416 THE BEAUTIES OF NATUEE CHAP. 



Ordinary observation, of course, is powerless 

 to inform us whether they are moving towards 

 or away from us. Spectrum analysis, how- 

 ever, enables us to solve the problem, and 

 we know that some are approaching, some 

 receding. 



Blue Red 



Fig. 55. Displacement of the hydrogen line in the spectrum of Rigel. 



If a star, say for instance Sirius, were 

 motionless, or rather if it retained a constant 

 distance from the earth, Fraunhofer's lines 

 would occupy exactly the same position in 

 the spectrum as they do in that of the Sun. 

 On the contrary, if Sirius were approaching, 

 the lines would be slightly shifted towards the 

 blue, or if it were receding towards the red. 

 Fig. 55 shows the displacement of the hydro- 

 gen line in the spectrum of Rigel, due to the 

 fact that it is receding from us at the rate of 

 39 miles a second. The Sun affords us an 

 excellent test of this theory. As it revolves 

 on its axis one edge is always approaching 

 and the other receding from us at a known 



