PROMSIOXAL SOLAR-CONSTAXT \ ALUES, AUGUST, 

 1920, TO NO\^EMBER, 1924' 



By C. G. abbot AND COLLEAGUES' 



The Smithsonian Institution has employed the appropriations made 

 by Congress for the support of the Astrophysical Observatory to con- 

 tinue observations of the solar constant. of radiation. This means, in 

 other words, the intensity of the sun's heat as it would be found m 

 free space at the earth's mean solar distance. Though called the 

 " solar constant," we find the sun's output of radiation really variable. 

 A growing interest is apparent among scientific men- and the public 

 generally in these observations. They are held by many to offer 

 promise of usefulness, in connection with weather reports, for the 

 study of the dependence of weather and chmates on the variations of 

 the solar radiation. 



As the variation of the sun is seldom large, there is great difficulty 

 in maintaining a sufficiently high standard of accuracy in the solar 

 measurements to give the magnitudes of the changes closely enough 

 for these purposes. It is true that our investigations have indicated 

 an extreme range of solar-constant values of lo per cent or even 

 more, but as Professor ]\Iarvin has pointed out, the apparent range 

 of results has somewhat diminished as we have improved our methods. 

 For several years the extreme range has not often exceeded 5 per 

 cent. The well-supported march of daily values frequently indicates 

 solar changes of i, 2, or 3 per cent, but larger changes than these are 

 so infrequent as to be looked upon as exceptional. One notable case 

 of a change of 6 per cent occurred in ]\Iarch, 1920, accompanying the 

 immense sun-spot group which appeared then. 



Situated as the observer must be, underneath an atmosphere loaded 

 with dust, water vapor, and other variable constituents, it would be 

 impossible to follow solar changes as small as i per cent unless the 



' With Monthly and Decade Cleans from 1918. 



' At present, Messrs. L. B. Aldrich and F. A. Greeley at Montezuma ; jMessrs. 

 A. F. Moore and H. B. Freeman at Harqua Hala ; Mr. F. E. Fowle, Miss M. A. 

 Neill and Mrs. A. Bond at Washington. During the interval, January, 1921, 

 to December, 1922, the Montezuma station was occupied by L. H. Abbot and 

 P. E. Greeley. 



Smithsonian Miscellaneous Collections, Vol. 77, No. 3 



