The Romance of the Heavens 25 



of all speeds to measure, speed in the line of sight. Movement at 

 right angles to the direction in which one is looking is, if there is 

 sufficient of it, easy to detect, and, if the distance of the moving 

 body is known, easy to measure. But movement in the line of 

 vision is both difficult to detect and difficult to measure. Yet, even 

 at the enormous distances with which astronomers have to deal, 

 the spectroscope can detect such movement and furnish data for 

 its measurement. If a luminous body containing, say, sodium is 

 moving rapidly towards the spectroscope, it will be found that the 

 sodium lines in the spectrum have moved slightly from their usual 

 definite positions towards the violet end of the spectrum, the 

 amount of the change of position increasing with the speed of 

 the luminous body. If the body is moving away from the spectro- 

 scope the shifting of the spectral lines will be in the opposite direc- 

 tion, towards the red end of the spectrum. In this way we have 

 discovered and measured movements that otherwise would prob- 

 ably not have revealed themselves unmistakably to us for 

 thousands of years. In the same way we have watched, and 

 measured the speed of, tremendous movements on the sun, and so 

 gained proof that the vast disturbances we should expect there 

 actually do occur. 



IS THE SUN DYING? 



3 



Now let us return to our consideration of the sun. 



To us on the earth the most patent and most astonishing 

 fact about the sun is its tremendous energy. Heat and light in 

 amazing quantities pour from it without ceasing. 



Where does this energy come from? Enormous jets of red 

 glowing gases can be seen shooting outwards from the sun, like 

 flames from a fire, for thousands of miles. Does this argue fire, 

 as we know fire on the earth? On this point the scientist is sure. 

 The sun is not burning, and combustion is not the source of its 



