112 FROM NEBULA TO NEBULA 



assumed to be the normal thing, whereas a condition 

 of passive equilibrium ought to seem so much more 

 natural and likely? So far as we know, Mercury (the 

 smallest of the planets) and the only satellites of other 

 planets susceptible of sufficiently definite telescopic ex- 

 amination (namely, some of Jupiter's) exhibit the same 

 idiosyncrasy of motion as does the moon. It is posi- 

 tively unthinkable that such a uniformity of motion 

 can be the result of mere chance; but, on the contrary, 

 it must be due, not only to a similarity of causes, but to 

 such causes as inevitably lead to the one result. 



As a matter of fact, the moon doesn't rotate on its 

 axis in any true sense; that is to say, it hasn't an in- 

 herent motion of that character any more than a balloon 

 could be said to have were it also to circumnavigate 

 our globe. Not having any fluid oceans our satellite has 

 simply settled into a position of stable equilibrium, bal- 

 last down. 



In this attitude the moon makes a circuit about the 

 earth every -T ! /> days, the plane of its orbit being ap- 

 proximately the same as that of the earth's around the 

 sun, so that we have what are known as the lunar phases. 



Now the moon has a peculiar trick, in rounding 

 from full to last quarter, of seeming to turn g'ently to 

 the east so as to hide a part of that edge, and simul- 

 taneously expose an equal segment or crescent around 

 the westerly limb. After passing 1 the quarter, the body 

 swings just as gradually backward until at "new,'' were 

 it then visible, we should see its face precisely as it 

 is at full. In the last half of its circuit the same pro- 

 cess is repeated, except that here we get to see an extra 

 crescent around the other or easterly edge. This phe- 

 nomenon is what is known as the "longitudinal libra - 

 tion." 



In order to solve this peculiarity, it will be neces- 

 sary to mention another fact or two by way of pre- 



