OUR OWN SYSTEM
Our sun is clearly middle-aged. It bears none of the marks associated
with juvenility in stars, while its decrepitude is in the distant
future. It is crossing, most likely, a level tract where recuperation
so nearly balances expenditure that radiation can be maintained for an
indefinite time at a high and fairly uniform standard. Stars of the
solar type pursue the even tenor of their way with particularly few
interruptions. They show little tendency to intrinsic variability.
Their periodicity, when it exists, is due to the presence of a
companion. Variables, in other words, belonging to the spectral family
of our sun, are binary systems; and they are usually, if not always,
non-eclipsing binaries, on the pattern of δ Cephei. Light changes can
thus be impressed upon sunlike stars by external influence; they do
not conspicuously arise through native instability.
Our planet, accordingly, is attached to a safe and steady luminary,
one subject, not to destructive spasms, but to vicissitudes so mild as
to evade distinct meteorological recognition.[102] It is, moreover,
governed by a polity settled on a broad basis of tranquillity and
permanence. All this is as it should be. The conditions specified were
a pre-requisite to the unfolding of human destinies. Nor can it be
confidently asserted that they have been realized anywhere else. Our
system may be unique; while, on the other hand, replicas of it might,
imperceptibly to us, be profusely scattered through the wide realms of
space. It is certain that a telescopic observer on Sirius or α Centauri
would see our sun unattended; not even Jupiter could be brought into
view by optical appliances in any degree comparable to those at our
disposal.
There are, nevertheless, strict limitations to the possible diffusion
of planetary worlds like those that wander amid the zodiacal
constellations. We have become aware of incapacitating circumstances,
by which a multitude of stars are precluded from maintaining retinues
of subordinate globes. Spectroscopic discoveries have compelled a
revision of ideas as to cosmical arrangements. Especially the large
proportion established by them of binary to single stars makes it
impossible any longer to regard the solar system as a pattern copied at
large throughout the sidereal domain. We cannot, then, compare it with
any other; the mechanism of which the earth forms part must, perforce,
be studied in itself and by itself, and it may, for aught that appears,
be the outcome of special and peculiar design.
The machine in question is self-sustaining and self-regulating; no
extraneous power noticeably affects its working. This immunity from
disturbance is the fortunate consequence of its isolation. A great
void surrounds it. The span of Neptune's orbit is but a hand-breadth
compared with the tremendous unoccupied gulf outside--unoccupied, that
is to say, by bodies of substantial mass. The feebleness of starlight
relatively to sunlight affords some kind of measure of the impotence of
stellar attractions to compete with the over-ruling gravitational power
that sways the planetary circulation. This it is which gives to it such
remarkable stability. The incomparable superiority of the sun over his
dependent orbs not only safeguards them against foreign interference,
but reduces to insignificance their mutual perturbations. Hence the
strong concentration of force exemplified in our system--the absolutely
despotic nature of the authority exercised--makes for a settled order
by excluding subversive change.
The organization of the solar kingdom, as disclosed by modern research,
is greatly more varied and complex than Laplace took it to be. His
genetic scheme was, indeed, no sooner promulgated than deviations from
the regularity and unanimity of movement upon which it was based began
to assert their inconvenient reality. They have since multiplied; and,
emerging to notice under the most unlikely aspects, they occasion
incongruities which tax, for their explanation, all the resources and
audacities of the most inventive cosmogonists. Let us briefly consider
their nature.
The swarm of asteroids that bridge the gap between Mars and Jupiter
revolve, it is true, with the general swirl of planetary movement; but
they use a large license as regards the shape and lie of their orbits.
And their partial exemption from the rules of the road becomes entire
for comets and meteors, which have nevertheless proved themselves
to be aboriginal in our system by their full participation in its
proper motion. Finally, several of the major planets set convention at
defiance in the arrangement of their several households, and thereby
intimate departures from the supposed normal course of development so
frequent and so considerable as to shake belief even in its qualified
prevalence. Thus, the anomalously short period of the inner satellite
of Mars, besides throwing doubt over its own mode of origin, tends
to obscure the history of its more sedately circulating associate.
Deimos cannot have been thrown off from its primary under conditions
materially different from those attending the birth of Phobos.
The sub-systems of Uranus and Neptune exhibit, moreover, eddies of
retrograde movement suggesting primitive disturbances of a fundamental
kind; while the surprising disclosures connected with Saturn's
firstborn and furthest satellite, photographically detected by
Professor W. H. Pickering in 1898, have added one more knotted thread
to the tangled skein we would fain unravel. Until acquaintance was made
with Phœbe, counterflows of revolution within the same satellite-family
were unknown, and, if contemplated at all, would have been scouted
as impossible. One ternary star, to be sure--ξ Scorpii--had been
recognised as probably owning an immediate and a more remote attendant,
in oppositely directed orbital movement;[103] but the cases are in many
ways disparate, and the analogy, though instructive, is imperfect.
If the ninth Saturnian moon is to be regarded as sprung from the
condensing mass of the planet, a total change in the state of the
parent body must have supervened during the long interval between
its separation and that of its successor, Iapetus. The change, in
Professor W. H. Pickering's opinion,[104] was nothing less than a
reversal of axial movement. The nebulous spheroid destined to develop
into the wonderful Saturnian system had, presumably, when Phœbe
became detached from it, a diameter of sixteen million miles, and
gyrated tranquilly from east to west, in a period of about a year and
a half. But the action of sun-raised tides availed first to destroy
and finally to invert this movement; for the natural outcome of tidal
friction is synchronism, and this implies agreement, both in period and
direction, between the rotation and revolution of the body acted upon.
Acceleration through contraction did the rest; and by the time another
satellite was ready to separate, the originating globe span normally in
seventy-nine days, the actual revolutionary period of Iapetus. The view
that such was the course of events is plausible at first sight; yet the
doubt remains whether the cause alleged was adequate to the effect
produced. At the distance of Saturn, solar tidal friction exerts only
about 1/20000 its power on the earth;[105] its efficacy would, on the
other hand, be greatly enhanced by the distension of the mass subjected
to it; but approximately to what extent, our powers of calculation are
impotent to determine.
This is not all. Exhaustive photographic research promises to unfold
intricacies of construction in secondary systems demanding the
patient industry of many generations for their complete unravelment.
The families of the great planets will perhaps be found to include
crowds of inferior members which pay slight heed in their circulatory
arrangements to the trammels of convention. In those of both
Jupiter and Saturn the phenomenon has lately been brought to light
of 'asteroidal' satellites, as they may be termed, minute bodies
travelling round their primaries at nearly the same mean distances,
each group evidently representing the unagglomerated materials of a
single full-sized satellite. The pigmy components of such groups
doubtless exist in multitudes; each great planet, most likely,
is encompassed by at least one zone of moonlets; but so far only
specimen-objects have been picked up. The tenth Saturnian satellite,
discovered, like its predecessor, by Professor W. H. Pickering, is thus
associated, by its period and locality, with Hyperion, the seventh and
least prominent of Saturn's visual train, the apparent insignificance
of which suggested to Sir John Herschel that it might have many
co-occupants of the wide gap between Titan and Iapetus.[106] But the
surmise had to await verification until methods were intensified beyond
what seemed possible in the middle of the nineteenth century.
The corresponding Jovian pair found by Professor Perrine circulate
far outside the boundaries of the original Galilean realm, in
orbits which interlock as a consequence of their marked difference
in eccentricity.[107] They are mutually inclined at an angle of
27 degrees, nor are they supposed actually to intersect, so that
collisions are apparently out of the question. Direct movement is
indicated, but cannot yet be claimed to belong quite certainly to both
objects. We are only beginning to make acquaintance with the submerged
populations of the Saturnian and Jovian kingdoms; they are perhaps
multitudinous; they are certainly peculiar, and we await impatiently
and curiously the further developments of their remarkable behaviour.
The one certain inference derivable from the diversity of facts
ascertained within the last hundred years is that our world is not (so
to speak) machine-made. The _modus operandi_ employed to disengage
the planets from their nebulous matrix was not of cast-iron rigidity;
it was adaptable to circumstances; it left room for the display of
boundless inventiveness in details. This was made, nevertheless, to
consist with the perfect preservation of the main order, both in design
and operation. The general plan is broadly laid down and unmistakable;
the springs of the machine are undisturbed in their free play, and for
the primary reason that departures from regularity, which might, in any
way, prove a menace to stability, affect bodies of negligible mass.
The great swing of settled movement goes on irrespectively of them. 'De
minimis non curat lex.'
So the erratic procedure of comets is harmless only because of their
insignificance. If imitated by substantially attractive masses, it
could not fail to jeopardize the planetary adjustments. Even the
asteroids would be unsafe neighbours but for their impotence; and it
is remarkable that Mercury, by far the smallest of the major planets,
circulates along a track of the asteroidal type. It would seem as if an
important size carried with it an obligation to revolve in an orbit of
small eccentricity, inclined at a low angle to the principal plane of
the system. The reason why this should be so is not obvious; but were
it otherwise the equilibrium, now so firmly established, would subsist
precariously, or not at all.
The assertion, indeed, that it is firmly established can only be
made under reserve. We are ignorant of any causes tending towards
its overthrow; yet they may supervene, or be already imperceptibly
active. One such lurking possibility is the presence of a resisting
medium in interplanetary space. Waifs and strays of matter must,
beyond doubt, be encountered there--outlawed molecules, self expelled
from the gaseous envelopes of feeble globes; thin remnants of cometary
paraphernalia, driven off amid the fugitive splendours of perihelion;
products of ionic dissociation set flying by the impact of ultra-violet
light--and all disseminated through an ethereal ocean, which 'is cut
away before and closes from behind' as moving bodies traverse it. That
its indifference is shared by ordinary material substances, when in the
last stage of attenuation, is a plausible but unverified conjecture. It
is only safe to say that retardation of velocity in what may pass for
empty space is insensible or null.
There may, nevertheless, be springs of decadence in the solar system.
Some of them have been discussed by M. Poincaré,[108] whose confidence
in the reassuring demonstrations of Laplace and Lagrange is inversely
proportional to the magnitude of the terms they were forced to neglect.
They dealt with fictitious globes, devoid of appreciable dimensions,
and swayed by the strict Newtonian law. But the real planets and
their satellites are acted on by other forces as well, frictional,
magnetic, radio-repulsive, the joint effects of which may not be
wholly evanescent. The tidal drag on rotation undoubtedly occasions
a small but irretrievable loss of energy. The moon, for instance, as
M. Poincaré states, now gains, by the reactive consequences of tidal
friction in widening its orbit, no more than 1/28 the _vis viva_ of
which the earth is deprived by the infinitesimal slowing down of its
rotation; and the remaining 27/28, being dissipated abroad as heat, are
finally abstracted from the system.
The ultimate state, we are told, towards which the planetary
mechanism tends is that of the synchronous revolution, in a period
of about twelve years, of all its members. This might, apart from
the possibility of a resisting medium, have indefinite permanence;
otherwise precipitation to the centre would gradually ensue, and
one solitary sphere, cold, stark, and unilluminated, would replace
the radiant orb of our cerulean skies, with its diversified and
exquisitely poised cortège. Unsecured drafts upon futurity, however,
are not among the most valuable assets of science, and a consummation
so immeasurably remote may be anticipated by a score of unforeseen
contingencies. What can be and has been ascertained is the relative
durability of the scheme with which the visible destinies of the human
race are so closely connected. It will unquestionably last long enough
for their accomplishment. Curiosity that would seek to pierce the
ulterior darkness is likely to remain ungratified.
But there is a further outlook. Other and incalculable items remain
to be taken into account. The sun, although an autocrat within his
own dominion, is himself subject to external influences. As a star,
he is compelled to follow whithersoever the combined attractions of
his fellow-stars draw him; nor can we thoroughly interpret the summons
which he obeys. The immediate upshot in the transport of the solar
system towards the constellation Lyra has, it is true, been determined,
but the eventual scope and purpose of the journey remain profoundly
obscure. The pace is to be reckoned as leisurely: twelve miles a
second is little more than half the average stellar speed. We should,
however, probably suffer no inconvenience from being whirled through
the ether in the train of such a stellar thunderbolt as Arcturus. Only
the excessive velocities of any adventitious bodies we might happen to
pick up would betray to ordinary experience the fact of our own swift
progress. As it is, our sweepings from space appear to be scanty.
If shreds from inchoate worlds, or dust of crumbled worlds, strewed
the path of our system, they should be annexed by it in its passage,
temporarily or completely, and we should then expect to find the apex
of the sun's way marked, if no otherwise, by the predominant inflow
from that quarter of comets and meteors. Yet there is no trace of such
a preference in the distribution of their orbits. Hence the enforced
conclusion that the sun has attached to him, besides the members of his
immediate household, an indefinite crowd of distant retainers, which,
by their attendance upon his march, claim with him original corporate
unity. To this rule there may be a few exceptions. An occasional
aerolite probably enters the earth's atmosphere with hyperbolic
velocity, and takes rank accordingly as, in the strictest sense, a
foreign intruder; but the broad truth can scarcely be challenged that
the sun travels through a virtual void.
We can, however, see no necessity why he should for ever continue to do
so. Widely different conditions seem to prevail near the centre and out
towards the circumference of the sidereal world. What may be designated
the interior vacuity of the Milky Way is occupied mainly by stars of
the solar type, including one to our apprehension super-eminent over
the rest; they are separated by vast, apparently clear intervals; they
are non-nebulous, and of stable constitution. This secure habitat is
ours for the present, although it may at some future time be exchanged
for one less exempt from disturbance. The shape and size of the sun's
orbit are utterly unknown; the changes of environment, accordingly,
that will accompany the description of it defy conjecture. Our actual
course is inclined at a small angle to the plane of the Milky Way. It
will presumably become deflected, but perhaps not sufficiently to keep
our system clear of entanglement with the galactic star-throngs. In
our ignorance of their composition no forecast of the results can be
attempted: they are uncertain and exorbitantly remote. Moreover, the
comparative slowness of the sun's motion in a manner guarantees the
permanence of his subsisting cosmical relations. For anything that
science can tell, they may ultimately be subverted by some preordained
catastrophe; but the possibility lies outside the sphere of rational
forecast.
The universe, as reflected in the mind of man, gains extent as the
mirror acquires polish. Early astronomers conceived of but one solar
system and one 'dædal earth,' upon which the 'pale populace of heaven'
rained influences sinister or propitious. Later, human egotism took
another form. The whole universe was assimilated to our particular
little settlement in it. Terrestrial conditions were universalized.
None divergent from them were counted admissible or profitable. But one
answer seemed possible to the perpetual _Cui bono?_ with which restless
thought assailed the heavens. But one purpose was regarded as worthy
of fulfilment, that of multiplying, in distant sidereal climes, copies
of our own planet, and of providing suitable locations for myriads
of intellectual beings, as little alien to ourselves as might be
compatible with the minimum of diversity in their material surroundings.
The spread of this astral philanthropy has been in some measure
checked by the advance of knowledge. Our position and circumstances
have been shown by it to be, if not quite peculiar, at any rate very
far from inevitable. It has reduced, by a process of exclusions, to
a relatively limited number the class of stars that can fairly be
regarded as possible centres of vitality; it has immensely widened the
scope of discernible variety in cosmical arrangements, and held out
warnings against errors of exposition due to inborn prejudices. And we
shall surely not wander from the truth by recognising our inability to
penetrate all the depths and complexities of Infinite Design.
FOOTNOTES:
[Footnote 102: The uncertainty affecting the best attainable
results in weather-cycle investigation is rendered strikingly
apparent by a comparison of the able and laborious papers by H. W.
Clough (_Astrophysical Journal_, vol. xxii., p. 42), and C. Easton
(Petermann's _Geogr. Mittheilungen_, 1905, Heft VIII., and _Proceedings
Amsterdam Academy of Sciences_, June 24, 1905).]
[Footnote 103: R. T. A. Innes, _Reference Catalogue_, p. 155A.]
[Footnote 104: _Harvard Annals_, vol. liii., p. 61, where, however, the
reversal is explained by a shifting of the plane of rotation.]
[Footnote 105: G. H. Darwin, _Philosophical Transactions_, vol.
clxxii., p. 526; Moulton, _Astrophysical Journal_, vol. xi., p. 110.]
[Footnote 106: _Monthly Notices_, vol. ix., p. 91.]
[Footnote 107: F. E. Ross, _Lick Bulletin_, No. 82.]
[Footnote 108: _Annuaire du Bureau des Longitudes_, 1898.]