Problems of Astronomy 



uring the parallax of a number of stars, the most 

 recent investigations show that there are very 

 few, perhaps hardly more than a score of stars of 

 which the parallax, and therefore the distance, 

 has been determined with any approach to cer- 

 tainty. Many parallaxes, determined by ob- 

 servers about the middle of the century, have 

 had to disappear before the powerful tests ap- 

 plied by measures with the heliometer; others 

 have been greatly reduced, and the distances of 

 the stars increased in proportion. So far as 

 measurement goes, we can only say of the dis- 

 tances of all the stars, except the few whose 

 parallaxes have been determined, that they are 

 immeasurable. The radius of the earth's orbit, 

 a line more than ninety millions of miles in 

 length, not only vanishes from sight before we 

 reach the distance of the great mass of stars, 

 but becomes such a mere point that, when magni- 

 fied by the powerful instruments of modern 

 times, the most delicate appliances fail to make 

 it measurable. Here the solar motion comes to 

 our help. This motion, by which, as I have 

 said, we are carried unceasingly through space, 

 is made evident by a motion of most of the stars 

 in the opposite direction, just as, passing through 

 a country on a railway, we see the houses on the 

 right and on the left of us being left behind. 

 It is clear enough that the apparent motion will 

 be more rapid the nearer the object. We may, 

 therefore, form some idea of the distance of the 

 stars when we know the amount of the motion. 

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