54 FKOM NEBULA TO NEBULA 



to be explained later on) that the sum of the momenta, 

 axial and orbital together, of any given system remains 

 the same from year to year. (Upon this rock it was that 

 the Nebular Hypothesis met final shipwreck, but of this in 

 a later chapter). They find, for example, that the plan- 

 ets, in their revolutions around the sun, do not follow 

 their courses without regard to what the others are doing, 

 but, on the contrary, rhythmically swing in and out from 

 their aphelion to perihelion in such a way that, no matter 

 where a particular planet may be in its orbit, whether 

 going fast at perihelion or slow at aphelion, the aggregate 

 momentum of the group continues unchanged: just as 

 though there were an understanding among them, or as if 

 they were controlled by some guiding hand. Now, 

 astronomers do not make any pretense of understanding 

 this mystery; all they know is that it is a fact of observa- 

 tion. Unfortunately, however, they presume to build 

 upon it, construing it not as the manifestation of an un- 

 known physical cause but as a sort of teleological ordina- 

 tion transcending natural law. 



To illustrate how arbitrary and supernatural they 

 conceive this law to be : Suppose, to take the case of the 

 earth-moon system, we call the earth 's axial momentum 

 A, the moon's B, and the momentum of the moon's orbital 

 motion C, then A+B+C must always, under the "law", 

 equal an invariable quantity M. Suppose, further, say 

 they, that from any cause whatsoever (as, for instance, 

 by colliding with a comet by a glancing stroke), the 

 earth's rotation, A, should be greatly accelerated, then B 

 and C, or one or other of them, would have to slow up cor- 

 respondingly so that A' +B'+C', the new values, might 

 still be equal to M - - not to M', but to M. On the other 

 hand, they assert, that if the comet should hit the moon, 

 sending her out fifty-thousand miles or so further, the 

 earth would obediently rotate faster of its own accord, 

 like a creature of sense, to make up for the moon's slower 

 orbital motion, at the new distance ! To the astronomer 

 the phenomenon is an uncanny one, without rhyme or 

 reason ; a mystery as cryptic and capricious as luck at 

 cards. 



