PERIODIC SOLAR VARIATION 



By C. G. abbot 



Research Associate, Smithsonian Institution 



Many stars are variable, some intrinsically, others by reason of 

 bodies interposing between them and us, as with the eclipsing vari- 

 ables. Our star, the sun, is rarely eclipsed. Astronomers question 

 whether the sun's intrinsic variations are perceptible. I propose to 

 offer observational evidence. 



The great obstacle to such an investigation of our star, the sun, 

 is the earth's atmosphere. It offers no such difficulty in measuring 

 variations of the other stars, for the star to be investigated may be 

 compared with many neighboring stars, whose rays suffer nearly the 

 same atmospheric obstruction. Daylight sun drowns out all its neigh- 

 boring stars. 



The planets, which all shine by reflected sunlight, might indeed be 

 compared with neighboring stars. But they are all too imperfect mir- 

 rors to yield highly accurate results on the sun's variation. We are 

 therefore forced to make absolute determinations of intensity of the 

 sun's rays, day after day, with proper allowances for the losses they 

 suffer in passing through the atmosphere. Many scientists think it is 

 impossible thus to achieve sufficient accuracy to determine the sun's 

 small percentage variation. 



For many years the Smithsonian Astrophysical Observatory has 

 carried on measurements of the intensity of the sun's rays as they 

 would be found at mean solar distance outside our atmosphere. This 

 is called "the solar constant of radiation." The methods and results 

 are described in detail in volumes 2 to 7 of the Annals of the Astro- 

 physical Observatory. We shall not dwell on the methods here. The 

 observations have been made daily, as far as possible, from mountains 

 6 to 10 thousand feet in elevation, in arid lands, from the year 1918 

 to the present time. Our stations have been in both hemispheres. 

 Careful comparisons of the measures show no appreciable systematic 

 differences between them, owing to the southern winter occurring in 

 northern summertime. Automatically recorded measurements from 

 balloons at 15 miles elevation show that the solar-constant values are 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. 128, NO. 4 



