STUDYING SUN RAYS IN AFRICA, CHILE, AND 

 CALIFORNIA 



By C. G. abbot, 



Secretary of tlic SiiiitJisoiiiaii Institution, anil Director of the 

 Astropliysical Ol^ser^'atory 



On Mount Montezuma, in the Atacama Desert of Chile, at 9,000 

 feet elevation, and on Tal)le Mountain, California, at 7,500 feet, over- 

 Iookin,q- the Mojave Desert, the Smithsonian Astrophysical 01)serva- 

 tory has two stations for measuring- the intensity of sun rays on 

 which all life depends. A third station on Mount lirukkaros, within 

 a Hottentot reservation of South West Africa, is su])ported by 

 funds of the National Geograj^hic Society hut administered l)y the 

 Smithsonian. These three stations are coo|)erating to measure solar 

 radiation on every ]iossihle day of the year. The observations are 

 made and reduced in a manner to indicate the daily intensity of sun 

 rays as they are outside our atmosphere, and to reveal any changes 

 therein. 



If one should ask: Are the sun's rays growing weaker or stronger, 

 or are they fluctuating in a manner to produce ])redictahle changes in 

 weather, it would be impossible to answer because it is only recently 

 that records of solar heat measurements have been available. Not 

 until the Chilean observatory was founded by the Smithsonian in 

 1918 did daily measurements throughout the year Ijegin. It is hoped 

 to carry on the observations for a long term of years, so that mete- 

 orologists of the next generation can consult these basic measures, 

 and, comparing them with climate and weather all over the world, 

 perhaps be in position to forecast conditions with great advantage. 



Within the past year, several results of promise have appeared. 

 First, the march of the monthly mean values of solar radiation from 

 1918 to 1926, inclusive, shows evidence: (a) That increased solar 

 radiation attends increased sun-spot activity; (b) that a regular 

 periodic change of solar radiation of considerable amplitude occurs 

 in a period of 25^ months ; { c ) that less important periodicities of 

 15 and II months also appear. 



Second, the observations of Dr. Pettit of Mount Wilson Observa- 

 tory on the changes in the extreme ultra-violet spectrum of the sun. 



