416 THE BEAUTIES OF NATURE 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. 
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 
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