78 The Realm of Nature CHAP. 



at noon, when they are faint. When a body giving out 

 light is in rapid motion toward the observer, the wave-length 

 of the light is apparently shortened and the lines of its 

 spectrum are shifted toward the violet end. In the light 

 of a rapidly receding body the lines are similarly shifted 

 toward the red end. At its equator the Sun's surface is 

 moving 70 miles a minute, toward an observer on one side 

 from him on the other. By causing a small image of the 

 solar disc to flit across the slit of the spectroscope several 

 times in a second, an observer analyses in quick succession 

 the light from the approaching and receding edges. Con- 

 sequently the most distinct solar absorption lines are seen 

 to oscillate slightly from side to side, being displaced alter- 

 nately toward the red and toward the violet, while the lines 

 produced in the Earth's atmosphere remain motionless 

 and can be readily distinguished. The elements which have 

 been detected in the Sun are identical with those found in 

 the Earth, but the spectrum shows that they are at an 

 enormously high temperature, so much so that some of the 

 solar lines not yet identified may be due to matter of 

 a simpler form than any elements known on the Earth 



( 47). 



1 1 8. The Heat of the Sun. The temperature of the 

 Sun is higher than any that has been produced on Earth, 

 and it does not perceptibly differ from year to year. If the 

 Sun were a heated solid or liquid globe it would be falling 

 in temperature as it radiated heat, unless the supply were 

 kept up in some way. There is no external source of heat 

 that is sufficient to account for the vast solar expenditure. 

 The collision of meteorites and many other theories have 

 been suggested, tested, and rejected, and we must look to 

 the Sun itself for an explanation. Sir William Thomson * 

 and Professor von Helmholz have shown that as the solar 

 atmosphere loses its heat the power of gravity draws its 

 particles closer together, and this shrinking transforms the 

 potential energy of separation ( 54, 56) into heat, which 

 is sufficient to maintain the diminished volume at the same 

 or even a higher temperature. The process will go on, 

 loss of heat being compensated, or more than compensated, 



