PROGRESS OF MODERN ASTRONOMY 



through whose filmy tail the earth passed in 1861 was 

 itself fourteen millions of miles away. The ordinary 

 mind, schooled to measure space by the tiny stretches 

 of a pygmy planet, cannot grasp the import of such 

 distances; yet these are mere units of measure com- 

 pared with the vast stretches of sidereal space. Were 

 the comet which hurtles past us at a speed of, say, a 

 hundred miles a second to continue its mad flight un- 

 checked straight into the void of space, it must fly on 

 its frigid way eight thousand years before it could 

 reach the very nearest of our neighbor stars ; and even 

 then it would have penetrated but a mere arm's-length 

 into the vistas where lie the dozen or so of sidereal resi- 

 dents that are next beyond. Even to the trained mind 

 such distances- are only vaguely imaginable. Yet the 

 astronomer of our century has reached out across this 

 unthinkable void and brought back many a secret 

 which our predecessors thought forever beyond human 

 grasp. 



A tentative assault upon this stronghold of the stars 

 was being made by Herschel at the beginning of the 

 century. In 1802 that greatest of observing astrono- 

 mers announced to the Royal Society his discovery that 

 certain double stars had changed their relative positions 

 towards one another since he first carefully charted 

 them twenty years before. Hitherto it had been sup- 

 posed that double stars were mere optical effects. Now 

 it became clear that some of them, at any rate, are 

 true "binary systems," linked together presumably by 

 gravitation and revolving about one another. Halley 

 had shown, three-quarters of a century before, that the 

 stars have an actual or "proper" motion in space; 



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