_THE RINGED PLANET SATURN._
Very different from the ruddy planet which approached so closely to him
in November, 1877, is Saturn, the ringed world, the most wonderful of
all the planets if the complexity of the system attending on him is
considered, and in size inferior only to the giant Jupiter.
It will have been noticed, perhaps, by those who are familiar with the
aspect of the planets, that the contrast between Mars and Saturn during
their late approach to us was not only greater than usual, but greater
than was to be expected even when account was taken of the unusual
lustre of Mars. I have often wondered whether the ancient astronomers
were ever perplexed by the varying lustre of Saturn. They recognised the
fact that Mars has an orbit of great eccentricity (see the picture of
the orbits of Mars, Venus, etc., at page 156); and there was nothing in
the varying lustre of Mars which could not be perfectly well explained
by his known variations of distance, whether the Ptolemaic or the
Copernican system were accepted. But with Saturn the case is different.
His distance at successive returns to our midnight skies is subject to
moderate changes only. Yet his brilliancy varies in a remarkable manner.
We now know that those changes are due to the opening and closing of
that marvellous system of rings which renders this planet the most
beautiful of all the objects of telescopic observation which the heavens
present to us. When the edge of the rings is turned towards the earth,
we see only the most delicate thread of light on either side of the
planet's disc. But when the rings are opened out to their full extent
they reflect towards us as much light as we receive from the disc. At
such times the planet presents a much more brilliant appearance than
when the ring is turned nearly edgewise; in fact to the naked eye he
seems very nearly twice as bright. Now at present the rings are turned
nearly edgewise towards the earth. In July and August, 1869, the planet
presented in the telescope the appearance presented in fig. 30, where it
will be seen that the shorter axis of the oval into which any one of the
ring-outlines is thrown is nearly equal to half the larger axis. Since
then the rings have been slowly closing up; and at present the rings
are so little open that the corresponding shorter axis, if it could be
directly seen, would appear to be about one-sixteenth only of the larger
axis. The rings were turned exactly edgewise towards the sun at two in
the afternoon, on St. Valentine's day, 1878, according to calculations
which I made in 1864, and published in a table under the head "Passages
of the Rings plane through the Sun between the years 1600 and 2000," in
my treatise entitled "Saturn and its System." The _Nautical Almanac_ for
1878, indeed makes the passage of the rings plane through the sun occur
somewhat earlier, stating that at noon on February 14 the sun's centre
would pass south of the ring's horizon by about one-fifth of its
apparent diameter (as seen by us). But my own calculation took into
account certain small details which, in matters of this sort, the
_Nautical Almanac_ computers neglect. After all, it mattered very little
to terrestrial observers whether the sun's light passed from the
northern to the southern side of the rings a few hours earlier or later:
the moment when it passed could not possibly be observed, even if it had
occurred during the night hours. In the present instance it occurred at
midday, and unfortunately none of the interesting phenomena presented in
powerful telescopes when the rings are turned edgewise to the sun or
earth could be observed, for they occurred when Saturn and the sun were
nearly in the same part of the heavens, and when the planet therefore
was utterly lost in the splendour of the solar rays.
[Illustration: Fig. 30.--The planet Saturn in July and August, 1869.]
But now let us briefly consider what is known or may be surmised
respecting the noble planet which was so far outshone in November, 1877,
by the comparatively minute orb of Mars.
Saturn travels at a distance from the sun exceeding rather more than
nine and a half times that of our own earth. The second figure of orbits
(see page 157) shows the wide span of his orbit compared with the
earth's, and yet it will be seen that the orbits of Uranus and Neptune,
planets unknown to the ancients, are so wide that the path of Saturn
becomes in turn small by comparison.
Saturn has a globe about 70,000 miles in diameter, where it bulges out
at the equator; but he is somewhat flattened at his poles, so that his
polar diameter is about 7000 miles less than the equatorial diameter. In
volume he exceeds our earth about 700 times; but in mass only about
ninety times: for his mean density is but about 13/100 of the earth's.
In fact, if we could imagine an ocean of water wide enough and deep
enough for the planets to be all set in it, Saturn would float with
about one-fourth of his bulk out of water,--always supposing that no
change took place in his density directly after he was immersed. Saturn,
indeed, would float highest of all the planets, or rather all of them
would sink except Saturn and Neptune, and Saturn would float higher than
Neptune. Uranus would just sink. Jupiter is half as heavy again as he
should be to float. All the terrestrial planets, Mercury, Venus, the
Earth, and Mars, would go to the bottom at once.
It is almost impossible to regard any feature of Saturn as better
deserving to be considered first than his ring system. Yet for the sake
of preserving a due sequence of ideas we must first consider his globe.
We find ourselves at once in presence of difficulties like those we
encountered when we considered the planet Jupiter. How is it that the
mighty mass of a planet like Saturn, constructed, we have every reason
to believe, of materials resembling those which constitute our earth,
has so failed to gather in its substance that the mean density is much
less than that of the earth's globe? It must be remembered that gravity
prevails throughout the frame of Saturn as throughout our earth's frame.
Every particle of that enormous globe is drawn towards the centre with a
force almost exactly the same as would be exerted by the attraction of
the entire mass of that portion of the planet which lies nearer to the
centre than the particle, if this mass were collected at the centre. But
this is not all. It is not merely the attraction exerted on each
particle of Saturn's mass which has to be considered, but the entire
_weight_ of all the superincumbent matter. The distinction between
attraction and weight, by the way, is very commonly overlooked in
considering the planets' interiors. I think it was Sir David Brewster
who argued that as attraction can easily be shown to diminish downwards
towards the centre, it is possible to conceive that the interior of a
planet may be hollow. The error is readily perceived, if we take a
familiar instance where the attraction is the same yet the effect of
pressure very much greater. (Without voyaging to the centre of the
earth, which is troublesome, and certainly not a familiar experience, we
cannot reach places where the attraction of gravity is greatly less
than at the surface.) Take a massive arch of brickwork: the bricks near
the top are subject to the same attraction as those belonging to the
foundation; but the pressure to which the foundation bricks are exposed
is very much greater than that affecting the upper bricks. So again with
a deep sea: the particles at the bottom of such a sea are subject to no
greater attraction than the particles near the top; but we know that a
strong hollow case of metal which near the top of such a sea would be
scarcely pressed at all, and would suffer no change of shape, will be
crushed perfectly flat under the tremendous pressure to which it will be
exposed when sunk to the bottom.
There is, in fact, no escape from the conclusion that the interior
portions of a planet like Saturn or Jupiter, nay, even of a body like
our earth or the moon, must be subject to tremendous pressure, a
pressure exceeding many hundred-fold the greatest which we can obtain
experimentally, and that under that enormous pressure the density of the
materials composing those central parts must be increased. How is it
then that Saturn is of much smaller density than the earth? I can
imagine no other explanation at once so natural and so complete as this,
that an intense heat pervading the entire frame of the planet enables it
to resist the tremendous pressure due to mere weight. The planet's mass
is expanded by the heat; large portions which at ordinary temperatures
would be solid are liquified or even vaporised; matters which are liquid
on our earth are vaporised; and, in fine, the planet assumes (as seen
from our distant station) the appearance of being very much larger than
it really is. We measure not the true globe, which, for aught that is
known, may be exceedingly dense, but the dimensions of cloud-layers
floating high in the planet's atmosphere.
In describing Jupiter, I considered the changes which have been noticed
in that planet's outline, and observed that it is impossible adequately
to explain the evidence, without assuming that the changes of outline
are real. The outline is not that of a solid globe, however, but of
cloud-layers surrounding such a globe, and probably at a great distance
from its surface.
[Illustration: Fig. 31.--Saturn's square-shouldered aspect.]
In Saturn's case we have very singular evidence to the same purpose. It
was observed by Sir W. Herschel in April, 1805, that Saturn occasionally
appears distorted, as though bulging out in the latitudes midway between
the pole and the equator of the planet. Fig. 31 represents the
appearance of the planet so far as shape is concerned, but the ring was
not, when Sir W. Herschel observed it, so narrow as it is shown in fig.
31. In fact the ring had been turned edgewise to the earth two years
before; and when Herschel noticed the abnormal appearance of Saturn,
the rings had begun to open out, though their outermost outline was
still far within the regions of the planet which seemed to project as
shown. Fig. 31 in fact represents Saturn as seen by Schröter in 1803,
when, as he said, the planet did not seem to present a truly spheroidal
figure. Herschel tested his observations by using several telescopes of
different dimensions,--ten, seven, twenty, and forty feet in length. In
1818, when the rings were scarcely visible, Kitchener saw the planet as
shown in fig. 31, or "square-shouldered," as some have called it. On one
occasion the present Astronomer-Royal saw the planet of that figure. In
January, 1855, Coolidge, using the fine refractor of the Cambridge U.S.
Observatory, noticed that the equatorial diameter was not the greatest;
on the 9th the planet seemed of its usual shape; but on December 6,
Coolidge writes, "I cannot persuade myself that it is an optical
illusion which makes the maximum diameter of the ball intersect the limb
half-way between the northern edge of the equatorial belt, and the inner
ellipse of the inner bright ring." This was at a time when the rings
were nearly at their greatest opening; so that, including Schröter's
observation, we have Saturn out of shape when his ring has presented
every shape between that shown in fig. 30 and that shown in fig. 31.
Again, in the report of the Greenwich Observatory for 1860-61, when the
ring was nearly closed, it is stated that "Saturn has _sometimes_
appeared to assume the square-shouldered aspect." Lastly, the eminent
observers, G. Bond and G. P. Bond, father and son, have seen Saturn
abnormally shaped, flattened unduly in the north polar regions in 1848,
when the ring was turned edgewise towards us, and unsymmetrical in
varying ways in 1855-57, when the ring was most widely opened.
Yet the planet's outline is usually a perfect oval, and has been shown
to be so by careful measurements effected in some instances by the same
observers, who, making equally careful measurements, have found the
planet to be distorted.
Does it not seem abundantly clear that the great cloud-layers which
float in the atmosphere of Saturn have a widely varying range in height,
and that therefore as we see and measure the outline of the
cloud-layers, we see and measure in effect a planet which is variable in
figure? This seems so natural and complete an explanation of the
observed peculiarities that it appears idle on the one hand to reject
the evidence of some among the most skilful observers who have ever
lived; or, on the other, to imagine that the solid frame of the planet
has undergone changes so tremendous as would be involved by the observed
variations of outline if they really signified that a solid planet had
changed in shape.
* * * * *
The mighty globe of Saturn turns upon its axis nearly as quickly as
Jupiter. It will be remembered that the Jovian day lasts only 9½ of our
hours, and as the diameter of Jupiter is about ten times the earth's,
the equatorial parts of the giant planet travel some twenty-six or
twenty-seven times as fast as those of our own earth, which move
(rotationally) at the rate of more than a thousand miles an hour.
Saturn's equatorial parts do not move quite so fast,--in fact, in this
respect, Jupiter comes first of all the members of the solar system,
including the sun himself. Saturn's equatorial circuit being almost nine
times the earth's, while his day is little more than five-twelfths of
the earth's, it follows that his equatorial parts move twelve-fifths of
nine times, or nearly twenty-two times faster than the earth's. Their
actual rotational rate is rather more than 22,000 miles an hour, or 367
miles a minute, or more than six miles a second. This is a wonderful
rate of motion. It always seems to me one of the most striking results
of modern astronomical research that we have to recognise in bodies like
that dull looking star,--the heavy slow-moving Saturn, as the ancients
called him,[14]--motions of such tremendous swiftness. The planet is not
only rushing bodily along through space with a velocity of nearly six
miles per second, but his equatorial parts are being carried round with
a velocity somewhat exceeding six miles per second. (The coincidence
must be regarded as accidental, but it has this curious effect, that the
equatorial parts of Saturn near the middle of the disc we see are
actually almost at rest with respect to the sun, being carried forward
with the planet at the rate of about five miles and nine-tenths per
second, and backward round the planet at the rate of about six miles
and one-tenth per second. In fact there are always two points on the
disc which are almost exactly at rest with respect to the sun, viz.,
those two points north and south of the equator where the rotational
velocity is about five miles and nine-tenths per second, the velocity of
Saturn in his orbit.)[15]
But let us turn from the contemplation of Saturn's globe, interesting
though it undoubtedly is, to study those marvellous objects, the
Saturnian rings.
The history of their discovery is interesting, but must not here detain
us long. Briefly, it runs as follows:--
Galileo, in July, 1610, observing the planet Saturn with a telescope not
powerful enough to show the rings, imagined at first that Saturn had two
companion planets, one on either side of him, as though helping the
planet along upon his road. (From a table relating to the rings, in my
treatise on "Saturn and its System," the aspect of the ring, at the time
of any such observation, can at once be inferred. In the present case,
for example, it will be seen from the table that the rings were closing
up as the time of their disappearance, December 28, 1612, drew near.) A
year and a half later, Galileo looked again at Saturn, and lo! the
companion planets were gone. He was perplexed beyond measure. "What is
to be said concerning so strange a metamorphosis?" he asked. "Are the
two lesser stars consumed after the manner of the solar spots? Have they
vanished or suddenly fled? Has Saturn, perhaps, devoured his own
children? Or were the appearances indeed an illusion or fraud, with
which the glasses have so long deceived me, as well as many others to
whom I have shown them? Now, perhaps, is the time come to revive the
well-nigh withered hopes of those who, guided by more profound
contemplations, have discovered the fallacy of the new observations, and
demonstrated the utter impossibility of their existence. I do not know
what to say in a case so surprising, so unlooked for, and so novel. The
shortness of the time, the unexpected nature of the event, the weakness
of my understanding, and the fear of being mistaken, have greatly
confounded me."
Hevelius was similarly perplexed by the constantly varying appearance of
the planet. "Saturn," he informed his contemporaries, "presents five
various figures to the observer, to wit--first, the mono-spherical;
secondly, the tri-spherical; thirdly, the spherico-ansated; fourthly,
the elliptico-ansated; fifthly, and finally, the spherico-cuspidated;"
of which we can only say, like Mr. Gilbert's Ferdinando, that "we know
it's very clever; but we do not understand it."
It was not till 1659 that Huyghens, using a telescope _forty yards
long_, was able to make out the real meaning of the appendages which had
so perplexed Galileo and Hevelius. He announced to the world, in an
anagram, his discovery that Saturn is girdled about by a flat ring
nowhere touching the planet.
Huyghens also discovered the largest of Saturn's moons. He looked for no
more, having the idea that, since six planets and six moons were now
known, no more moons existed.
In 1663 the Brothers Ball discovered that the rings are divided into
two, or, at any rate, that a broad black stripe, such as is shown in
fig. 30, separates the outer portion of the ring from the inner. Two
years later these observers saw the stripe on the northern side of the
rings, when the rings had so shifted in position that observers saw
their southern side. Dominic Cassini recognised a corresponding stripe
on the southern side. This was regarded as proving that there is a real
division between the rings. The width of the gap thus separating the
outside of the inner ring from the inside of the outer cannot be less
than 1,600 miles.
Cassini also detected another Saturnian moon in October, 1671, and,
later, he discovered three others, making five Saturnian moons in all.
Sir W. Herschel observed the rings with great care. He confirmed the
discovery of the great division between the rings; but rejected the idea
which was beginning to be entertained in his time, that there are many
divisions. He found reasons for suspecting, but never actually proved,
that the outer ring turns round in about 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.