The Evolution of the Sciences 



in 1718, this variation in the case of Aldebaran, 

 Sirius and Arcturus. Cassini, twenty years 

 later, placed it beyond doubt in the case of the 

 last star by comparing the observations made by 

 Richter in 1672 with his own. Since then the 

 researches of the great astronomer Herschel 

 and of many observers have enabled the altera- 

 tions in the positions of a great number of stars 

 to be traced on the celestial sphere. Naturally, 

 the brightest, and consequently nearest, stars 

 present the greatest angular displacements. 

 The stars situated at the limits of our heavens 

 appear practically motionless; this fact 

 enables the motions of the stars to be deter- 

 mined by the method which we have already 

 explained for the measurement of parallaxes, 

 as it is possible and more exact to determine 

 the motion of a near and bright star by re- 

 ferring its position to reference stars in its 

 vicinity, sufficiently small to enable their 

 angular motion on the sky to be neglected. 



On the other hand, the radial velocity of 

 the stars may be measured by the spectroscope 

 by making use of the same principle which 



enabled the sun's rotation around its axis to be 



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