THE MOON 327 



PROBLEM OF THE MOON'S MOTION 



It is truly singular that axial rotation, the cause 

 whereof savants have never yet managed to guess, has 

 been assumed to be the natural thing, whereas a condi- 

 tion of inertness and stability is all their dynamical pre- 

 mises give them the right to expect. So far as we know, 

 Mercury (the smallest of the planets) and the only satel- 

 lites of other planets susceptible of sufficiently definite 

 telescopic examination (namely, some of Jupiter's) ex- 

 hibit the same idiosyncrasy of motion as does the moon. 

 It is positively unthinkable that such a uniformity of ro- 

 tation can be the result of mere chance ; but, on the con- 

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

 but to such causes as inevitably lead to the one result. 

 Astronomers have invariably approached this problem by 

 assuming initial rapid rotations (not attempting to ex- 

 plain them) and thence toning these down, with their 

 imaginary tidal brakes, to fit the observations. 



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 inher- 

 ent 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, on the familiar principle of the loaded die. 



In this attitude the moon makes a circuit about the 

 earth every 27-J4 days, the plane of its orbit being ap- 

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

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



Now, the moon has a peculiar trick, in rounding 

 from full to last quarter, of seeming to turn gently to 

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

 ously expose an equal segment or crescent around the 

 westerly limb. After passing 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 latter half of its circuit the same maneuver 

 is repeated, except that there we get to see an extra cres- 

 cent around the other, or easterly, edge. This phenome- 

 non is what is known as the "longitudinal libration". 



