Myths and Marvels of Astronomy
the outer ring had long been observed to be less bright than the inner.
Of course it did not seem impossible that the outer ring might be made
of different materials; yet there was something bizarre in the
supposition that two rings forming the same system were thus different
in substance. It would not have been at all noteworthy if different
parts of the same ring differed in luminosity--in fact, it was much more
remarkable that each zone of the system seemed uniformly bright all
round. But that one zone should be of one tint, another of an entirely
different tint, was a strange circumstance so long as the only available
interpretation seemed to be that one zone was made (throughout) of one
substance, the other of another. If this was strange when the difference
between the inner and outer bright rings was alone considered, how much
stranger did it seem when the multitudinous divisions in the rings were
taken into account! Why should the ring-system, 30,000 miles in width,
be thus divided into zones of different material? An arrangement so
artificial is quite unlike all that is elsewhere seen among the
subjects of the astronomer's researches. But when the rings are regarded
as made up of multitudes of small bodies, we can quite readily
understand how the nearly circular movements of all of these, at
different rates, should result in the formation of rings of aggregation
and rings of segregation, appearing at the earth's distance as bright
rings and faint rings. The dark ring clearly corresponds in appearance
with a ring of thinly scattered satellites. Indeed, it seems impossible
otherwise to account for the appearance of a dusky belt across the globe
of the planet where the dark ring crosses the disc. If the material of
the dark ring were some partly transparent solid or fluid substance, the
light of the planet received through the dark ring added to the light
reflected by the dark ring itself, would be so nearly equivalent to the
light received from the rest of the planet's disc, that either no dark
belt would be seen, or the darkening would be barely discernible. In
some positions a bright belt would be seen, not a dark one. But a ring
of scattered satellites would cast as its shadow a multitude of black
spots, which would give to the belt in shadow a dark grey aspect. A
considerable proportion of these spots would be hidden by the satellites
forming the dark ring, and in every case where a spot was wholly or
partially hidden by a satellite, the effect (at our distant station
where the separate satellites of the dark ring are not discernible)
would simply be to reduce _pro tanto_ the darkness of the grey belt of
shadow. But certainly more than half the shadows of the satellites would
remain in sight; for the darkness of the ring at the time of its
discovery showed that the satellites were very sparsely strewn. And
these shadows would be sufficient to give to the belt a dusky hue, such
as it presented when first discovered.[37]
The observations which have recently been made by Mr. Trouvelot
indicate changes in the ring-system, and especially in the dark ring,
which place every other theory save that to which we have thus been led
entirely out of the question. It should be noted that Mr. Trouvelot has
employed telescopes of unquestionable excellence and varying in aperture
from six inches to twenty-six inches, the latter aperture being that of
the great telescope of the Washington Observatory (the largest refractor
in the world).
He has noted in the first place that the interior edge of the outer
bright ring, which marks the outer limit of the great division, is
irregular, but whether the irregularity is permanent or not he does not
know. The great division itself is found not to be actually black, but,
as was long since noted by Captain Jacob, of the Madras Observatory, a
very dark brown, as though a few scattered satellites travelled along
this relatively vacant zone of the system. Mr. Trouvelot has further
noticed that the shadow of the planet upon the rings, and especially
upon the outer ring, changes continually in shape, a circumstance which
he attributes to irregularities in the surface of the rings. For my own
part, I should be disposed to attribute these changes in the shape of
the planet's shadow (noted by other observers also) to rapid changes in
the deep cloud-laden atmosphere of the planet. Passing on, however, to
less doubtful observations, we find that the whole system of rings has
presented a clouded and spotted aspect during the last four years. Mr.
Trouvelot specially describes this appearance as observed on the parts
of the ring outside the disc, called by astronomers the _ansae_ (because
of their resemblance to handles), and it would seem, therefore, that the
spotted and cloudy portions are seen only where the background on which
the rings are projected is black. This circumstance clearly suggests
that the darkness of these parts is due to the background, or, in other
words, that the sky is in reality seen through those parts of the
ring-system, just as the darkness of the slate-coloured interior ring is
attributed, on the satellite theory, to the background of sky visible
through the scattered flight of satellites forming the dark ring. The
matter composing the dark ring has been observed by Mr. Trouvelot to be
gathered in places into compact masses, which prevent the light of the
planet from being seen through those portions of the dark ring where the
matter is thus massed together. It is clear that such peculiarities
could not possibly present themselves in the case of a continuous solid
or fluid ring-system, whereas they would naturally occur in a ring
formed of multitudes of minute bodies travelling freely around the
planet.
The point next to be mentioned is still more decisive. When the dark
ring was carefully examined with powerful telescopes during the ten
years following its discovery by Bond, at which time it was most
favourably placed for observation, it was observed that the outline of
the planet could be seen across the entire breadth of the dark ring. All
the observations agreed in this respect. It was, indeed, noticed by
Dawes that outside the planet's disc the dark ring showed varieties of
tint, its inner half being darker than its outer portion. Lassell,
observing the planet under most favourable conditions with his two-feet
mirror at Malta, could not perceive these varieties of tint, which
therefore we may judge to have been either not permanent or very
slightly marked. But, as I have said, all observers agreed that the
outline of the planet could be seen athwart the entire width of the
dark ring. Mr. Trouvelot, however, has found that during the last four
years the planet has not been visible through the whole width of the
dark ring, but only through the inner half of the ring's breadth. It
appears, then, that either the inner portion is getting continually
thinner and thinner--that is, the satellites composing it are becoming
continually more sparsely strewn--or that the outer portion is becoming
more compact, doubtless by receiving stray satellites from the interior
of the inner bright ring.
It is clear that in Saturn's ring-system, if not in the planet itself,
mighty changes are still taking place. It may be that the rings are
being so fashioned under the forces to which they are subjected as to be
on their way to becoming changed into separate satellites, inner members
of that system which at present consists of eight secondary planets.
But, whatever may be the end towards which these changes are tending, we
see processes of evolution taking place which may be regarded as
typifying the more extensive and probably more energetic processes
whereby the solar system itself reached its present condition. I
ventured more than ten years ago, in the preface to my treatise upon the
planet Saturn, to suggest the possibility 'that in the variations
perceptibly proceeding in the Saturnian ring-system a key may one day be
found to the law of development under which the solar system has reached
its present condition.' This suggestion seems to me strikingly confirmed
by the recent discoveries. The planet Saturn and its appendages, always
interesting to astronomers, are found more than ever worthy of close
investigation and scrutiny. We may here, as it were, seize nature in the
act, and trace out the actual progress of developments which at present
are matters rather of theory than of observation.
VIII.
_COMETS AS PORTENTS_
The blazing star,
Threat'ning the world with famine, plague, and war;
To princes death; to kingdoms many curses;
To all estates inevitable losses;
To herdsmen rot; to ploughmen hapless seasons;
To sailors storms; to cities civil treasons.
Although comets are no longer regarded with superstitious awe as in old
times, mystery still clings to them. Astronomers can tell what path a
comet is travelling upon, and say whence it has come and whither it will
go, can even in many cases predict the periodic returns of a comet, can
analyse the substance of these strange wanderers, and have recently
discovered a singular bond of relationship between comets and those
other strange visitants from the celestial depths, the shooting stars.
But astronomy has hitherto proved unable to determine the origin of
comets, the part they perform in the economy of the universe, their real
structure, the causes of the marvellous changes of shape which they
undergo as they approach the sun, rush round him, and then retreat. As
Sir John Herschel has remarked: 'No one, hitherto, has been able to
assign any single point in which we should be a bit better or worse off,
materially speaking, if there were no such thing as a comet. Persons,
even thinking persons, have busied themselves with conjectures; such as
that they may serve for fuel for the sun (into which, however, they
never fall), or that they may cause warm summers, which is a mere fancy,
or that they may give rise to epidemics, or potato-blights, and so
forth.' And though, as he justly says, 'this is all wild talking,' yet
it will probably continue until astronomers have been able to master the
problems respecting comets which hitherto have foiled their best
efforts. The unexplained has ever been and will ever be marvellous to
the general mind. Just as unexplored regions of the earth have been
tenanted in imagination by
anthropophagi and men whose heads
Do grow beneath their shoulders,
so do wondrous possibilities exist in the unknown and the ill-understood
phenomena of nature.
In old times, when the appearance and movements of comets were supposed
to be altogether uncontrolled by physical laws, it was natural that
comets should be regarded as signs from heaven, tokens of Divine wrath
towards some, and of the interposition of Divine providence in favour of
others. As Seneca well remarked: 'There is no man so dull, so obtuse, so
turned to earthly things, who does not direct all the powers of his mind
towards things Divine when some novel phenomenon appears in the heavens.
While all follows its usual course up yonder, familiarity robs the
spectacle of its grandeur. For so is man made. However wonderful may be
what he sees day after day, he looks on it with indifference; while
matters of very little importance attract and interest him if they
depart from the accustomed order. The host of heavenly constellations
beneath the vault of heaven, whose beauty they adorn, attract no
attention; but if any unusual appearance be noticed among them, at once
all eyes are turned heavenwards. The sun is only looked on with
interest when he is undergoing eclipse. Men observe the moon only under
like conditions.... So thoroughly is it a part of our nature to admire