I'KESIDKNT S ADDRESS. 7 



Fortunately there is an intlirect method of attack which, in 

 tlie course of time, will tell us the distances of all the stars. 



Basically this method depends upon a knowledge of the 

 jjroper motions of the stars. If by its annual motion around 

 the sun, the earth causes the stars to be displaced, it is obvious 

 that the progressive motion of the sun through space must cause 

 a progressive displacement. If for the moment we assume the 

 stars to be at rest, they will seem to sufifer two displacements — 

 one purely periodic in a year, the other progressive, due respect- 

 ively to the earth's orbital motion and the sun's motion through 

 space. 



Here is a diagram of such a double motion, which was or- 

 ginally published by Mr. Slocum. It plots the measures exactly 

 made in the case of Perseus and its companion. 



- 400 days -500 -200 - 100 o 100 200 300 ^ 00 da ys 



Parallax observations of 6 Perseus (1) and its companion (2) 

 (after Slocum, Popular Astronomy, May, 1915). 



That the lines are zigzags is due to the parallactic displace- 

 ment caused by the orbital motion of the earth, whilst the pro- 

 gressive movement is due to the motion of the sun — at least in 

 part ; wholly if the stars were stationary in space — partly if, as is 

 to be expected, both stars and the sun are in motion. The crosses 

 mark the actual observations made, so that at a glance one can see 

 both the scale of the parallax and the inevitable uncertainties of 

 observation. 



The earth's orbital motion being periodic has no cumulative 

 effect, but the sun's progressive motion is cumulative. The 

 amplitude of the earth's periodic motion is about 300,000,000 

 kilometres, and all the best and most recent results show that 

 the sun is moving through space with a velocity of about 18 

 kilometres a second ; hence in a year the sun, and with it, of 

 course, the earth and the rest of the Solar System, move over a 

 distance of 550,000,000 kilometres ; roughly this is already twice 

 the earth's annual displacement, and, as already stated, it is cumu- 

 lative ; thus, in six years, the progressive displacement is already 



