NO. 7 ASTROPHYSICAL OBSERVATORY, I904-I953 — ABBOT 



PUBLICATIONS 



Quarto volumes 2-6 of the Annals, prepared by C. G. Abbot, F. E. 

 Fowle, and L. B. Aldrich, and volume 7, prepared by L. B. Aldrich 

 and W. H. Hoover, were published 1908-1953. About 150 papers, 

 mostly published in Smithsonian Miscellaneous Collections, were 

 written by members of the staff.^ 



THEORETICAL FINDINGS 



Langley showed about 1880 that, to determine the solar constant, 

 the spectral transmission of the atmosphere must be measured. This 

 was always done by us with a clock-operated recording spectro- 

 bolometer in Langley's Method (now often called the "long method"). 

 Langley did not publish a complete theory of this method, and he 

 used it erroneously in publishing his Mount Whitney Expedition of 

 1881. This error gave his preferred solar constant value 3.07 instead 

 of 2.0 calories. The full theoretical demonstration, and examples of 

 the correct application of Langley's method are published in Annals, 

 volume 2. 



Although Langley's method is fundamental, it requires several 

 hours of observing while the sun's air mass or atmospheric path 

 changes from (say) 3.5 to 1.5 times that for vertical sun. During this 

 considerable time the transparency of the atmosphere always changes. 

 If it grows clearer for small air masses the resulting solar constant 

 is too large, and vice versa. Hence only large groups of solar constant 

 measures by Langley's method can be trusted ; these give mean values 

 nearly correctly. 



A brief method for measuring spectral transparency was required, 

 so that several values of the solar constant could be obtained daily, 

 with little atmospheric variation affecting any one of them. Then 

 the day's mean value would be good. 



From 1920 to 1924 the A.P.O. developed the "short method." This 

 is empirical, depending on the fact that the brightness of the sky near 

 the sun is greater as the sky becomes more hazy. It is found pos- 

 sible to draw curves suited at all times to yield transmission coefficients 

 suited to each station occupied for all of the 40 wavelengths used in 

 Langley's method. A "short method" measurement requires only 

 one reading of the pyranometer of sky-brightness near the sun. So 

 in the 10 minutes employed for making a spectrobolometric graph of 



1 1 will use the abbreviations "A.P.O." for Astrophysical Observatory, and 

 "Pub." for publication. 



