462 Sir George H. Darwin [April 1^6, 



for he made incidentally a discovery of another kind and of at least 

 equal interest. 



The earth moves round the sun at a distance of 93 million miles, 

 so that in sis months we shift our position by 186 million miles. If, 

 then, there are two stars of which one is relatively near to and the 

 other far from the sun, but so situated as to appear to us very close 

 together, the near one ought to shift its position relatively to the 

 distant one in the course of each six months. The amount of this 

 change of position, called by astronomers annual parallax, should 

 furnish the distance of the nearer of the pair, provided that the other 

 is very far off. This idea is as old as the time of Galileo, but no one 

 had been able to make successful use of it. 



As I have already said, the only general test of the distance of a 

 star is its brightness, and therefore Herschel chose pairs of stars of 

 very different brilliancy. He thought, at least at first, that it was 

 mere chance which brought the stars so near to one another, and 

 there are undoubtedly such pairs now known as " optically double 

 stars." But Herschel's mode of attack was bound to fail if the seem- 

 ingly neighbouring stars were really so, and were linked together by 

 their mutual gravitation. Already as early as 1707 Michel had sug- 

 gested the existence of such true double stars, but it was Herschel 

 who proved their existence. His first catalogues of double stars, 

 published in 1782, contained 203 cases of such doublets, and he 

 already suspected a community in their motions explicable only by 

 their real association ; but by 1802 he had become certain. In many 

 cases the two components of a binary pair were found to be moving 

 in nearly the same direction and at the same speed, but superposed 

 on this motion of the system as a whole there was an orbital motion 

 of one star round the other. Herschel even lived long enough to 

 see some of his pairs of stars perform half a revolution about one 

 another. 



After his death Savary took the matter one stage further, and 

 showed that the revolution was governed by the laws of gravity, and 

 thereby confirmed the truth of Herschel's belief. Thus the failure 

 to measure the distance of stars led to the proof that gravity reigns 

 amongst the stars as in the solar system. 



Arago thought that of all Herschel's discoveries this was the one 

 that had the greatest future, and his prophecy has proved singularly 

 correct. Every year adds to the number of double stars, whose orbits 

 are now accurately determinable. These systems are found to be 

 very unlike our own solar system, for the component stars are, in 

 many cases, far larger than the sun and revolve about one another in 

 periods which, in various cases, may be either many years or only a 

 few hours. 



The spectroscope has, moreover, added enormously to our know- 

 ledge, for the speed of approach or recession of a star from the sun 

 can now be determined as so many kilometres per second. Thus 



