Myths and Marvels of Astronomy
among many millions of suns, any one of which (though all but a few
thousands are actually invisible) would become visible to the naked eye,
if exposed to the same conditions as have affected the suns in flames
described in the preceding pages. Seeing, then, that during the last
two thousand years or thereabouts, only a few instances of the kind,
certainly not so many as twenty, have been recorded, while there is
reason to believe that some of these relate to the same star which has
blazed out more than once, we may fairly consider the chance exceedingly
small that during the next two thousand, or even the next twenty
thousand years, our sun will be exposed to a catastrophe of the kind.
We might arrive at this conclusion independently of any considerations
tending to show that our sun belongs to a safe class of system-rulers,
and that all, or nearly all, the great solar catastrophes have occurred
among suns of a particular class. There are, however, several
considerations of the kind which are worth noting.
In the first place, we may dismiss as altogether unlikely the visit of a
comet from the star-depths to our sun, on a course carrying the comet
directly upon the sun's surface. But if, among the comets travelling in
regular attendance upon the sun, there be one whose orbit intersects the
sun's globe, then that comet must several times ere this have struck the
sun, raising him temporarily to a destructive degree of heat. Now, such
a comet must have a period of enormous length, for the races of animals
now existing upon the earth must all have been formed since that comet's
last visit--on the assumption, be it remembered, that the fall of a
large comet upon the sun, or rather the direct passage of the sun
through the meteoric nucleus of a large comet, would excite the sun to
destructive heat. If all living creatures on the earth are to be
destroyed when some comet belonging to the solar system makes its next
return to the sun, that same comet at its last visit must have raised
the sun to an equal, or even greater intensity of heat, so that either
no such races as at present exist had then come into being, or, if any
such existed, they must at that time have been utterly destroyed. We
may fairly believe that all comets of the destructive sort have been
eliminated. Judging from the evidence we have on the subject, the
process of the formation of the solar system was one which involved the
utilisation of cometic and meteoric matter; and it fortunately so
chanced that the comets likely otherwise to have been most
mischievous--those, namely, which crossed the track of planets, and
still more those whose paths intersected the globe of the sun--were
precisely those which would be earliest and most thoroughly used up in
this way.
Secondly, it is noteworthy that all the stars which have blazed out
suddenly, except one, have appeared in a particular region of the
heavens--the zone of the Milky Way (all, too, on one half of that zone).
The single exception is the star in the Northern Crown, and that star
appeared in a region which I have found to be connected with the Milky
Way by a well-marked stream of stars, not a stream of a few stars
scattered here and there, but a stream where thousands of stars are
closely aggregated together, though not quite so closely as to form a
visible extension of the Milky Way. In my map of 324,000 stars this
stream can be quite clearly recognised; but, indeed, the brighter stars
scattered along it form a stream recognisable with the naked eye, and
have long since been regarded by astronomers as such, forming the stars
of the Serpent and the Crown, or a serpentine streak followed by a loop
of stars shaped like a coronet. Now the Milky Way, and the outlying
streams of stars connected with it, seem to form a region of the stellar
universe where fashioning processes are still at work. As Sir W.
Herschel long since pointed out, we can recognise in various parts of
the heavens various stages of development, and chief among the regions
where as yet Nature's work seems incomplete, is the Galactic
zone--especially that half of it where the Milky Way consists of
irregular streams and clouds of stellar light. As there is no reason for
believing that our sun belongs to this part of the galaxy, but on the
contrary good ground for considering that he belongs to the class of
insulated stars, few of which have shown signs of irregular variation,
while none have ever blazed suddenly out with many hundred times their
former lustre, we may fairly infer a very high degree of probability in
favour of the belief that, for many ages still to come, the sun will
continue steadily to discharge his duties as fire, light, and life of
the solar system.
VII.
_THE RINGS OF SATURN._
The rings of Saturn, always among the most interesting objects of
astronomical research, have recently been subjected to close scrutiny
under high telescopic powers by Mr. Trouvelot, of the Harvard
Observatory, Cambridge, U.S. The results which he has obtained afford
very significant evidence respecting these strange appendages, and even
throw some degree of light on the subject of cosmical evolution. The
present time, when Saturn is the ruling planet of the night, seems
favourable for giving a brief account of recent speculations respecting
the Saturnian ring-system, especially as the observations of Mr.
Trouvelot appear to remove all doubt as to the true nature of the rings,
if indeed any doubt could reasonably be entertained after the
investigations made by European and American astronomers when the dark
inner ring had but recently been recognised.
It may be well to give a brief account of the progress of observation
from the time when the rings were first discovered.
In passing, I may remark that the failure of Galileo to ascertain the
real shape of these appendages has always seemed to me to afford
striking evidence of the importance of careful reasoning upon all
observations whose actual significance is not at once apparent. If
Galileo had been thus careful to analyse his observations of Saturn, he
could not have failed to ascertain their real meaning. He had seen the
planet apparently attended by two large satellites, one on either side,
'as though supporting the aged Saturn upon his slow course around the
sun.' Night after night he had seen these attendants, always similarly
placed, one on either side of the planet, and at equal distances from
it. Then in 1612 he had again examined the planet, and lo, the
attendants had vanished, 'as though Saturn had been at his old tricks,
and had devoured his children.' But after a while the attendant orbs had
reappeared in their former positions, had seemed slowly to grow larger,
until at length they had presented the appearance of two pairs of mighty
arms encompassing the planet. If Galileo had reasoned upon these changes
of appearance, he could not have failed, as it seems to me, to interpret
their true meaning. The three forms under which the rings had been seen
by him sufficed to indicate the true shape of the appendage. Because
Saturn was seen with two attendants of apparently equal size and always
equi-distant from him, it was certain that there must be some appendage
surrounding him, and extending to that distance from his globe. Because
this appendage disappeared, it was certain that it must be thin and
flat. Because it appeared at another time with a dark space between the
arms and the planet, it was certain that the appendage is separated by a
wide gap from the body of the planet. So that Galileo might have
concluded--not doubtfully, but with assured confidence--that the
appendage is a thin flat ring nowhere attached to the planet, or, as
Huyghens said some forty years later, Saturn '_annulo cingitur tenui,
plano, nusquam cohaerente_.' Whether such reasoning would have been
accepted by the contemporaries of Galileo may be doubtful. The
generality of men are not content with reasoning which is logically
sound, but require evidence which they can easily understand. Very
likely Huyghens' proof from direct observation, though in reality not a
whit more complete and far rougher, would have been regarded as the
first true proof of the existence of Saturn's ring, just as Sir W.
Herschel's observation of one star actually moving round another was
regarded as the first true proof of the physical association of certain
stars, a fact which Michell had proved as completely and far more neatly
half a century earlier, by a method, however, which was 'caviare to the
general.'
However, as matters chanced, the scientific world was not called upon to
decide between the merits of a discovery made by direct observation and
one effected by means of abstract reasoning. It was not until Saturn had
been examined with much higher telescopic power than Galileo could
employ, that the appendage which had so perplexed the Florentine
astronomer was seen to be a thin flat ring, nowhere touching the planet,
and considerably inclined to the plane in which Saturn travels. We
cannot wonder that the discovery was regarded as a most interesting one.
Astronomers had heretofore had to deal with solid masses, either known
to be spheroidal, like the earth, the sun, the moon, Jupiter, and Venus,
or presumed to be so, like the stars. The comets might be judged to be
vaporous masses of various forms; but even these were supposed to
surround or to attend upon globe-shaped nuclear masses. Here, however,
in the case of Saturn's ring, was a quoit-shaped body travelling around
the sun in continual attendance upon Saturn, whose motions, no matter
how they varied in velocity or direction, were so closely followed by
this strange attendant that the planet remained always centrally poised
within the span of its ring-girdle. To appreciate the interest with
which this strange phenomenon was regarded, we must remember that as yet
the law of gravity had not been recognised. Huyghens discovered the
ring (or rather perceived its nature) in 1659, but it was not till 1666
that Newton first entertained the idea that the moon is retained in its
orbit about the earth by the attractive energy which causes unsupported
bodies to fall earthwards; and he was unable to demonstrate the law of
gravity before 1684. Now, in a general sense, we can readily understand
in these days how a ring around a planet continues to travel along with
the planet despite all changes of velocity or direction of motion. For
the law of gravity teaches that the same causes which tend to change the
direction and velocity of the planet's motion tend in precisely the same
degree to change the direction and velocity of the ring's motion. But
when Huyghens made his discovery it must have appeared a most mysterious
circumstance that a ring and planet should be thus constantly
associated--that during thousands of years no collision should have
occurred whereby the relatively delicate structure of the ring had been
destroyed.
Only six years later a discovery was made by two English observers,
William and Thomas Ball, which enhanced the mystery. Observing the
northern face of the ring, which was at that time turned earthwards,
they perceived a black stripe of considerable breadth dividing the ring
into two concentric portions. The discovery did not attract so much
attention as it deserved, insomuch that when Cassini, ten years later,
announced the discovery of a corresponding dark division on the southern
surface, none recalled the observation made by the brothers Ball.
Cassini expressed the opinion that the ring is really divided into two,
not merely marked by a dark stripe on its southern face. This conclusion
would, of course, have been an assured one, had the previous observation
of a dark division on the northern face been remembered. With the