A HISTORY OF SCIENCE 



Bradley 's studies led him to discover also the libra- 

 tory motion of the earth's axis. "As this appear- 

 ance of 7 Draconis indicated a diminution of the 

 inclination of the earth's axis to the plane of the 

 ecliptic," he says; "and as several astronomers have 

 supposed that inclination to diminish regularly; if this 

 phenomenon depended upon such a cause, and amount- 

 ed to 1 8" in nine years, the obliquity of the ecliptic 

 would, at that rate, alter a whole minute in thirty 

 years; which is much faster than any observations, 

 before made, would allow. I had reason, therefore, to 

 think that some part of this motion at the least, if not 

 the whole, was owing to the moon's action upon the 

 equatorial parts of the earth; which, I conceived, might 

 cause a libratory motion of the earth's axis. But as I 

 was unable to judge, from only nine years observations, 

 whether the axis would entirely recover the same 

 position that it had in the year 1727, I found it 

 necessary to continue my observations through a 

 whole period of the moon's nodes; at the end of 

 which I had the satisfaction to see, that the stars re- 

 turned into the same position again; as if there had 

 been no alteration at all in the inclination of the earth's 

 axis; which fully convinced me that I had guessed 

 rightly as to the cause of the phenomena. This cir- 

 cumstance proves likewise, that if there be a gradual 

 diminution of the obliquity of the ecliptic, it does not 

 arise only from an alteration in the position of the 

 earth's axis, but rather from some change in the plane 

 of the ecliptic itself ; because the stars, at the end of the 

 period of the moon's nodes, appeared in the same 

 places, with respect to the equator, as they ought to 



12 



