24 



KNOWLEDGE & SCIENTIFIC NEWS. 



[Feb., 1905. 



Modern Cosmogonies. 



XII.— Our Own System, 



Bv Miss .Agn-es M. Clerke. 



Olr sun is clearly middle-aged. It bears none of 

 the marks associated with juvenility in stars; and its 

 decrepitude is in the distant future. It is crossing, 

 probably, 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 ten- 

 our 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. Light-changes can thus be impressed 

 upon them by external influence; they do not con- 

 Kpicuously 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 meteoro- 

 logical recognition. It is, moreover, governed by a 

 polity settled on a broad basis of tranquillity and per- 

 manence. 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 realised anywhere else. Our system may 

 be unique; while, on the other hand, replicas 

 of it might, imperceptibly to us, be profusely 

 scattered throupV the wide realms of space. It 

 is certain that a telescopic observer on Sirius or a 

 Centauri would see our sun unattended; not even Jupi- 

 ter could be brought into view by optical appliances 

 in any degree comparable to those at our disposal. 

 There are, nevertheless, strict limitations to the possi- 

 ble 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 re- 

 tinues of subordinate globes. Spectroscopic dis- 

 coveries have compelled a revision of ideas as to cos- 

 mical 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- 

 rrgulating; no extraneous influence noticeably affects 

 its working. This exemption from disturbance is the 

 fortunate consequence of its i.solafion. A great void 

 surrounds it. The span of Neptune's orbit is but a 

 h.md-breaflth rompircd with the trcmendf)us unoccupied 

 gulf outside— unoccupied, that is to sav, by bodies of 

 substantial ma.ss. The feebleness of star-light relative- 

 ly to sun-light affords some kind of measure of the 

 impotence of stellar attractions to compete with the 

 over-ruling gravitational power that sways the planet- 

 ary circulation. This it is which gives to it such re- 

 markable stability. The incomparable superiority of 

 the sun over his dependant orbs not only safeguards 

 them against foreign interference, but reduces to in- 

 significance their mutual perturbations. Hence, the 

 strong concentration of force exemplified in our .system 



— the absolutely despotic nature of the authority exer- 

 cised — makes for a settled order by excluding subver- 

 sive change. 



The organisation of the solar kingdom, as disclosed 

 by modern research, is greatly more varied and com- 

 plex 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 cos- 

 mogonists. 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 use a large licence 

 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 proved them- 

 selves, nevertheless, 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 I'hobos, 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. The sub-systems of 

 Uranus and Neptune exhibit, moreover, eddies of re- 

 trograde movement, suggesting primitive disturbances 

 of a fundamental kind; while the surprising disclosures 

 connected with .Saturn's first-born, and furthest satel- 

 lite, have added one more knotted thread to the tangled 

 skein we would fain unravel. L'ntil acquaintance was 

 made with Phcebe, counter-flows of revolution within 

 the same satellite-family were unknown, and, if con- 

 templated at all, would have been scouted as impossi- 

 ble. One ternary star, to be sure — t Scorpii — had been 

 recognised as probably owning an immediate and a 

 more remote attendant, in oppositely directed orbital 

 movement; but the cases are in many ways disparate, 

 and the analog}-, though instructive, is imperfect. 



If the ninth Saturnian moon is to be regarded as 

 sprung from the mass of its primary, a total change 

 in the condition of the parent body must have super- 

 \ened during the long interval between its separation 

 and that of its successor lapcfus. The change, in 

 Professor W. H. Pickering's opinion,* was nothing 

 less than a reversal in the sense of axial rotation. The 

 nebulous spheroid destined to develop into the wonder- 

 ful .Saturnian system had a di.'imcter, when Phoebe 

 was thrown off from it, of sixteen million miles, and 

 g}Tatcd tranquilly from east to west, in a period of 

 about a year and a half. The action of sun-raised 

 tides, however, 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, betwct-n the rotation and 

 revolution of the body acted upon. Acceleration 

 through contraction did the rest; and before lapctus 

 entered on its separate career, the originating globe 

 span normally in seventy-nine days. The view that 

 such was the course of events is plausible at first sight; 



'Harvard Annals, Vol. LIII. p. 61, where, however, the 

 reversal is explained by a shifting of the axis of rotation. Tlie 

 mode of action described in the text was long ago suggested by 

 Kirkwood. 



