SATURN
At nearly double the distance of Jupiter from the sun circles the
second largest planet of our system, unique, so far as human knowledge
goes, in the character of its appendages. The orbit of Saturn has a
mean radius of 886,000,000 miles, but owing to its eccentricity, his
distance may be diminished to 841,000,000 or increased to 931,000,000.
This large variation may not play so important a part in his economy
as might have been supposed, owing to the fact that the sun heat
received by him is not much more than 1/100th of that received by the
earth. The planet occupies twenty-nine and a half years in travelling
round its immense orbit. Barnard's measures with the Lick telescope
give for the polar diameter 69,770, and for the equatorial 76,470
miles. Saturn's polar compression is accordingly very great, amounting
to about 1/12th. Generally speaking, however, it is not so obvious
in the telescopic view as the smaller compression of Jupiter, being
masked by the proximity of the rings.
[Illustration: PLATE XXIV.
Saturn, July 2, 1894. E. E. Barnard, 36-inch Equatorial.]
Saturn is the least dense of all the planets; in fact, this enormous
globe, nine times the diameter of the earth, would float in water.
This fact of extremely low density at once suggests a state of matters
similar to that already seen to exist, in all likelihood, in the case
of Jupiter; and all the evidence goes to support the view that Saturn,
along with the other three large exterior planets, is in the condition
of a semi-sun.
The globe presents, on the whole, similar characteristics to those
already noticed as prevailing on Jupiter, but, as was to be expected,
in a condition enfeebled by the much greater distance across which
they are viewed and the smaller scale on which they are exhibited.
It is generally girdled by one or two tropical belts of a grey-green
tone; the equatorial region is yellow, and sometimes, like the
corresponding region of Jupiter, bears light spots upon it and a
narrow equatorial band of a dusky tone; the polar regions are of a
cold ashy or leaden colour. Professor Barnard's fine drawing (Plate
XXIV.) gives an admirable representation of these features as seen
with the 36-inch Lick telescope. Altogether, whether from greater
distance or from intrinsic deficiency, the colouring of Saturn is by
no means so vivid or so interesting as that of his larger neighbour.
The period of rotation was, till within the last few years, thought
to be definitely and satisfactorily ascertained. Sir William Herschel
fixed it, from his observations, at ten hours sixteen minutes.
Professor Asaph Hall, from observations of a white spot near the
equator, reduced this period to ten hours fourteen minutes twenty-four
seconds. Stanley Williams and Denning, in 1891, reached results
differing only by about two seconds from that of Hall; but the former,
discussing observations of 1893, arrived at the conclusion that there
were variations of rotation presented in different latitudes and
longitudes of the planet's surface, the longest period being ten hours
fifteen minutes, and the shortest ten hours twelve minutes forty-five
seconds. Subsequently Keeler obtained, by spectroscopic methods, a
result exactly agreeing with that of Hall. It appeared, therefore,
that fairly satisfactory agreement had been reached on a mean period
of ten hours fourteen minutes twenty-four seconds.
In 1903, however, a number of bright spots appeared in a middle north
latitude which, when observed by Barnard, Comas Solà, Denning, and
other observers, gave a period remarkably longer than that deduced
from spots in lower latitudes--namely, about ten hours thirty-eight
minutes. Accordingly, it follows that the surface of Saturn's
equatorial regions rotates much more rapidly than that of the regions
further north--a state of affairs which presents an obvious likeness
to that prevailing on Jupiter. But in the case of Saturn the
equatorial current must move relatively to the rest of the surface at
the enormous rate of from 800 to 900 miles an hour, a speed between
three and four times greater than that of the corresponding current on
Jupiter!
The resemblance between the two great planets is thus very marked
indeed. Great size, coupled with small density; very rapid rotation,
with its accompaniment of large polar compression; and, even more
markedly in the case of the more distant planet than in that of
Jupiter, a variety of rotation periods for different markings, which
indicates that these features have been thrown up from different
strata of the planet's substance--such points of likeness are too
significant to be ignored. It is not at all likely that Saturn has
any solidity to speak of, any more than Jupiter; the probabilities all
point in the direction of a comparatively small nucleus of somewhat
greater solidity than the rest, surrounded by an immense condensation
shell, where the products of various eruptions are represented.
Were this all that can be said about Saturn, the planet would scarcely
be more than a reduced and somewhat less interesting edition of
Jupiter. As it is, he possesses characteristics which make him
Jupiter's rival in point of interest, and, as a mere telescopic
picture, perhaps even his superior. When Galileo turned his telescope
upon Saturn, he was presented with what seemed an insoluble enigma.
It appeared to him that, instead of being a single globe, the planet
consisted of three globes in contact with one another; and this
supposed fact he intimated to Kepler in an anagram, which, when
rearranged, read: 'Altissimum planetam tergeminum observavi'--'I
have observed the most distant planet to be threefold.' Under better
conditions of observation, he remarked subsequently, the planet
appeared like an olive, as it still does with low powers. This was
sufficiently puzzling, but worse was to follow. After an interval,
on observing Saturn again, he found that the appearances which had so
perplexed him had altogether disappeared; the globe was single, like
those of the other planets. In his letter to Welser, dated December
4, 1612, the great astronomer describes his bewilderment, and his
fear lest, after all, it should turn out that his adversaries had been
right, and that his discoveries had been mere illusions.
Then followed a period when observers could only command optical power
sufficient to show the puzzling nature of the planet's appendages,
without revealing their true form. It appeared that Saturn had 'ansæ,'
or handles, on either side of him, between which and his body the
sky could be seen; and many uncouth figures are still preserved which
eloquently testify to the bewilderment of those who drew them, though
some of them are wonderfully accurate representations of the planet's
appearance when seen with insufficient means. The bewilderment was
sometimes veiled, in amusing cuttle-fish fashion, under an inky cloud
of sesquipedalian words. Thus Hevelius describes the aspects of Saturn
in the following blasting flight of projectiles: 'The mono-spherical,
the tri-spherical, the spherico-ansated, the elliptico-ansated,
and the spherico-cuspidated,' which is very beautiful no doubt, but
scarcely so simple as one could wish a popular explanation to be.
In the year 1659, however, Huygens, who had been observing Saturn with
a telescope of 2-1/3 inches aperture and 23 feet focal length, bearing
a magnifying power of 100, arrived at the correct solution of the
mystery, which he announced to, or rather concealed from, the world
in a barbarous jumble of letters, which, when properly arranged, read
'annulo cingitur, tenui, plano, nusquam cohaerente, ad eclipticam
inclinato'--'he (Saturn) is surrounded by a thin flat ring, nowhere
touching (him, and) inclined to the ecliptic.' Huygens also discovered
the first and largest of Saturn's satellites, Titan. His discoveries
were followed by those of Cassini, who in 1676 announced his
observation of that division in the ring which now goes by his name.
From Cassini's time onwards to the middle of the nineteenth century,
nothing was observed to alter to any great extent the conception
of the Saturnian system which had been reached; though certain
observations were made, which, though viewed with some suspicion,
seemed to indicate that there were more divisions in the ring than
that which Cassini had discovered, and that the system was thus a
multiple one. In particular a marking on the outer ring was detected
by Encke, and named after him, though generally seen, if seen at all,
rather as a faint shading than as a definite division. (It is not
shown in Barnard's drawing, Plate XXIV.). But in 1850 came the last
great addition to our knowledge of the ring system, W. C. Bond in
America, and Dawes in England making independently the discovery of
the faint third ring, known as the Crape Ring, which lies between the
inner bright ring and the globe.
The extraordinary appendages thus gradually revealed present a
constantly varying aspect according to the seasons of the long
Saturnian year. At Saturn's equinoxes they disappear, being turned
edgewise; then, reappearing, they gradually broaden until at the
solstice, 7-1/3 years later, they are seen at their widest expansion;
while from this point they narrow again to the following equinox,
and repeat the same process with the opposite side of the ring
illuminated, the whole set of changes being gone through in 29 years
167·2 days. Barnard's measures give for the outer diameter of the
outer ring 172,310 miles; while the clear interval between the inner
margin of the Crape Ring and the ball is about 5,800 miles, and the
width of the great division in the ring-system (Cassini's) 2,270
miles. In sharp contrast to these enormous figures is the fact
that the rings have no measurable thickness at all, and can only be
estimated at not more than 50 miles. They disappear absolutely when
seen edgewise; even the great Lick telescope lost them altogether for
three days in October, 1891.[*]
The answer to the question of what may be the constitution of these
remarkable features may now be given with a moderate approach to
certainty. It has been shown successively that the rings could not be
solid, or liquid, and in 1857 Clerk-Maxwell demonstrated that the only
possible constitution for such a body is that of an infinite number
of small satellites. The rings of Saturn thus presumably consist of
myriads of tiny moonlets, each pursuing its own individual orbit
in its individual period, and all drawn to their present form of
aggregation by the attraction of Saturn's bulging equator. The
appearances presented by the rings are explicable on this theory, and
on no other. Thus the brightness of the two rings A and B would arise
from the closer grouping of the satellites within these zones; while
the semi-darkness of the Crape Ring arises from the sparser sprinkling
of the moonlets, which allows the dark sky to be seen between them.
Cassini's division corresponds to a zone which has been deprived of
satellites; and as it has been shown that this vacant zone occupies a
position where a revolving body would be subject to disturbance from
four of Saturn's satellites, the force which cleared this gap in the
ring is obvious. It has been urged as an objection to the satellite
theory that while the thin spreading of the moonlets would account for
the comparative darkness of the Crape Ring when seen against the sky,
it by no means accounts for the fact that this ring is seen as a dark
stripe upon the body of the planet. Seeliger's explanation of this is
both satisfactory and obvious, when once suggested--namely, that the
darkness of the Crape Ring against the planet is due to the fact that
what we see is not the actual transits of the satellites themselves,
but the perpetual flitting of their shadows across the ball. The final
and conclusive argument in favour of this theory of the constitution
of the rings was supplied by the late Professor Keeler by means of the
spectroscope. It is evident that if the rings were solid, the speed
of rotation should increase from their inner to their outer
margin--_i.e._, the outer margin must move faster, in miles per
second, than the inner does. If, on the contrary, the rings are
composed of a great number of satellites, the relation will be exactly
reversed, and, owing to the superior attractive force exercised upon
them by the planet through their greater nearness to him, the inner
satellites will revolve faster than the outer ones. Now, this point
is capable of settlement by spectroscopic methods involving the
application of the well-known Doppler's principle, that the speed of a
body's motion produces definite and regular effects upon the pitch of
the light emitted or reflected by it. The measurements were of extreme
delicacy, but the result was to give a rate of motion of 12-1/2 miles
per second for the inner edge of ring B, and of 10 miles for the outer
edge of A, thus affording unmistakable confirmation of the satellite
theory of the rings. Keeler's results have since been confirmed by
Campbell and others; and it may be regarded as a demonstrated fact
that the rings, as already stated, consist of a vast number of small
satellites.
It has been maintained that the ball of Saturn is eccentrically placed
within the ring, and further, that this eccentricity is essential to
the stability of the system; while the suggestion has also been made
that the ring-system is undergoing progressive change, and that the
interval between it and the ball is lessening. It has to be noticed,
however, that the best measures, those of Barnard, indicate that the
ball is symmetrically placed within the rings; and the suggestion of a
diminishing interval between the ring-system and the ball receives no
countenance from comparison of the measures which have been made at
different times.
There can be no question that of all objects presented to observation
in the solar system, there is not one, which, for mere beauty and
symmetry can be for a moment compared with Saturn, even though, as
already indicated, Mars and Jupiter present features of more lasting
interest. To quote Proctor's words: 'The golden disc, faintly striped
with silver-tinted belts; the circling rings, with their various
shades of brilliancy and colour; and the perfect symmetry of the
system as it sweeps across the dark background of the field of
view, combine to form a picture as charming as it is sublime and
impressive.' Fortunately the main features of this beautiful picture
are within the reach of very humble instruments. Webb states that
when the ring system was at its greatest breadth he has seen it with
a power of about twenty on only 1-1/3-inch aperture. A beginner cannot
expect to do so much with such small means; but at all events a 2-inch
telescope with powers of from 50 to 100 will reveal the main outlines
of the ring very well indeed, and, with careful attention will show
the shadow of the ring upon the ball, and that of the ball upon the
ring. When we come to the question of the division in the ring, we are
on somewhat more doubtful ground. Proctor affirms that 'the division
in the ring (Cassini's) can be seen in a good 2-inch aperture in
favourable weather.' One would have felt inclined to say that the
weather would require to be very favourable indeed, were it not that
Proctor's statement is corroborated by Denning, who remarks that
'With a 2-inch refractor, power about ninety, not only are the rings
splendidly visible, but Cassini's division is readily glimpsed, as
well as the narrow dark belt on the body of the planet.' The student
may, however, be warned against expecting that such statements will
apply to his own individual efforts. There are comparatively few
observers whose eyes have had such systematic training as to qualify
them for work like this, and those who begin by expecting to see all
that skilled observers see with an instrument of the same power are
only laying up for themselves stores of disappointment. Mr. Mee's
frank confession may be commended to the notice of those who hope to
see at the first glance all that old students have learned to see
by years of hard work. 'The first time I saw Saturn through a large
telescope, a fine 12-inch reflector, I confess I could not see
the division (Cassini's), though the view of the planet was one of
exquisite beauty and long to be remembered, and notwithstanding the
fact that the much fainter division of Encke was at the moment visible
to the owner of the instrument!' It is extremely unlikely that the
beginner will see the division with anything much less than 3 inches,
and even with that aperture he will not see it until the rings are
well opened. The writer's experience is that it is not by any means so
readily seen as is sometimes supposed. Three inches will show it under
good conditions; with 3-7/8 it can be steadily held, even when the
rings are only moderately open (steady holding is a very different
thing from 'glimpsing'), but even with larger apertures the division
becomes by no means a simple object as the rings close up (Fig. 26).
In fact, there is nothing better fitted to fill the modern observer's
mind with a most wholesome respect for the memory of a man like
Cassini, than the thought that with his most imperfect appliances this
great observer detected the division, a much more difficult feat than
the mere seeing it when its existence and position are already known,
and discovered also four of the Saturnian satellites. As for the minor
divisions in the ring, if they are divisions, they are out of the
question altogether for small apertures, and are often invisible even
to skilled observers using the finest telescopes. Barnard's drawing
(Plate XXIV.), as already noted, shows no trace of Encke's division;
but nine months later the same observer saw it faintly in both ansæ of
the ring. The conclusion from this and many similar observations seems
to be that the marking is variable, as may very well be, from the
constitution of the ring. The Crape Ring is beyond any instrument
of less than four inches, and even with such an aperture requires
favourable circumstances.
[Illustration: FIG. 26.
SATURN, 3-7/8-inch.]
With regard to a great number of very remarkable details which of
late years have been seen and drawn by various observers, it may be
remarked that the student need not be unduly disappointed should his
small instrument fail, as it almost certainly will, to show these.
This is a defect which his telescope shares with an instrument of such
respectable size and undoubted optical quality as the Lick 36-inch.
Writing in January, 1895, concerning the beautiful drawing which
accompanies this chapter, Professor Barnard somewhat caustically
observes: 'The black and white spots lately seen upon Saturn by
various little telescopes were totally beyond the reach of the
36-inch--as well as of the 12-inch--under either good or bad
conditions of seeing.... The inner edge (of the Crape Ring) was a
uniform curve; the serrated or saw-toothed appearance of its inner
edge which had previously been seen with some small telescopes
was also beyond the reach of the 36-inch.' Such remarks should be
consoling to those who find themselves and their instruments unequal
to the remarkable feats which are sometimes accomplished, or recorded.
So far as one's personal experience goes, Saturn is generally the
most easily defined of all the planets. Of late years he has been very
badly placed for observers in the Northern Hemisphere, and this has
considerably interfered with definition. But when well placed the
planet presents a sharpness and steadiness of outline which render
him capable of bearing higher magnifying powers than Jupiter, and even
than Mars, though a curious rippling movement will often be noticed
passing along the rings. It can scarcely be said, however, that
there is much work for small instruments upon Saturn--the seeing
of imaginary details being excluded. Accordingly, in spite of the
undoubted beauty of the ringed planet, Jupiter will on the whole be
found to be an object of more permanent interest. Yet, viewed merely
as a spectacle, and as an example of extraordinary grace and symmetry,
Saturn must always command attention. The sight of his wonderful
system can hardly fail to excite speculation as to its destiny; and
the question of the permanence of the rings is one that is almost
thrust upon the spectator. With regard to this matter it may be noted
that, according to Professor G. H. Darwin, the rings represent merely
a passing stage in the evolution of the Saturnian system. At present
they are within the limit proved by Roche, in 1848, to be that within
which no secondary body of reasonable size could exist; and thus the
discrete character of their constituents is maintained by the strains
of unequal attraction. Professor Darwin believes that in time the
inner particles of the ring will be drawn inwards, and will eventually
fall upon the planet's surface, while the outer ones will disperse
outwards to a point beyond Roche's limit, where they may eventually
coalesce into a satellite or satellites--a poor compensation for the
loss of appendages so brilliant and unique as the rings.
Saturn's train of satellites is the most numerous and remarkable in
our system. As already mentioned, Huygens, the discoverer of the true
form of the ring, discovered also the first and brightest satellite,
Titan, which is a body somewhat larger than our own moon, having a
diameter of 2,720 miles. A few years later came Cassini's discoveries
of four other satellites, beginning in 1671 and ending in 1684. For
more than 100 years discovery paused there, and it was not until
August and September, 1789, that Sir William Herschel added the sixth
and seventh to our knowledge of the Saturnian system.
In 1848 Bond in America and Lassell in England made independently the
discovery of the eighth satellite--another of the coincidences which
marked the progress of research upon Saturn, and in both of which Bond
was concerned. Then followed another pause of fifty years broken by
the discovery, in 1898, by Professor Pickering, of a ninth, whose
existence was not completely confirmed till 1904. The motion of this
satellite has proved to be retrograde, unlike that of the earlier
discovered members of the family, so that its discovery has introduced
us to a new and abnormal feature of the Saturnian system. The
discoverer of Ph[oe]be, as the ninth satellite has been named, has
followed up his success by the discovery of a tenth member of Saturn's
retinue, known provisionally as Themis. Accordingly the system, as at
present known, consists of a triple ring and ten satellites. The last
discovered moons are very small bodies, the diameter of Ph[oe]be, for
instance, being estimated at 150 miles; while its distance from Saturn
is 8,000,000 miles. From the surface of the planet Ph[oe]be would
appear like a star of fifth or sixth magnitude; to observers on our
own earth its magnitude is fifteenth or sixteenth. The ten satellites
have been named as follows: 1, Titan, discovered by Huygens; 2,
Japetus; 3, Rhea; 4, Dione; 5, Tethys, all discovered by Cassini; 6,
Enceladus; and 7, Mimas, Sir William Herschel; 8, Hyperion, Bond and
Lassell; 9, Ph[oe]be; and 10, Themis, W. H. Pickering. Titan, the
largest satellite, has been found to be considerably denser than
Saturn himself.
The most of these little moons are, of course, beyond the power of
small glasses; but a 2-inch will show Titan perfectly well. Japetus
also is not a difficult object, but is much easier at his western than
at his eastern elongation, a fact which probably points to a surface
of unequal reflective power. Rhea, Dione, and Tethys are much more
difficult. Kitchiner states that a friend of his saw them with
2-7/10-inch aperture, the planet being hidden; but probably his friend
had been amusing himself at the quaint old gentleman's expense.
Noble concludes that with a first-class 3-inch and under favourable
circumstances four, or as a bare possibility even five, satellites may
be seen; and I have repeatedly seen all the five with 3-7/8-inches.
The only particular advantages of seeing them are the test which they
afford of the instrument used, and the accompanying practice of the
eye in picking up minute points of light. There is a considerable
interest in watching the gradual disappearance of the brilliant disc
of Saturn behind the edge of the field, or of the thick wire which may
be placed in the eye-piece to hide the planet, and then catching the
sudden flash up of the tiny dots of light which were previously lost
in the glare of the larger body. For purposes of identification,
recourse must be had to the 'Companion to the Observatory,' which
prints lists of the elongations of the various satellites and a
diagram of their orbits which renders it an easy matter to identify
any particular satellite seen. Transits are, with the exception
of that of Titan, beyond the powers of such instruments as we are
contemplating. The shadow of Titan has, however, been seen in transit
with a telescope of only 2-7/8-inch aperture.
[Footnote *: The plane of the rings passes through the earth
on April 13, and through the sun on July 27, 1907, at
which periods it is probable that the rings will altogether
disappear.]