118 Dr. Frank Watson Dyson [April 24, 



WEEKLY EYEXING MEETING, 



Friday, April 24, 1914. 



Alexander Siemens, M.Inst.C.E. M.I.E.E., Secretary and 

 Vice-President, in the Chair. 



Frank Watson Dyson, M.A. LL.D. F.R.S. F.R.A.S. 



(The Astronomer Royal). 



The Stars around the North Pole. 



1. My purpose to-night is to put before you some of the methods 

 employed to determine the distances of the stars. A knowledge of 

 the distances of the stars from us is of fundamental importance in 

 any attempt to describe the stellar universe. It is required ])efore 

 answers can be given to questions on the average distances of stars 

 from one another, their brightness compared with the sun, and the 

 extent to which they reach in space. There are not more than 100 

 or 150 stars whose distances have been measured with any degree of 

 accuracy. Although this number is being steadily increased, it is 

 only the stars which are comparatively near to the sun which can be 

 treated individually. For the greater number we have to be content 

 with average values, which apply to groups of stars, and I propose to 

 show you how these may be obtained. 



2. A map or a photograph of the stars gives us only their bearings, 

 that is to say their directions as seen from the earth. It gives no 

 information whatever about their distances. One star may be a 

 hundred times as far away as its neighbour on the map. But if 

 two maps are made, separated by a sufficient interval of time, some 

 differences will be found in the relative positions of the stars. These 

 indicate movements either of the stars .themselves or of the point 

 from which they are viewed. But the movements which are observed 

 are merely changes of angular position. We cannot tell directly from 

 them either the actual velocities or distances of the stars, but only the 

 ratio between these quantities. It is, however, from the geometrical 

 study of these small angular motions, supplemented by the information 

 obtained from the spectroscope as to the velocities of stars in the line 

 of sight, that our knowledge of their distances is derived. 



8. The problem is in many ways analogous to one which has 

 been completely solved. In the early days of astronomy the move- 

 ments of the wandering stars or planets were noted. The essential 

 characteristics of the movements were next embodied in geometrical 

 formulae by the Greeks. In the course of time Copernicus showed 



