220 



ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1931 



For more than a century and a half records of the numbers of sun 

 spots have been kept and afford data for a study of their periodicity 

 over a range of about fifteen 11-year cycles. For more than a cen- 

 tury records of the variation in the earth's magnetism have been 

 made and preserved. The remarkable correlation of sun spots with 

 magnetic changes on the earth is at once apparent when we make a 

 graph of the number of sun spots and compare this with a similar 

 graph for changes of the compass needle (fig. 6). Simultane- 

 ously with the so-called magnetic storms, which are wont to sweep 



FiGUHB 6. 



-Graph showing correlation of sun-spot numbers to magnetic effects 

 on the earth 



the earth upon the appearance of great sun-spot activity, we witness 

 frequent and brilliant displays of the aurora borealis. 



The auroral light is due to an electronic discharge in the upper 

 and highly rarified atmosphere of the earth and is most probably 

 activated by charged particles of electricity emanating from the sun, 

 whose activity varies with the sun-spot cycle. It seems probable 

 that the magnetic vertical whirl of sun spots acts as a directing field 

 in guiding electrons escaping from the sun. When a conspicuous 

 spot appears near the center of the solar disk, and is therefore ap- 

 proximately in line with the earth and the sun's center, there is 



