1916] The Movements of the Earth's Pole 673 



WEEKLY EVENING MEETING, 



Friday, May 19, 1916. 



Tjie Right Hon. Lord Wrenbury, P.C. M.A., 

 Vice-President, in the Chair. 



Colonel Ed:\iond H. Hills, C.M.G. D.Sc. F.R.S., Sec. R.I. 

 The Movements of the Earth's Pole. 



More than a century ago it was shown by the mathematician Euler 

 that if the axis round which the earth was rotating were not 

 coincident with the axis of figure, which latter in the case of a 

 spheroidally flattened earth is the shortest axis that can be drawn, 

 the axis of rotation will revolve about the axis of figure in a period 

 which, upon certain assumptions, can be precisely predicted. The 

 time of one revohition of the pole of rotation around the pole of 

 figure depends only upon the shape and degree of elasticity of the 

 earth. In Euler's days the supposition that the solid earth had any 

 appreciable elasticity was so far outside the range of experience that 

 it was not considered by him. He calculated the period of the 

 polar rotation on the assumption that the earth was perfectly rigid, 

 and showed that this period would be about 305 days. 



If we determine the latitude of a point on the earth's surface by 

 observations of the stars we are in effect measuring the angular 

 distance between the axis of rotation of the earth and the vertical 

 line, or line through the zenith, at the point of observation. If, 

 now, this axis of rotation moves the observed latitude of the place 

 will change and, if we prolong the observations over a sufficient 

 time, we ought to find that this observed latitude fluctuates back- 

 wards and forwards about a mean value with the same periodicity as 

 that in which the earth's pole of rotation moves round the pole of 

 figure. 



Every observer who is engaged on making observations to deter- 

 mine the precise positions of the stars, a class of observation which 

 up to a few years ago occupied a very large fraction of the time 

 and energies of astronomers, is actually continually determining and 

 redetermining the latitude of his instrument. There is thus an 

 enormous mass of latitude observations available for examination, 

 and it should prove a not too difficult task to analyse these with the 

 object of detecting a periodic variation. Two causes, however, 

 militated against success in this inquiry : firstly, the very small 

 Vol. XXI. (No. 110) 2 y 



