ADVISORY COMMITTEE ON ASTRONOMY 1 45 



•universe ; and, again, we may study the Sun as the source of that 

 radiation which makes the Earth habitable. In this case we have 

 a problem which concerns all humanity and all life on Earth, and 

 one, however, strangely neglected, which is incomparably the most 

 important to man of anything which Astronomy has to offer. 



Our concern in this case is to determine how large an amount of 

 radiation the Sun emits ; what is the nature of these rays ; what 

 modifications they undergo in their passage through the Sun's en- 

 velope and the Earth's atmosphere ; what, if any, variations occur 

 in the total radiation of the Sun, and all this with final reference to 

 the effects of these variations on life upon the Earth. The abstract 

 and the utilitarian interests are closely joined, but it is cliieily with 

 refereuce to the latter that the following remarks are offered. 



The total radiation of the Sun, usually measured by the heating 

 effect of the rays falling perpendicularly upon a square centimeter 

 of area, has been determined by numerous observers at manA' stations 

 on the Earth's surface. Many- of these measurements have been 

 corrected according to more or less plausible theories of the absorp- 

 tion of the Earth's atmosphere. The result so reached, which pur- 

 ports to be the heat equivalent of the solar radiation falling perpen- 

 dicularly upon a square centimeter of surface outside the Earth's 

 atmosphere, is termed the " solar constant." Whether or not the 

 name is well chosen is doubtful, for we do not yet know, after a 

 hundred years of careful actinometry; even within wide limits, 

 whether the emission of the Sun is constant or variable. Indeed, if 

 we should trust the experimental results implicitly, we should regard 

 the ■' solar constant " as one of the most variable things in nature; 

 for the most authoritative values range from less than 2 calories up 

 to over 4 calories per minute. 



The great discrepancy in these results does not necessarily imply 

 an actual variation in the amount of solar emission, but is chiefly 

 caused by differences in the method of reducing the obsen'ations to 

 eliminate the absorption of the Earth's atmosphere. Thus it was 

 for a long time customary to treat the atmosphere as if one portion 

 of it were just like another in its absorption. Upon this basis, if, 

 for example, a layer thick enough to exert a barometric pressure of 

 I decimeter transmitted A per cent of the incident beam a second 



* To prevent confusion, it may be stated that a solar constant of 3 calories per 

 minute indicates that the heat equivalent of the solar radiation falling perpen- 

 dicularly upon a surface of i squsre centimeter area for t minute of time is 3 

 times the amount required to raise the temperature of i gram of water tlirough 

 I degree centigrade. 



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