176 AN EMPIRICAL STUDY OF GYRATING BODIES. 



For a time the argument for a gradual decrease of the 

 inclination of the earth's axis seemed unanswerable. 

 Two facts, however, stood in the way. It seemed impos- 

 sible, with the refined methods of the present day, that 

 such a movement should escape notice. 



This, however, presented no insurmountable obstacle, 

 for, until the contrary was shown, it could be answered 

 by the obvious fact that the movement might be so small 

 as not to have accumulated sufficiently to show itself in 

 the very brief period — cosmically speaking — of which we 

 have any record. 



The other fact presented far greater difficulties, and 

 could not be turned by any such answer. This fact was 

 the existence of that up-and-down movement of the pole 

 which astronomers style nutation. 



The earth's axis has been proved by careful measure- 

 ments to describe in its p>recessional revolution not a 

 regular curve, but one scolloped, as it were (fig. 43), the 

 pole actually rising and falling, now nearer to, now far- 

 ther from, the pole of the ecliptic, and consequently the 

 equator making corresponding changes in reference to 

 that circle. 1 The movement towards the ecliptic was all 

 right enough. It was just exactly what ought to happen 

 as the result of the tilting force ; but the movement from 

 the ecliptic, that was directly in the face of our experi- 

 ence thus far. Evidently, no further progress was possi- 

 ble till this movement was accounted for. 



Whatever the explanation, the existence of such a 

 back-and-forth movement proved that there was a force 

 somewhere which, at least to a certain extent, counter- 

 acted the tilting effect of sun and moon. As a complete 

 nutation requires about nineteen years, the same time 



1 R, fig. 43, is the pole of the ecliptic. The ellipse is the path which the pole of the earth, 

 P, ought to describe, if, as General Barnard says, a gyroscopic body without friction would 

 keep up forever. A line not quite returning into itself, but forming a slowly lessening spi- 

 ral, would seem to represent the actual path, since the attraction of the sun and moon and 

 planets draws the equatorial belt towards the ecliptic. But observation proves that the real 

 path is approximately represented by the little curves. 



ieo 



