ASTRONOMY 



count the number of persons passing you in different 

 directions. You will probably find that there are as 

 many going north as south, or east or west. There 

 is no apparent preferential direction of motion when 

 you are stationary. If, however, 

 you walk westward towards 

 Piccadilly you will meet more 

 persons going eastward, and 

 fewer will pass you going west- 

 ward ; and the more rapidly 

 you walk the greater will be 

 the difference in the numbers 

 of those two classes of persons. 

 There will also, relative to 

 yourself, be changes in the 

 direction of motion of those 

 persons going north and south. 

 Now apply this to the stars. 

 If in any region they were 

 moving in a perfectly haphazard manner, and from a 

 point lines were drawn whose lengths were propor- 

 tional to the number moving in the direction indicated 

 by the line, we should expect all these lengths to be 

 equal, so that their extremities would lie on a circle. 

 If, however, the motions are referred to a point itself 

 in motion, for example, one of the stars themselves, 

 we find that the diagram loses its symmetry (Fig. 3). 

 It becomes elongated in a direction opposite to that 

 in which the base point of reference is moving, 



57 



= 1-5 



Fig. 3. T HEORETICAL 



CURVES SHOWING THE NUM- 

 BER OF STARS MOVING IN 

 DIFFERENT DIRECTIONS 



V is the ratio of the velocity 

 of the base point to the 

 mean velocity of the stars. 



