C. B. WARRIXO. |?7 



that the moon requires to go through all possible posi- 

 tions, it seemed most probable that it depended on the 

 moon's movements, as indeed we have all read in our 

 astronomies. But how could the moon produce such an 

 effect? How could it make the equator move from the 

 ecliptic when its pull is always towards it ? As the moon 

 is sometimes on the upper side of the equator, and 

 sometimes on the lower side, and consequently, pulls 

 the equatorial protuberance alternately north and south, 

 it may be thought that in this alternation lies the true 

 explanation of nutation. In fact, this appears to many 

 to be the true cause of this phenomenon, and it is inter- 

 esting that this conclusion is apparently confirmed by 

 the fact just mentioned, that a complete nutation is 

 identical in duration with that period which the moon 

 occupies in going through its various changes, till it has 

 given equal portions of its time and influence to each 

 side of the equator. 



It is true that the moon is sometimes north and some- 

 times south of the equator ; but it is not true that it 

 sometimes makes the equator go towards the ecliptic, 

 and sometimes from it. Wherever it is, and however it 

 acts, it always pulls toward the ecliptic, as can easily be 

 shown. 



Fig. 41ir shows the moons orbit, at different epochs, 

 say nine years apart. When the moon is at either A or 

 B, it is north of the equator, and when it is at either A' 

 or B', it is south of the equator. Whether at A or B, 

 or at A' or B', the attraction on the equatorial protuber- 

 ance tends to move it toward the ecliptic, E E'. We may 

 put the moon in all possible positions with the same 

 result. 



It follows, therefore, in the actual arrangement of our 

 system, that when the equator is drawn north it goes to- 

 wards the ecliptic, and when it is drawn south, it still 

 goes towards the ecliptic. 



161 



