186 AXNUAL REPORT SMITHSONIAN INSTITUTION, 1931 



aloiiii' the solar diaiiicler. For it seemed probable that if tliere are 

 changes in the intensity of sohir radiation, there must be associated 

 changes in the distribution of brightness along the diameter of the 

 solar disk. 



We first conducted these measurements with a horizontal telescope 

 of 140 feet focus at Washington in 190T. In 1913 we equipped a 

 tower telescope of 60 feet focus at Mount Wilson. From 1913 to 1920 

 Ave made drifts across the sun's disk in several wave lengths of radia- 

 tion on every day on which we conducted solar-constant observa- 

 tions. In this way we discovered indications of slight alterations in 

 the brightness distribution along the solar diameter. These changes 



Brightness Distribution Along Sun's DiAf-iETER 

 Fob Diffehknt Colors 



-i 





Figure 9. — Distribution of energy of radiation along diameter of solar disk 



seemed to be associated with the observed variations of the solar 

 constant. Thus in 1907 and in 1914, j-ears of numerous sunspots and 

 high solar radiation, there was greater contrast between the center 

 and edge of the solar disk than in 1913, a year of minimum sun spots 

 and low solar-constant values. The observed change of contrast was 

 greater for shorter wave lengths. Our determination of the distri- 

 bution of radiation over the sun's disk has been used in England, 

 Europe, and Japan as a check on which to test solar theories. 



SOI.AR VARIATION AND THE WEATHER 



In the year 1917, H. H. Clayton, then chief forecaster of Argentina, 

 wrote that he had found relations between our observed variation of 

 the sun and the weather of the world. He employed averages of 

 many Mount Wilson solar-constant values in his studies, thus largely- 

 eliminating atmospheric errors. We were so keenly impressed by 

 Clayton's results that we undertook to maintain daily solar-constant 

 work throughout the year. At the recommendation of Dr. Walter 

 Knoche, then in charge of the Chilean weather service, we located a 

 new solar-constant station at Calama, in the Atacama desert of 

 northern Chile. Later by the generous aid of John A. Roebling, we 

 i^moved this station to Mount Montezuma, at 9,000 feet elevation. 



