THE PROBLEM' OF THE MOON'S ORIGIN. 



By B. G. HARRISON, F.R.A.S. 



The air of mystery which always enshrouds the 

 Moon compels even the most unreflective of us to 

 speculate upon its past history. A world devoid of 

 life, one pictures the vast upheavals and intense 

 convulsions that must have taken place to have 

 moulded the landscape into its present chaotic form, 

 which bears witness to its former tempestuous exis- 

 tence. Seen through the telescope, one is impressed 

 by the sense of utter loneliness and desolation that 

 seems to prevail everywhere, while the apparent 

 permanence of each feature on the lunar surface 

 makes one wonder how long this changeless aspect 

 has continued. Indeed, there are few problems con- 

 nected with astronomy so fascinating as that of the 

 origin of our satellite, more especially since any 

 knowledge of its past would be of infinite assistance 

 in helping us to frame the general theory of cosmical 

 evolution. 



Apart from the fact that the Moon is our nearest 

 neighbour and the most familiar of all the celestial 

 bodies, the evidence of its formation is considered to 

 be of a far more conflicting nature than in the case 

 of any other portion of the solar system, and for this 

 reason alone presents features of the greatest 

 interest. 



There are, of course, only two ways to which our 

 satellite could have attained its present position. It 

 may once have been a minor planet which ventured 

 too near the Earth and was consequently captured 

 by the latter ; or else it must at some remote period 

 have formed part of our globe and have been separated 

 from it by rapidity of rotation. Before considering 

 these alternatives in detail it will be perhaps as well 

 briefly to outline the most usually accepted hypo- 

 thesis of planetary evolution. 



The general features of Laplace's celebrated 

 nebular theory are well known. This illustrious 

 mathematician suggested that our system was 

 originally a nebulous cloud or " firemist" of glowing 

 gas, which gradually contracted, throwing off rings 

 in the process. From these rings the planets were 

 evolved and the nucleus eventually condensed to 

 form our sun. Laplace himself advanced this theory 

 with considerable mistrust, and recent research has 

 caused several modifications to be made in his 

 hypothesis. 



In the first place, the original high temperature 

 with which he endowed his nebula is now considered 

 unnecessary. It is realised that the actual contrac- 

 tion of the gaseous mass would provide all the heat 

 required to account for the present observed tempera- 

 ture of the sun and planets, even after due allowance 

 is made for loss by radiation entailed in the process. 

 Secondly, it is thought very improbable that the 

 nebula could ever have had a sufficiently rapid rotation 

 to detach the rings. Even if these were detached, 

 the result has not been quite what we should expect. 



Theory demands that the width of these rings should 

 be infinitesimal, since otherwise the outer portion 

 would revolve more rapidly than the inner and thus 

 upset their dynamical equilibrium. As the accelera- 

 tion of rotation would probably proceed at a uniform 

 rate there should have been a vast number of these 

 situated at intervals bearing a fixed ratio to each 

 other, thus giving the transformed nebula a perfectly 

 symmetrical appearance. Even if it were possible 

 for the rings to condense into planets, which seems 

 doubtful, we should expect their masses to bear a 

 more constant proportion to one another than is 

 actually the case. Perhaps a still more unfavourable 

 argument to this idea of our system's development 

 lies in the fact that amongst the vast numbers 

 of nebulae which have been discovered, none 

 afford any convincing evidence of concentric ring 

 formation. 



It is generally far more easy to detect faults in the 

 theories of others than to provide a thoroughly satis- 

 factory hypothesis in their place, and critics have 

 found the case in point no exception to the rule. 

 Nevertheless, numerous suggestions of more or less 

 merit have been advanced from time to time, but as 

 space will not permit of their consideration in detail, 

 we will confine ourselves to outlining what is perhaps 

 the most feasible theory of planetary evolution. To 

 do this it will be necessary to go back an epoch 

 further than Laplace did, and consider the actual 

 .formation of the primordial nebula. It is thought 

 probable that this was caused by the near approach 

 of another star to our sun, which at that remote 

 period may have been simply a dark body. It is by 

 no means necessary for an actual collision to have 

 taken place, since the disruptive action caused by 

 the different portions of each sphere being at varying 

 distances from the common centre of gravity, and 

 thus being pulled with unequal force, may have been 

 quite sufficient to tear them both apart and scatter a 

 portion of their contents into surrounding space. 



Now there are three possible courses for matter 

 thus ejected to pursue. Either to follow a path by 

 which no return to the system would ever be 

 possible, to fall back to the Sun itself, or to revolve 

 around him in elliptical orbits. If our luminary 

 were travelling in a straight line, and in the absence 

 of any other perturbing force, all matter would 

 follow one of the first two courses, according to the 

 force of ejection. It is necessary, therefore, to 

 assume that either owing to the action of the 

 disturbing stranger, or else through collisions 

 amongst themselves, the paths of some of the 

 particles were altered, and they were forced to 

 permanently revolve round the Sun, thus forming 

 our embryo solar system. It is probable, in any 

 case, that a large proportion must have followed one 

 of the other alternatives, and so have either increased 



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