The Evolution of the Sciences 



necessary is to sight the star at both extremities 

 of the earth's annual orbit, which are situated 

 three hundred million kilometres apart. 



Notwithstanding the dimensions of this base 

 the efforts of astronomers, from Tycho-Brahe 

 to the middle of the nineteenth century, could 

 not remove a disquieting uncertainty with regard 

 to the value and to the very existence of the 

 stellar parallaxes, because the numbers found 

 were so small that it was possible to attribute 

 them to inevitable errors of measurement. 

 But fortunately a method exists, already 

 pointed out by Galileo and applied for the first 

 time by Struve in 1835, which enables the 

 measurements to be made with greater exactness 

 and consequently removes all doubts. 



Let us consider the group formed by a 

 bright star and by two or three feebler ones, 

 which seem to surround it closely. There will 

 be a presumption, even though we may know 

 nothing certain regarding their distance from 

 the earth, that the bright star is nearer to us 

 and that the others are much farther removed. 

 Starting from this hypothesis, we may infer 



that the small stars, whose parallax is negligible, 



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