328 FROM NEBULA TO NEBULA 



In order to solve this peculiarity it will be necessary 

 to mention another fact or two by way of preliminary. 

 The gravitational attraction of the sun on the moon, after 

 all allowances as to distances are made, is a little more 

 than twice that of the earth. Hence it may be asked why 

 the moon does not settle with its center of gravity turned 

 sunward instead of earthward, in which case it would 

 show every side of itself to us, but keep one side per- 

 petually directed toward the sun. The answer to this is 

 not difficult. The test is not one of degree of power 

 alone, but of differentiation of power. For the sun being 

 some 400 times more distant than the earth, the angle 

 formed by two lines drawn from his center to the two 

 points marking respectively the center of figure and the 

 center of gravity of the moon (for the two are by no 

 means identical) will be only 1/400 as great as a similar 

 angle formed by lines so drawn from the earth's center. 

 Hence the sun cannot bring his greater strength so well 

 to bear. It is a question of leverage, or twist. Now, as 

 the moon progresses in her orbit she keeps shifting her 

 angular position relatively to these rival attractions of 

 sun and earth, and this it is that produces the libration 

 in question. Thus, when she is at new, and again at full, 

 the sun and earth act along the same straight line, but 

 when she is at the quarters their attractions conflict most, 

 and the moon swings east or west accordingly. 



Here there arises an interesting problem on which 

 we may now gain some light : How far would the moon 

 need to be removed from the earth toward the sun in 

 order to convert her from a satellite of the earth into a 

 primary planet? As matters stand, when the moon 

 passes between the other two, her motion is relatively 

 retrograde with reference to the sun, but direct with ref- 

 erence to the earth. To effect the conversion, then, from 

 satellite into planet, she must exactly reverse this order 

 of motion a process which obviously would not take ef- 

 fect spasmodically but by infinitely slow degrees, and 

 during which she would gradually become more and more 

 distinctly " retrograde" in character. In order for the 

 sun to succeed in " wrenching" the earth and moon 



