FOOTNOTES:
[100] It must at the same time be borne in mind that their total darkness is not proved. All that is certain is that their spectra are not bright enough to leave any impression on the exposed plates.
[101] Thomson and Tait, Natural Philosophy, Appendix E, p. 494, edition 1890.
[CHAPTER XIV]
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.