies just inside the inner of the two well-known rings,
and extends to about half the distance towards the body of the planet.
It seems to be of a totally different character from the two other rings
in so far as they present a comparatively substantial appearance. We
shall, indeed, presently show that they are not solid--not even liquid
bodies--but still, when compared with the third ring, the others were of
a substantial character. They can receive and exhibit the deeply-marked
shadow of Saturn, and they can throw a deep and black shadow upon Saturn
themselves; but the third ring is of a much less compact texture. It has
not the brilliancy of the others, it is rather of a dusky,
semi-transparent appearance, and the expression "crape ring," by which
it is often designated, is by no means inappropriate. It is the
faintness of this crape ring which led to its having been so frequently
overlooked by the earlier observers of Saturn.
It has often been noticed that when an astronomical discovery has been
made with a good telescope, it afterwards becomes possible for the same
object to be observed with instruments of much inferior power. No doubt,
when the observer knows what to look for, he will often be able to see
what would not otherwise have attracted his attention. It may be
regarded as an illustration of this principle, that the crape ring of
Saturn has become an object familiar to those who are accustomed to work
with good telescopes; but it may, nevertheless, be doubted whether the
ease and distinctness with which the crape ring is now seen can be
entirely accounted for by this supposition. Indeed, it seems possible
that the crape ring has, from some cause or other, gradually become more
and more visible. The supposed increased brightness of the crape ring is
one of those arguments now made use of to prove that in all probability
the rings of Saturn are at this moment undergoing gradual
transformation; but observations of Hadley show that the crape ring was
seen by him in 1720, and it was previously seen by Campani and Picard,
as a faint belt crossing the planet. The partial transparency of the
crape ring was beautifully illustrated in an observation by Professor
Barnard of the eclipse of Iapetus on November 1st, 1889. The satellite
was faintly visible in the shadow of the crape ring, while wholly
invisible in the shadow of the better known rings.
The various features of the rings are well shown in the drawing of
Trouvelot already referred to. We here see the inner and the outer ring,
and the line of division between them. We see in the outer ring the
faint traces of the line by which it is divided, and inside the inner
ring we have a view of the curious and semi-transparent crape ring. The
black shadow of the planet is cast upon the ring, thus proving that the
ring, no less than the body of the planet, shines only in virtue of the
sunlight which falls upon it. This shadow presents some anomalous
features, but its curious irregularity may be, to some extent, an
optical illusion.
There can be no doubt that any attempt to depict the rings of Saturn
only represents the salient features of that marvellous system. We are
situated at such a great distance that all objects not of colossal
dimensions are invisible. We have, indeed, only an outline, which makes
us wish to be able to fill in the details. We long, for instance, to see
the actual texture of the rings, and to learn of what materials they are
made; we wish to comprehend the strange and filmy crape ring, so unlike
any other object known to us in the heavens. There is no doubt that much
may even yet be learned under all the disadvantageous conditions of our
position; there is still room for the labour of whole generations of
astronomers provided with splendid instruments. We want accurate
drawings of Saturn under every conceivable aspect in which it may be
presented. We want incessantly repeated measurements, of the most
fastidious accuracy. These measures are to tell us the sizes and the
shapes of the rings; they are to measure with fidelity the position of
the dark lines and the boundaries of the rings. These measures are to be
protracted for generations and for centuries; then and then only can
terrestrial astronomers learn whether this elaborate system has really
the attributes of permanence, or whether it may be undergoing changes.
We have been accustomed to find that the law of universal gravitation
pervades every part of our system, and to look to gravitation for the
explanation of many phenomena otherwise inexplicable. We have good
reasons for knowing that in this marvellous Saturnian system the law of
gravitation is paramount. There are satellites revolving around Saturn
as well as a ring; these satellites move, as other satellites do, in
conformity with the laws of Kepler; and, therefore, any theory as to the
nature of Saturn's ring must be formed subject to the condition that it
shall be attracted by the gigantic planet situated in its interior.
To a hasty glance nothing might seem easier than to reconcile the
phenomena of the ring with the attraction of the planet. We might
suppose that the ring stands at rest symmetrically around the planet. At
its centre the planet pulls in the ring equally on all sides, so that
there is no tendency in it to move in one way rather than another; and,
therefore, it will stay at rest. This will not do. A ring composed of
materials almost infinitely rigid might possibly, under such
circumstances, be for a moment at rest; but it could not remain
permanently at rest any more than can a needle balanced vertically on
its point. In each case the equilibrium is unstable. If the slightest
cause of disturbance arise, the equilibrium is destroyed, and the ring
would inevitably fall in upon the planet. Such causes of derangement are
incessantly present, so that unstable equilibrium cannot be an
appropriate explanation of the phenomena.
Even if this difficulty could be removed, there is still another, which
would be quite insuperable if the ring be composed of any materials with
which we are acquainted. Let us ponder for a moment on the matter, as it
will lead up naturally to that explanation of the rings of Saturn which
is now most generally accepted.
Imagine that you stood on the planet Saturn, near his equator; over your
head stretches the ring, which sinks down to the horizon in the east and
in the west. The half-ring above your horizon would then resemble a
mighty arch, with a span of about a hundred thousand miles. Every
particle of this arch is drawn towards Saturn by gravitation, and if the
arch continue to exist, it must do so in obedience to the ordinary
mechanical laws which regulate the railway arches with which we are
familiar.
The continuance of these arches depends upon the resistance of the
stones forming them to a crushing force. Each stone of an arch is
subjected to a vast pressure, but stone is a material capable of
resisting such pressure, and the arch remains. The wider the span of the
arch the greater is the pressure to which each stone is exposed. At
length a span is reached which corresponds to a pressure as great as the
stones can safely bear, and accordingly we thus find the limiting span
over which a single arch of masonry can be constructed. Apply these
principles to the stupendous arch formed by the ring of Saturn. It can
be shown that the pressure on the materials of the arch capable of
spanning an abyss of such awful magnitude would be something so enormous
that no materials we know of would be capable of bearing it. Were the
ring formed of the toughest steel that was ever made, the pressure would
be so great that the metal would be squeezed like a liquid, and the
mighty structure would collapse and fall down on the surface of the
planet. It is not credible that any materials could exist capable of
sustaining a stress so stupendous. The law of gravitation accordingly
bids us search for a method by which the intensity of this stress can be
mitigated.
One method is at hand, and is obviously suggested by analogous phenomena
everywhere in our system. We have spoken of the ring as if it were at
rest; let us now suppose it to be animated by a motion of rotation in
its plane around Saturn as a centre. Instantly we have a force developed
antagonistic to the gravitation of Saturn. This force is the so-called
centrifugal force. If we imagine the ring to rotate, the centrifugal
force at all points acts in an opposite direction to the attractive
force, and hence the enormous stress on the ring can be abated and one
difficulty can be overcome.
We can thus attribute to each ring a rotation which will partly relieve
it from the stress the arch would otherwise have to sustain. But we
cannot admit that the difficulty has been fully removed. Suppose that
the outer ring revolve at such a rate as shall be appropriate to
neutralise the gravitation on its outer edge, the centrifugal force will
be less at the interior of the ring, while the gravitation will be
greater; and hence vast stresses will be set up in the interior parts of
the outer ring. Suppose the ring to rotate at such a rate as would be
adequate to neutralise the gravitation at its inner margin; then the
centrifugal force at the outer parts will largely exceed the
gravitation, and there will be a tendency to disruption of the ring
outwards.
To obviate this tendency we may assume the outer parts of each ring to
rotate more slowly than the inner parts. This naturally requires that
the parts of the ring shall be mobile relatively to one another, and
thus we are conducted to the suggestion that perhaps the rings are
really composed of matter in a fluid state. The suggestion is, at first
sight, a plausible one; each part of each ring would then move with an
appropriate velocity, and the rings would thus exhibit a number of
concentric circular currents with different velocities. The
mathematician can push this inquiry a little farther, and he can study
how this fluid would behave under such circumstances. His symbols can
pursue the subject into the intricacies which cannot be described in
general language. The mathematician finds that waves would originate in
the supposed fluid, and that as these waves would lead to disruption of
the rings, the fluid theory must be abandoned.
But we can still make one or two more suppositions. What if it be really
true that the ring consist of an incredibly large number of concentric
rings, each animated precisely with the velocity which would be suitable
to the production of a centrifugal force just adequate to neutralise the
attraction? No doubt this meets many of the difficulties: it is also
suggested by those observations which have shown the presence of several
dark lines on the ring. Here again dynamical considerations must be
invoked for the reply. Such a system of solid rings is not compatible
with the laws of dynamics.
We are, therefore, compelled to make one last attempt, and still further
to subdivide the ring. It may seem rather startling to abandon entirely
the supposition that the ring is in any sense a continuous body, but
there remains no alternative. Look at it how we will, we seem to be
conducted to the conclusion that the ring is really an enormous shoal of
extremely minute bodies; each of these little bodies pursues an orbit of
its own around the planet, and is, in fact, merely a satellite. These
bodies are so numerous and so close together that they seem to us to be
continuous, and they may be very minute--perhaps not larger than the
globules of water found in an ordinary cloud over the surface of the
earth, which, even at a short distance, seems like a continuous body.
Until a few years ago this theory of the constitution of Saturn's rings,
though unassailable from a mathematical point of view, had never been
confirmed by observation. The only astronomer who maintained that he had
actually seen the rings rotate was W. Herschel, who watched the motion
of some luminous points on the ring in 1789, at which time the plane of
the ring happened to pass through the earth. From these observations
Herschel concluded that the ring rotated in ten hours and thirty-two
minutes. But none of the subsequent observers, even though they may have
watched Saturn with instruments very superior to that used by Herschel,
were ever able to succeed in verifying his rotation of these appendages
of Saturn. If the ring were composed of a vast number of small bodies,
then the third law of Kepler will enable us to calculate the time which
these tiny satellites would require to travel completely round the
planet. It appears that any satellite situated at the outer edge of the
ring would require as long a period as 13 hrs. 46 min., those about the
middle would not need more than 10 hrs. 28 min., while those at the
inner edge of the ring would accomplish their rotation in 7 hrs. 28 min.
Even our mightiest telescopes, erected in the purest skies and employed
by the most skilful astronomers, refuse to display this extremely
delicate phenomenon. It would, indeed, have been a repetition on a grand
scale of the curious behaviour of the inner satellite of Mars, which
revolves round its primary in a shorter time than the planet itself
takes to turn round on its own axis.
[Illustration: Fig. 66.--Prof. Keeler's Method of Measuring the
Rotation of Saturn's Ring.]
But what the telescope could not show, the spectroscope has lately
demonstrated in a most effective and interesting manner. We have
explained in the chapter on the sun how the motion of a source of light
along the line of vision, towards or away from the observer, produces a
slight shift in the position of the lines of the spectrum. By the
measurement of the displacement of the lines the direction and amount of
the motion of the source of light may be determined. We illustrated the
method by showing how it had actually been used to measure the speed of
rotation of the solar surface. In 1895 Professor Keeler,[26] Director of
the Allegheny Observatory, succeeded in measuring the rotation of
Saturn's ring in this manner. He placed the slit of his spectroscope
across the ball, in the direction of the major axis of the elliptic
figure which the effect of perspective gives the ring as shown by the
parallel lines in Fig. 66 stretching from E to W. His photographic
plate should then show three spectra close together, that of the ball of
Saturn in the middle, separated by dark intervals from the narrower
spectra above and below it of the two handles (or ansae, as they are
generally called) of the ring. In Fig. 67 we have represented the
behaviour of any one line of the spectrum under various suppositions as
to rotation or non-rotation of Saturn and the ring. At the top (1) we
see how each line would look if there was no rotatory motion; the three
lines produced by ring, planet, and ring are in a straight line. Of
course the spectrum, which is practically a very faint copy of the solar
spectrum, shows the principal dark Fraunhofer lines, so that the reader
must imagine these for himself, parallel to the one we show in the
figure. But Saturn and the ring are not standing still, they are
rotating, the eastern part (at E) moving towards us, and the western
part (W) moving away from us.[27] At E the line will therefore be
shifted towards the violet end of the spectrum and at W towards the red,
and as the actual linear velocity is greater the further we get away
from the centre of Saturn (assuming ring and planet to rotate together),
the lines would be turned as in Fig. 67 (2), but the three would remain
in a straight line. If the ring consisted of two independent rings
separated by Cassini's division and rotating with different velocities,
the lines would be situated as in Fig. 67 (3), the lines due to the
inner ring being more deflected than those due to the outer ring, owing
to the greater velocity of the inner ring.
[Illustration: Fig. 67.--Prof. Keeler's Method of Measuring the Rotation
of Saturn's Ring.]
Finally, let us consider the case of the rings, consisting of
innumerable particles moving round the planet in accordance with
Kepler's third law. The actual velocities of these particles would be
per second:--
At outer edge of ring 10.69 miles.
At middle of ring 11.68 miles.
At inner edge of ring 13.01 miles.
Rotation speed at surface of planet 6.38 miles.
The shifting of the lines of the spectrum should be in accordance with
these velocities, and it is easy to see that the lines ought to lie as
in the fourth figure. When Professor Keeler came to examine the
photographed spectra, he found the lines of the three spectra tilted
precisely in this manner, showing that the outer edge of the ring was
travelling round the planet with a smaller linear velocity than the
inner one, as it ought to do if the sources of light (or, rather, the
reflectors of sunlight) were independent particles free to move
according to Kepler's third law, and as it ought not to do if the ring,
or rings, were rigid, in which case the outer edge would have the
greatest linear speed, as it had to travel through the greatest
distance. Here, at last, was the proof of the meteoritic composition of
Saturn's ring. Professor Keeler's beautiful discovery has since been
verified by repeated observations at the Allegheny, Lick, Paris, and
Pulkova Observatories; the actual velocities resulting from the observed
displacements of the lines have been measured and found to agree well
(within the limits of the errors of observation) with the calculated
velocities, so that this brilliant confirmation of the mathematical
deductions of Clerk Maxwell is raised beyond the possibility of doubt.
The spectrum of Saturn is so faint that only the strongest lines of the
solar spectrum can be seen in it, but the atmosphere of the planet seems
to exert a considerable amount of general absorption in the blue and
violet parts of the spectrum, which is especially strong near the
equatorial belt, while a strong band in the red testifies to the density
of the atmosphere. This band is not seen in the spectrum of the rings,
around which there can therefore be no atmosphere.
As Saturn's ring is itself unique, we cannot find elsewhere any very
pertinent illustration of a structure so remarkable as that now claimed
for the ring. Yet the solar system does show some analogous phenomena.
There is, for instance, one on a very grand scale surrounding the sun
himself. We allude to the multitude of minor planets, all confined
within a certain region of the system. Imagine these planets to be
vastly increased in number, and those orbits which are much inclined to
the rest flattened down and otherwise adjusted, and we should have a
ring surrounding the sun, thus producing an arrangement not dissimilar
from that now attributed to Saturn.
It is tempting to linger still longer over this beautiful system, to
speculate on the appearance which the ring would present to an
inhabitant of Saturn, to conjecture whether it is to be regarded as a
permanent feature of our system in the same way as we attribute
permanence to our moon or to the satellites of Jupiter. Looked at from
every point of view, the question is full of interest, and it provides
occupation abundant for the labours of every type of astronomer. If he
be furnished with a good telescope, then has he ample duties to fulfil
in the task of surveying, of sketching, and of measuring. If he be one
of those useful astronomers who devote their energies not to actual
telescopic work, but to forming calculations based on the observations
of others, then the beautiful system of Saturn provides copious
material. He has to foretell the different phases of the ring, to
announce to astronomers when each feature can be best seen, and at what
hour each element can be best determined. He has also to predict the
times of the movements of Saturn's satellites, and the other phenomena
of a system more elaborate than that of Jupiter.
Lastly, if the astronomer be one of that class--perhaps, from some
points of view, the highest class of all--who employ the most profound
researches of the human intellect to unravel the dynamical problems of
astronomy, he, too, finds in Saturn problems which test to the utmost,
even if they do not utterly transcend, the loftiest flights of analysis.
He discovers in Saturn's ring an object so utterly unlike anything else,
that new mathematical weapons have to be forged for the encounter. He
finds in the system so many extraordinary features, and such delicacy of
adjustment, that he is constrained to admit that if he did not actually
see Saturn's rings before him, he would not have thought that such a
system was possible. The mathematician's labours on this wondrous system
are at present only in their infancy. Not alone are the researches of so
abstruse a character as to demand the highest genius for this branch of
science, but even yet the materials for the inquiry have not been
accumulated. In a discussion of this character, observation must precede
calculation. The scanty observations hitherto obtained, however they may
illustrate the beauty of the system, are still utterly insufficient to
form the basis of that great mathematical theory of Saturn which must
eventually be written.
But Saturn possesses an interest for a far more numerous class of
persons than those who are specially devoted to astronomy. It is of
interest, it must be of interest, to every cultivated person who has the
slightest love for nature. A lover of the picturesque cannot behold
Saturn in a telescope without feelings of the liveliest emotion; while,
if his reading and reflection have previously rendered him aware of the
colossal magnitude of the object at which he is looking, he will be
constrained to admit that no more remarkable spectacle is presented in
the whole of nature.
We have pondered so long over the fascinations of Saturn's ring that we
can only give a very brief account of that system of satellites by which
the planet is attended. We have already had occasion to allude more than
once to these bodies; it only remains now to enumerate a few further
particulars.
It was on the 25th of March, 1655, that the first satellite of Saturn
was detected by Huyghens, to whose penetration we owe the discovery of
the true form of the ring. On the evening of the day referred to,
Huyghens was examining Saturn with a telescope constructed with his own
hands, when he observed a small star-like object near the planet. The
next night he repeated his observations, and it was found that the star
was accompanying the planet in its progress through the heavens. This
showed that the little object was really a satellite to Saturn, and
further observations revealed the fact that it was revolving around him
in a period of 15 days, 22 hours, 41 minutes. Such was the commencement
of that numerous series of discoveries of satellites which accompany
Saturn. One by one they were detected, so that at the present time no
fewer than nine are known to attend the great planet through his
wanderings. The subsequent discoveries were, however, in no case made by
Huyghens, for he abandoned the search for any further satellites on
grounds which sound strange to modern ears, but which were quite in
keeping with the ideas of his time. It appears that from some principle
of symmetry, Huyghens thought that it would accord with the fitness of
things that the number of satellites, or secondary planets, should be
equal in number to the primary planets themselves. The primary planets,
including the earth, numbered six; and Huyghens' discovery now brought
the total number of satellites to be also six. The earth had one,
Jupiter had four, Saturn had one, and the system was complete.
Nature, however, knows no such arithmetical doctrines as those which
Huyghens attributed to her. Had he been less influenced by such
prejudices, he might, perhaps, have anticipated the labours of Cassini,
who, by discovering other satellites of Saturn, demonstrated the
absurdity of the doctrine of numerical equality between planets and
satellites. As further discoveries were made, the number of satellites
was at first raised above the number of planets; but in recent times,
when the swarm of minor planets came to be discovered, the number of
planets speedily reached and speedily passed the number of their
attendant satellites.
It was in 1671, about sixteen years after the discovery of the first
satellite of Saturn, that a second was discovered by Cassini. This is
the outermost of the older satellites; it takes 79 days to travel round
Saturn. In the following year he discovered another; and twelve years
later, in 1684, still two more; thus making a total of five satellites
to this planet.
[Illustration: Fig. 68.--Transit of Titan and its Shadow, by F. Terby
Louvain, 12th April, 1892.]
The complexity of the Saturnian system had now no rival in the heavens.
Saturn had five satellites, and Jupiter had but four, while at least one
of the satellites of Saturn, named Titan, was larger than any satellite
of Jupiter.[28] Some of the discoveries of Cassini had been made with
telescopes of quite monstrous dimensions. The length of the instrument,
or rather the distance at which the object-glass was placed, was one
hundred feet or more from the eye of the observer. It seemed hardly
possible to push telescopic research farther with instruments of this
cumbrous type. At length, however, the great reformation in the
construction of astronomical instruments began to dawn. In the hands of
Herschel, it was found possible to construct reflecting telescopes of
manageable dimensions, which were both more powerful and more accurate
than the long-focussed lenses of Cassini. A great instrument of this
kind, forty feet long, just completed by Herschel, was directed to
Saturn on the 28th of August, 1789. Never before had the wondrous planet
been submitted to a scrutiny so minute. Herschel was familiar with the
labours of his predecessors. He had often looked at Saturn and his five
moons in inferior telescopes; now again he saw the five moons and a
star-like object so near the plane of the ring that he conjectured this
to be a sixth satellite. A speedy method of testing this conjecture was
at hand. Saturn was then moving rapidly over the heavens. If this new
object were in truth a satellite, then it must be carried on by Saturn.
Herschel watched with anxiety to see whether this would be the case. A
short time sufficed to answer the question; in two hours and a half the
planet had moved to a distance quite appreciable, and had carried with
him not only the five satellites already known, but also this sixth
object. Had this been a star it would have been left behind; it was not
left behind, and hence it, too, was a satellite. Thus, after the long
lapse of a century, the telescopic discovery of satellites to Saturn
recommenced. Herschel, as was his wont, observed this object with
unremitting ardour, and discovered that it was much nearer to Saturn
than any of the previously known satellites. In accordance with the
general law, that the nearer the satellite the shorter the period of
revolution, Herschel found that this little moon completed a revolution
in about 1 day, 8 hours, 53 minutes. The same great telescope, used with
the same unrivalled skill, soon led Herschel to a still more interesting
discovery. An object so small as only to appear like a very minute
point in the great forty-foot reflector was also detected by Herschel,
and was by him proved to be a satellite, so close to the planet that it
completed a revolution in the very brief period of 22 hours and 37
minutes. This is an extremely delicate object, only to be seen by the
best telescopes in the brief intervals when it is not entirely screened
from view by the ring.
Again another long interval elapsed, and for almost fifty years the
Saturnian system was regarded as consisting of the series of rings and
of the seven satellites. The next discovery has a singular historical
interest. It was made simultaneously by two observers--Professor Bond,
of Cambridge, Mass., and Mr. Lassell, of Liverpool--for on the 19th
September, 1848, both of these astronomers verified that a small point
which they had each seen on previous nights was really a satellite. This
object is, however, at a considerable distance from the planet, and
requires 21 days, 7 hours, 28 minutes for each revolution; it is the
seventh in order from the planet.
Yet one more extremely faint outer satellite was discerned by
photography on the 16th, 17th, and 18th August, 1898, by Professor W.H.
Pickering. This object is much more distant from the planet than the
larger and older satellites. Its motion has not yet been fully
determined, but probably it requires not less than 490 days to perform a
single revolution.
From observations of the satellites it has been found that 3,500 globes
as heavy as Saturn would weigh as much as the sun.
A law has been observed by Professor Kirkwood, which connects together
the movements of the four interior satellites of Saturn. This law is
fulfilled in such a manner as leads to the supposition that it arises
from the mutual attraction of the satellites. We have already described
a similar law relative to three of the satellites of Jupiter. The
problem relating to Saturn, involving as it does no fewer than four
satellites, is one of no ordinary complexity. It involves the theory of
Perturbations to a greater degree than that to which mathematicians are
accustomed in their investigation of the more ordinary features of our
system. To express this law it is necessary to have recourse to the
daily movements of the satellites; these are respectively--
SATELLITE. DAILY MOVEMENT.
I. 382 deg..2.
II. 262 deg..74.
III. 190 deg..7.
IV. 131 deg..4.
The law states that if to five times the movement of the first satellite
we add that of the third and four times that of the fourth, the whole
will equal ten times the movement of the second satellite. The
calculation stands thus:--
5 times I. equals 1911 deg..0
III. equals 190 deg..7 II. 262 deg..74
4 times IV. equals 525 deg..6 10
-------- --------
2627 deg..3 equal 2627 deg..4 nearly.
Nothing can be simpler than the verification of this law; but the task
of showing the physical reason why it should be fulfilled has not yet
been accomplished.
Saturn was the most distant planet known to the ancients. It revolves in
an orbit far outside the other ancient planets, and, until the discovery
of Uranus in the year 1781, the orbit of Saturn might well be regarded
as the frontier of the solar system. The ringed planet was indeed a
worthy object to occupy a position so distinguished. But we now know
that the mighty orbit of Saturn does not extend to the frontiers of the
solar system; a splendid discovery, leading to one still more splendid,
has vastly extended the boundary, by revealing two mighty planets,
revolving in dim telescopic distance, far outside the path of Saturn.
These objects have not the beauty of Saturn; they are, indeed, in no
sense effective telescopic pictures. Yet these outer planets awaken an
interest of a most special kind. The discovery of each is a classical
event in the history of astronomy, and the opinion has been maintained,
and perhaps with reason, that the discovery of Neptune, the more remote
of the two, is the greatest achievement in astronomy made since the time
of Newton.