The Milky Way 



the chaos of stars and to grasp their unity. 

 The double ring and the central cluster form 

 a whole; the Milky Way enfolds and includes 

 our entire universe. It forms the great nebula 

 to which we belong, with all stars accessible to 



our investigations. 



/ 



Hitherto we have tried to realise the solidarity 

 of the worlds constituting the Milky Way by 

 geometrical arguments; we shall now demonstrate 

 it by the more convincing proofs of mechanics. 



The fixity of the stars is only an illusion due 

 to their extreme remoteness. None of the stars 

 are motionless; all are animated by translatory 

 motions. Now we have to-day the means of 

 determining these motions in magnitude and in 

 direction, because we are able to compute the 

 velocity of a star, both at right angles to the 

 ray of light reaching us from it (this is called 

 the tangential velocity) and in the direction 

 of the ray itself (the radial velocity). 



The tangential motions can be ascertained 

 by angular measurements. A star moving in 

 this manner must vary in right ascension and 



in declination. Halley was the first to suspect, 



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