HOW THE SUN WARMS TPIE EARTH ABBOT 167 



Such is Langley's method for measuring what is called the solar 

 constant of radiation. I can explain its result as follows : Imagine 

 a cube of blackened water 1 centimeter on edge, or about the size of a 

 dice cube. Expose it on the moon with one face at right angles to 

 the sun's rays. Let the experiment be made in March when the earth 

 and moon are at mean solar distance. How much will the tem- 

 perature of the blackened water cube rise in 1 minute? The answer 

 is 1.94° C. That is called the solar constant of radiation. In usual 

 words, the intensity of solar heating is 1.94 calories per square 

 centimeter per minute at mean solar distance outside our atmosphere. 



THE SUN A VARIABLE STAR 



For 15 years Smithsonian observers have measured the solar con- 

 stant on every practicable day on one or more high desert mountains 

 in distant lands. Though called a constant, the value is not a 

 constant. Figure 10 shows how its monthly mean values have been 

 observed to vary from 1920 to 1930. The changes, as you see, are 

 not large. In 1922 there came the largest, reaching a range of 2.5 

 percent. Figure 11 shows the values measured simultaneously by 

 Smithsonian observers in California, Chile, and South-West Africa. 

 Their results, though not absolutely identical, are so nearly so as 

 to disclose fair agreement as to the variability of the sun. Its 

 variability seems to be altogether irregular. As we see it in figure 

 10, it seems quite devoid of any wavelike regular periodicity. 



PERIODICITIES IN SOLAR VARIATION 



In figure 12, however, I show that this first impression of entire 

 irregularity in solar variation is superficial. In reality the sun's 

 variation is a complex summation of not less than seven regularly 

 recurring waves, or periodicities. They are respectively of approxi- 

 mately 7, 8, 11, 21, 25, 45, and 68 months in their periods. 



The component periodicity curves are summed up into the curve B, 

 which is plotted in dots upon the original measured curve A. Even 

 in details the agreement of the two curves is striking. Thus we claim 

 that the sun is not a constant star but a variable one, whose appar- 

 ently irregular variation is really a complex of at least seven perio- 

 dicities. 



The interesting question at once occurs : Do the solar periodicities 

 impress themselves on the earth's weather? They do. Before dis- 

 cussing this subject I must speak of the sun spots. 



SUN SPOTS 



When Galileo in 1610 first used the telescope he saw that the sun'.s 

 surface carried dark spots (pi. 6) and that these moved along, indi- 

 cating that the sun rotates in about 26 days. From 1826 to 1868 



