THE SUN AND THE ATMOSPHERE 



By Harlan T. Stetson 

 Massachusetts Institute of Technology 



[With three plates] 



The subject assigned me for this evening is peculiarly gratifying, 

 for there is a new emphasis being placed upon the importance of 

 science to human welfare. Certainly of the cosmic elements neces- 

 sary for our well-being, no two are more vital to us than the sun's 

 radiation and the atmosphere we breathe. 



As we all know, the earth's atmosphere consists of nearly one- 

 quarter oxygen, three-quarters nitrogen, a sprinkling of carbon 

 dioxide, with a bit of seasoning of the noble condiments of argon, 

 neon, crypton, xenon, and a trace of helium. Here at the earth's 

 surface, we can count on a little more than 1 percent of moisture in 

 the form of water vapor to prevent the complete desiccation of acad- 

 emicians and others. For a thorough mixing of these elements of 

 the atmosphere and the maintenance of its temperature as well as 

 the variations of its temperature, we rely upon the sun. Rarely 

 more vividly have we had impressed upon us the relationship of our 

 atmosphere to the sun as a part of our cosmic environment than 

 has been evidenced lately by some magnificent displays of the North- 

 ern Lights. Flaming gorgeously red, a hundred or more miles high, 

 like neon signs they advertise the lofty air, swarming with the traffic 

 of electrons, ions, and particles, all jostling one another as they are 

 stimulated by radiations from the sun during a period of great sun- 

 spot activity. 



If we look at the sun through a telescope, we see a gaseous globe 

 approximately one million times the size of the earth in volume with 

 a hot radiating surface of about 6,000° C. in temperature and fre- 

 quently, as on the day when this photograph (pi. 1) was taken, be- 

 sprinkled with dark patches that are called sunspots. From watching 

 the motion of these spots, we learn that the sun rotates about an axis. 

 This rotation is not uniform but is most rapid at the equator where 

 one rotation is completed in about 25 days. Halfway between the 

 equator and the poles, observations indicate 3 extra days are con- 

 sumed in a single rotation. 



' The seventh Arthur Lecture, under the auspices of the Smithsonian Institution, February 24, 1938. 



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