g6 Master Minds of Modern Science 



most careful observations extending over more than a 

 century made it certain that this irregularity did exist. 



The irregularity noticed was this. The perihelion of 

 Mercury had advanced during the century between forty 

 and fifty seconds of arc farther than it should have 

 advanced. 



Here it may be well to explain the meaning of peri- 

 helion. A planet revolving around the sun does not 

 travel in a perfect circle, but in an ellipse — that is, in an 

 elongated curve of which one axis is longer than the 

 other. The perihelion of a planetary orbit is at one of 

 the end points of the major or longer axis. The orbit 

 of the planet, while always the same in relation to the sun, 

 is not of course the same in space, for the sun itself is 

 moving rapidly in one direction, dragging its attendant 

 planets with it. 



Now the question which puzzled astronomers was the 

 cause of this irregularity in the perihelion of Mercury. 

 For long they were of the opinion that there must be 

 another undiscovered planet even nearer to the sun than 

 Mercury, but search as they might this could not be found. 

 Another suggestion was that there was a ring of cosmic 

 matter distributed around the sun which disturbed Mer- 

 cury's orbit, but this theory too was presently abandoned. 

 It remained for Albert Einstein to supply a key to the 

 puzzle of Mercury's curious behaviour, and this he did 

 in a paper read in November 1915 before the Prussian 

 Academy of Sciences. 



What is Relativity ? There is no need to be frightened 

 by the word, which in itself is simple enough. There are 

 comparatively few things in this world that are absolute. 

 The number of people in a room, the number of coins in 

 a purse, the number of bricks in a wall — these are absolute. 

 But it is easy to find simple instances of relativity. 



Imagine two brothers, Jim and Bill, each with one 

 hundred pounds in his pocket. Settle both in London, 



