The Milky Way 



Milky Way it is necessary to abandon the old 

 idea of a spherical firmament, studded with 

 stars, and with the earth for its centre; the 

 true position of each star must be defined, not 

 only by its direction but also by its distance 

 from the earth. 



The method for determining the distance 

 of inaccessible objects is very simple in principle; 

 it consists in sighting the object from two 

 different points of view and measuring the 

 angle of the two lines of vision. This angle, 

 called the parallax, is greater according as 

 the object is nearer, and according as the 

 base, that is to say the distance between the 

 two points, is greater. This is in fact the 

 very method which we instinctively apply in 

 binocular vision, taking the distance between 

 our eyes for base. But the farther away the 

 point to be determined is, the greater must the 

 base be if the parallax obtained is to be measur- 

 able; in order to determine stellar distances, 

 which we know beforehand to be enormous, 

 no base could be too great. Fortunately the 

 earth, in its annual revolution, supplies us 



with the necessary stations, and all that is 



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