;138 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



thereon large black areas, blemishes on the face of a perfect, luminous 

 si)here. Today one may with fair frequency similarly observe with 

 the naked eye dark areas on the surface of the sun, provided the eye 

 is suitably protected by a piece of dark glass or an overexposed bit 

 of photographic film. These dark areas are the sunspots, imperma- 

 nent markings, subject to the variation in number previously out- 

 lined. They represent evidence of some deep-lying solar activity, 

 the exact nature of which astronomers do not yet know. 



Research has conclusively demonstrated that sunspots are huge 

 solar cyclones, whirlwinds in the solar atmosphere, accompanied by 

 pronounced magnetic conditions. Each sunspot, assumedly as the 

 result of the whirling of electrically charged particles within it, is 

 a huge magnet. Each such disturbed area of the solar atmosphere 

 may be simple or complex in appearance. One speaks of single 

 spots, the ultimate state of simplicity; of bipolar groups, whereby 

 is meant, in general, two neighboring disturbances, the one preced- 

 ing, the other following, along a solar parallel of latitude; and of 

 complex groups, of which the name is adequately descriptive. The 

 magnetic polarities of the individual centers of activity within an 

 area follow certain general rules that become of increasing complex- 

 ity of application as one proceeds from single spots to the complex 

 groups. 



Sunspots have been termed impermanent markings. Inasmuch as 

 they are of the nature of solar cyclones, they should be as imperma- 

 nent as their terrestrial counterpart. The average life of a dis- 

 turbance of this nature is 2 or 3 months. The longest-lived spot on 

 record is that of the years 1840 and 1841, which persisted for 18 

 months. The shortest-lived disturbance is usually of but a few 

 hours' duration. 



Such a variable phenomenon requires daily observation, even as 

 daily records are made of terrestrial weather. It is to a certain 

 extent the equivalent of keeping a daily account of the solar weather, 

 though there is no attempt at a detailed prediction of what the 

 morrow will bring. It is for this reason that one activity of the 

 Department of Astronomy at Harvard has been the daily photog- 

 raphy of the sun, when the weather permits, a procedure that has 

 been followed since the autumn of 1926. To the present over 1,800 

 photographs have been filed. 



Some attempts have been made to ascertain whether certain re- 

 gions of the solar surface represent weak areas wherein sunspots 

 are most likely to reoccur. The results are but poorly supported by 

 the direct evidence. Of one result, however, astronomers are com- 

 pletely certain. Sunspots occur only within certain limits of solar 

 latitude. Beyond the confines of the two belts defined on the ap- 



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