358 smithsonian miscelivaneous coi^lections vol. 52 



Solar Activity and Terrestrial Phenomena 



In the introductory part of this lecture reference was made to the 

 relationship between solar phenomena and terrestrial temperatures. 

 The fact that the temperature of our atmosphere- undergoes small 

 fluctuations which correspond with the Sun-spot period indicates 

 that the solar heat radiation varies with the number of Sun-spots. 

 Unfortunately, however, since the total area of Sun-spots is only 

 a very small fraction of that of the Sun's disk, and since intervals 

 of several weeks sometimes elapse during which no Sun-spots are 

 seen, the spot area may not prove to be the most reliable index of 

 the solar activity. The total area of the flocculi is always much 

 greater than that of the spots, and even at Sun-spot minimum these 

 objects are never entirely absent from the Sun. For this reason it 

 seems probable that measurements of their area will serve as the 

 best index to the state of the Sun and the surest means of detect- 

 ing rapid fluctuations in activity, which may be associated with 

 changes in the solar heat radiation or in terrestrial temperatures. 



The selection of the flocculi whose areas are to be measured is 

 necessarily a more or less arbitrary matter, depending upon the 

 judgment of the person engaged in the work. As will be seen 

 from Plate xxxii, the calcium flocculi range in size from extensive 

 regions covering a considerable area of the solar surface to minute 

 points barely discernible by the unaided eye on the original negatives. 

 Moreover, the range in brightness of the flocculi is almost as great 

 as the range in area. Evidently many of the fainter and smaller 

 flocculi must be excluded from consideration, especially as their 

 visibility depends upon the quality of the photographs, which dif- 

 fers from day to day with the conditions of the atmosphere. After 

 all has been said, however, the difficulties of selection appear to be 

 no greater than in the case of the facul?e measured on direct photo- 

 graphs at Greenwich. The faculre are clearly visible only in the 

 immediate neighborhood of the Sun's limb and gradually disappear 

 as they approach the center. Their total area, as measured on any 

 given photograph, is far less than the area of the calcium (Ho) 

 flocculi of the same date, and the eft'ect of atmospheric conditions 

 on their visibility is more marked than in the case of the flocculi. 



After experimenting with several methods of measuring the areas 

 of the flocculi, a simple photometric device was adopted. A piece 

 of clear glass is placed over the solar negative and the image of 

 each flocculus selected for measurement is painted over with opaque 



