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ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



manifests itself in familiar light and heat but in a wide variety of 

 electromagnetic disturbances, whose far-reaching effects we are just 

 beginning to appreciate. 



Studies during the last few years indicate that there are cosmic 

 causes at work which may profoundly influence the electrical state 

 of the earth's atmosphere which radio waves traverse. Probably the 

 sun is the one astronomical body most responsible for changes in 

 our terrestrial affairs. Every radio listener knows that daytime re- 

 ception is vastly poorer than night-time reception in the broadcasting 

 zone. Here is the most obvious exhibition of the effect of the sun's 

 rays upon radio. On the other hand, both day and night reception 

 vary greatly from time to time for what has often seemed no good 

 reason at all. It is from relatively very recent researches that we 

 have come to believe much of the cause for this varying degree of 



reception is to be found in 

 the sun's atmosphere itself. 

 When we examine the 

 sun's surface through the 

 telescope, we find that it 

 presents a strange mottled, 

 or granulated appearance. 

 In this mottled surface 

 there develop now and then 

 dark patches, often grow- 

 ing into huge black areas 

 surrounded by a somewhat 

 shaded region called the 

 penumbra. These dark 

 areas are the sun spots. Whatever may be ultimately accepted as the 

 best explanation of the si)ots, one can not go far wrong in picturing 

 a sun spot as a terrific storm in the sun's atmosphere, a cyclonic 

 whirlwind for which the most violent tropical hurricane would be a 

 microscopic illustration. 



One of the most extraordinary features of sun spots is the perio- 

 dicity with which they appear on the solar surface. For nearly a cen- 

 tury and a half sufficiently accurate records of the appearance of 

 sun spots have been made, so that if we plot the degree of spottedness 

 of the solar surface year by yeai-, we discover a periodic rise and fall 

 in the stormy condition of tlie sun's surface spanning approximately 

 11 years. We are now not far from Avhat we call a sun-spot maxi- 

 mum. About eight years ago sun spots were very scarce, and when 

 they occasionally appeared were very small and insignificant affairs. 

 Curiously enough, at the beginning of a sun-spot cycle the spots 

 appear on the sun's surface at relatively high latitudes and as the 



FiGuuB 1. — The sun-spot curve, according to Wolf's 

 sun-spot numbers. (From data by Wolfer) 



