SATURN.
Next beyond Jupiter, proceeding outwards from the Sun, we reach the
planet Saturn, which beyond any doubt is the most beautiful and most
interesting of all the planets. Nobody who has ever had a fairly good
chance of seeing it can have the least doubt that this is the case.
Briefly stated the three main features which constitute its claims
are:—(1) Its belts, (2) its rings, (3) its satellites.
The belts of Saturn resemble generally those of Jupiter, but they are
more faint and less changeable. Their physical cause, however, may be
assumed to be the same. Taking the planet as a whole, it may be said
that its ordinary colour is yellowish white, the belts inclining to
grayish white; though the dark belts have often been thought to exhibit
a greenish hue. Lassell considered that the south pole is generally
darker than the north pole and more blue in tinge.
[Illustration: Fig. 15.—Saturn, Jan. 26, 1889 (Antoniadi).]
There is one important particular in which the belts of Saturn differ
from those of Jupiter. Jupiter’s belts are straight, whereas Saturn’s
are sensibly curved. Supposing, as is probable, that Saturn’s belts are
parallel to the planet’s equator, then we must assume that the plane of
this equator makes a rather considerable angle with the ecliptic. Spots
on Saturn are very rare. Whether Saturn has an atmosphere seems
uncertain, or perhaps it may be said that one has not been proved to
exist but may exist. The question of polar snow is also uncertain, but
Sir W. Herschel thought he could trace changes of hue at the poles which
might be due to the melting of snow.
It is usual to speak of the planet itself under the name of the “Ball”
when it is not a question of referring to the whole Saturnian system
collectively. In consequence of its distance from the Sun, Saturn
undergoes no equivalent to a phase; or to be more exact, no phase can be
detected, though theoretically when the planet is in quadrature the disc
must undergo an infinitesimally small loss of light.
Though the point has now-a-days no scientific importance, it may perhaps
be desirable just to make a brief allusion to Sir W. Herschel’s curious
theory that Saturn was seen by him to be compressed not only at the
poles but at the equator, so that it resembled a parallelogram with the
corners rounded off. It is difficult to imagine what could have given
rise to this strange idea, though, of course, Herschel’s good faith in
advancing it cannot be called in question. I refer to it because it will
be found mentioned in so many books on astronomy, often under the name
of the “square-shouldered” figure of Saturn. As a theory it may be
regarded as quite exploded in consequence of accurate measures by
Bessel, Main and others having conclusively shown that the form of the
ball does not depart from that of a regular spheroid.
In referring to Saturn generally, we speak of its ring in the singular
number, but, in point of fact, there are several rings—three in
particular. The principal bright ring is really double, and within the
innermost bright ring there is a dusky one, perfect as a ring, but not
luminous as the outer rings are. By way of distinguishing one ring from
another, it is usual to adopt Struve’s nomenclature, whereby the
outermost bright ring is called A, the inner bright ring B, and the
dusky ring C.
A good engraving will convey more fully and more clearly an idea of what
the Saturnian system consists of than the fullest verbal description
will do. (See _Frontispiece_.)
To the earliest astronomers who possessed telescopes, Saturn proved a
great puzzle, because it seemed to undergo changes of shape which were
quite inexplicable on any principles then known. Galileo, when first he
saw it, thought it presented an oval outline which might be due to a
central planet having a smaller planet on each side of it, and
accordingly he announced to his friend, Kepler, that the most distant
planet was _tergeminum_ or tri-form. But greater magnifying power led
him to arrive at the conclusion that the planet was not a triple
combination of spheres, but one body, either oblong or oval in outline.
This conclusion, however, was soon found to be untenable, because the
two (supposed) tributary bodies gradually decreased in size until they
entirely disappeared. Galileo writing to his friend, Welser, in December
1612, thus expressed himself:—
“What is to be said concerning so strange a metamorphosis? Are the two
lesser stars consumed after the manner of the solar spots? Have they
vanished or suddenly fled? Has Saturn, perhaps, devoured his own
children? Or were the appearances indeed illusion or fraud, with which
the glasses have so long deceived me, as well as many others to whom I
have shewn them? Now, perhaps, is the time come to revive the well-nigh
withered hopes of those who, guided by more profound contemplations,
have discovered the fallacy of the new observations, and demonstrated
the utter impossibility of their existence. I do not know what to say in
a case so surprising, so unlooked for, and so novel. The shortness of
the time, the unexpected nature of the event, the weakness of my
understanding, and the fear of being mistaken have greatly confounded
me.”
[Illustration: Fig. 16.—General view of the Phases of Saturn’s Rings.]
Galileo seems to have become so out of heart in consequence of the
difficulty of determining what these changes really meant, that he gave
up altogether observing Saturn. In the course of time, but by very
gradual steps, astronomers came to realise what the facts were. The next
idea that was broached, was that the planet consisted of simply one
central ball, and that the excrescences which Galileo had been puzzled
by were merely handles as they were called, (_ansæ_) projecting like the
handles, say of a soup tureen, though why they should vary in size at
stated intervals remained as great a mystery as ever. It was not until
about 1656 that the true explanation was arrived at by a Dutchman, named
Christopher Huygens. It was the fashion in those days for scientific men
to intimate to the world discoveries which they had made by resort to
mysterious anagrams, which served in some degree the purpose which in
the present day is served by the law regulating copyright or patent
rights. Accordingly Huygens published the following singular
memorandum:—
aaaaaaa cccc d eeeee g h i iiiiii llll mm nnnnnnnnn oo oo pp q rr s
ttttt uuuuu.
These letters arranged in their proper order furnish the following Latin
sentence:—
_Annulo cingitur, tenui, plano, nusquam cohaerente, ad eclipticam
inclinato_; which Latin sentence becomes in the English tongue:—
“[The planet] is surrounded by a slender flat ring inclined to the
ecliptic, but which nowhere touches [the body of the planet.]”
Huygen’s discovery was not a mere piece of guesswork, for he spent
several years carefully observing the alterations of form which Saturn
underwent, before he came to the conclusion that it was only the
existence of a ring surrounding the planet which would explain the
various observed changes.
It was by way of guarding himself from being robbed of the fruits of his
discovery whilst he was accumulating the necessary proof of its truth,
that he buried his thoughts in the logogriph or anagram just quoted.
Having arrived at the conclusion which he did, he thought himself
sufficiently sure of his facts to predict that in July or August 1671,
the planet would again appear round, the ring becoming invisible. This
surmise proved practically correct, in so far, that in May 1671, or
within 2 months of the time predicted by Huygens, Cassini saw the planet
as a simple ball unaccompanied by any ring.
This is a convenient place at which to offer a brief explanation of the
changes of appearance as regards the ball and rings which Saturn
undergoes. These changes depend jointly on Saturn’s motion in its orbit
round the Sun, and on the corresponding motion of the Earth in its
orbit. Neither Saturn nor the Earth revolve round the Sun exactly in the
ecliptic, and this want of coincidence results in the fact, that twice
in the 29½ years occupied by Saturn in journeying round the Sun, the
plane of its ring is seen edgeways by us on the Earth; whilst at two
other periods intermediate but equi-distant the ring is seen opened out
to the widest possible extent; that is, so far as we on the Earth can by
any possibility have a chance of seeing it.
[Illustration: Fig. 17.—Phases of Saturn’s Rings at specified dates.]
The appearances presented by the rings when undergoing the
transformations to which they are subject, will be readily understood by
an inspection of the annexed engravings. Fig. 17, indicates the actual
appearances in the years specified, and these years may be considered as
carried forward and brought up to date by substituting 1877 for 1848,
1885 for 1855, 1891 for 1862, and 1898 for 1869.
Adverting to fig. 16, it will suffice to remark that the two central
phases of the rings, opened wide, are to be deemed co-related, or indeed
identical in a geometrical sense (so to speak) the difference being that
one of them is to be deemed to show the northern side of the ring (which
is now in view and will continue in view till 1907) whilst the other
represents the southern side, which was in view from 1877 till 1891. The
foregoing is a brief statement of the general principle involved in the
changes which take place, but the motions of the two planets introduce
certain technical complications into the details which would be seen by
an observer using a large telescope; with these, however, the ordinary
reader will not care to concern himself, and need not do so.
A great deal might be said with respect to the rings treated
descriptively. I will now mention a few matters of general interest.
Huygens regarded the appendage to Saturn, whose existence he
established, to be a single ring, but as far back as 1675, Cassini
determined that Huygen’s single ring was really made up of two, one
lying inside the other. Cassini in this conclusion outstepped not only
all the observers of his own century, but those of the succeeding
century, for Sir W. Herschel even 100 years after Cassini, was for a
long time unable to satisfy himself, even with his superior telescopes,
that the black streaks seen in the ring by Cassini, and regarded by him
as indicative of a severance of the ring into two parts, really implied
a severance. It is now, however, accepted as a fact that not only are
the rings which are known as A and B absolutely distinct, but that A
also is itself certainly duplex, that is, that it certainly consists of
two independent rings. In addition to this many competent observers
armed with powerful telescopes have obtained traces of other
sub-divisions, both in A and B; and though there is some want of harmony
in the details, as stated by the different observers, yet undoubtedly we
must speak of Saturn’s rings collectively as forming a _multiple_
system.
What the rings are is a highly debatable point, but the preponderating
idea is that they are not what they appear to be, namely solid masses of
matter, but are swarms of independent fragments of matter. Yet
“fragment” is not the best word to use, because it implies that
something has been broken up to make the fragments. Rather, perhaps, we
should say with Professor Young, that the rings are “composed of a swarm
of separate particles, each a little independent moon pursuing its own
path around the planet. The idea was suggested long ago, by J. Cassini
in 1715, and by Wright in 1750, but was lost sight of until Bond revived
it in connection with his discovery of the dusky ring. Professor
Benjamin Pierce soon afterwards demonstrated that the rings could not be
continuous solids; and Clerk Maxwell finally showed that they can be
neither solid nor liquid sheets, but that all the known conditions would
be answered by supposing them to consist of a flock of separate and
independent bodies, moving in orbits nearly circular, and in one
plane—in fact, a swarm of meteors.”
The thickness of the rings seen edgeways has been variously estimated.
Sir J. Herschel suggested 250 miles as an outside limit, which G. P.
Bond reduced to 40 miles. It is generally considered, however, that 100
miles is probably not far from the truth. Young has pointed out that if
a model of them were constructed on the scale of 1 inch to represent
10,000 miles, so that the outer ring of such a model would be nearly 17
inches in diameter, then the thickness of the ring would be represented
by that of an ordinary sheet of writing paper.
Considered as a system, the rings are distinctly more luminous than the
planet, and of the two bright rings, the inner one is brighter than the
outer one; and the inner one is less bright at its inner edge than
elsewhere. It is also to be noticed that when seen edgeways just about
the time of the Saturnian equinoxes, when the Sun is shifting over from
one side of the ring to the other, and the ring is dwindling down to a
narrow streak, its edges (forming the _ansæ_ as they are termed) do not
disappear and reappear at the same time, and are not always of the same
apparent extent. One ansa, indeed, is sometimes visible without the
other, and most commonly it is the Eastern one that is missing. To what
causes these various peculiarities are due is unknown.
Many physical peculiarities have been either noticed or suspected with
reference to the bright rings. For instance, on comparing one with
another, some persons have thought that their surfaces are convex, and
that they do not lie in the same plane. The existence of mountains on
their surface has more than once been suspected. Again, it has been
fancied that they are surrounded by an extensive atmosphere. It seems
hardly likely that the rings would have an atmosphere and not the ball
(or _vice versâ_), and, therefore, no wonder that we have no
observations which countenance the idea that the ball does really
possess an atmosphere. This, indeed, seems to flow from Trouvelot’s
observation, that the ball is less luminous at its circumference than at
its centre.
The circumstances of ring C, otherwise called the “Dusky” or “Crape”
ring are as curious historically, as they are mysterious physically. In
1838, Galle of Breslau, noticed what he thought to be a gradual shading
off of the interior bright ring towards the ball. Though he published a
statement of what he saw, the matter seems to have attracted little or
no notice. In 1850, G. P. Bond in America perceived something luminous
between the ring and the ball, and after repeated observations in
concert with his father, came to the conclusion that the luminous
appearance which he saw, was neither less nor more than an independent
and imperfectly illuminated ring lying within the old rings and
concentric with them. Before, however, tidings of Bond’s discovery
reached England, but a few days after the discovery in point of actual
date, Dawes suddenly noticed one evening as Bond had done, a luminous
shading within the bright rings, which he was not long in finding out to
be in reality a complete ring, except so far that a portion of it was of
course hidden from view behind the ball. He, and O. Struve likewise,
noticed that this new Dusky Ring was occasionally to be seen divided
into two or more rings. The Dusky Ring is transparent, though this fact
was not ascertained until 1852, or two years after Bond’s discovery of
the ring.
The Dusky Ring is now recognised as a permanent feature of Saturn, but
how far it used to be permanent, or how long it has been so, is a matter
wrapped in doubt. Recorded observations by Picard in 1673, and by Hadley
in 1723, made of course with telescopes infinitely less powerful than
those of the present day, seem to suggest that both the observers named
saw the Dusky Ring, without, however, being able to form a clear
conception that it was a ring. It is strange that during the long period
from 1723 to 1838, no one—not even Sir W. Herschel, with his various
telescopes—should have obtained or at least have recorded any suspicion
of its existence. There is, however, direct evidence that the Dusky Ring
is wider and less faint than formerly. This was directly confirmed by
Carpenter in 1863, who says he saw it “nearly as bright as the
illuminated ring, so much so, that it might easily have been mistaken
for a part of it.” In 1883, Davidson found a marked difference in the
brilliancy of the two ends (_ansæ_) of the ring.
In 1889 Barnard was fortunate enough to observe an eclipse of one of
Saturn’s satellites by the ring, but the eclipse, that is the
concealment of the satellite, was only effected when it passed behind
the bright rings; the dusky ring did not obliterate it, and hence there
was obtained a conclusive proof of the transparency of the dusky ring.
Barnard further concluded from his observations that there was no
separating space or division between the inner bright ring and the dusky
ring, as has frequently been represented in drawings. This transparency
of the Dusky Ring, as a matter of fact, is therefore undoubted; yet what
are we to consider to be the meaning of an observation by Wray in 1861,
that whilst looking at the dusky ring edgeways the impression was
conveyed to his eye that that ring was very much thicker than the bright
rings?
A very interesting question which has been much discussed has reference
to the stability of the rings. It is generally agreed that the
constituent particles of the rings must be in motion round the primary
or their equilibrium could not be maintained: almost equally certain is
it, and for the like reason, that the rings cannot be solid. Of actual
change in the rings as regards their dimensions, we have no satisfactory
proof, though authorities differ on the point, some thinking that the
rings are expanding inwards, so that ultimately they will come into
contact with the ball, whilst others consider no proof whatever of such
change can be obtained from any of the observations yet made in the way
of measurements.
We must now proceed to consider the satellites of Saturn. These are 8 in
number, 7 of which move in orbits whose planes coincide nearly with the
planet’s equator, whilst the remaining one is inclined about 12°
thereto. One consequence of this coincidence in the planes of these
satellites, which, it should be stated, are the 7 innermost, is that
they are always visible to the inhabitants of both hemispheres when they
are not actually undergoing eclipse in the shadow of Saturn. The
satellites are of various sizes, and succeed one another in the
following order, reckoning from the nearest, outwards:—Mimas, Enceladus,
Tethys, Dione, Rhea, Titan, Hyperion and Iapetus. Any good 2-inch
telescope will show Titan; a 3-inch will sometimes show Iapetus; a
4-inch will show Iapetus well, together with Rhea and Dione, but hardly
Tethys; all the others require large telescopes. If Saturn has any
inhabitants at all constituted like ourselves, which is highly
improbable, they will have a chance of seeing celestial phenomena of the
greatest interest. What with the rings surrounding the planet and 8
moons in constant motion, there will be an endless succession of
astronomical sights for them to study. The amount of light received from
the Sun cannot be much—barely 1/100th what the earth receives. The ring
and satellites will therefore be useful as supplementary sources of
light; yet the satellites will not furnish much, for it has been
calculated that the surface of the sky occupied by all the satellites
put together would to a dweller on Saturn only amount to 6 times the
area of the sky covered by our Moon; whilst the intrinsic brightness of
all put together would be no more than 1/16th part of the light which we
receive from our Moon.
The only physical fact worth noting here in connection with the
satellites concerns Iapetus. Cassini two centuries ago with his
indifferent telescopes thought he had ascertained that this satellite
was subject to considerable variations of brilliancy. Sir W. Herschel
confirmed Cassini as to this. He found that it was much less brilliant
when traversing the eastern half of its orbit than at other times. Two
conclusions have been drawn from this fact. One is that the satellite
rotates once on its axis in the same time that it performs one
revolution round its primary; and that there are portions of its surface
which are almost entirely incapable of reflecting the rays of the Sun.
This last named supposition may perhaps be well founded, but the former
needs more proof than is as yet forthcoming. Iapetus on the whole may be
said to shine as a star of the 9th magnitude. To this it may be added
that Titan is of the 8th magnitude, but all the others much smaller.
[Illustration: Fig. 18.—Saturn with the shadow of Titan on it, March 11,
1892 (Terby).]
Saturn revolves round the Sun in a little under 29½ years at a mean
distance of 886 millions of miles. Its apparent diameter varies between
15″ and 20″; its true diameter may be put at 75,000 miles. The
flattening of the poles, or “polar compression” as it is called, is
greater than that of any other planet, but is usually less noticeable
than in the case of Jupiter, because the ring is apt to distract the
eye, except when near the edgeways phase. The compression may be taken
at 1/9.