Solar Activity and Its Terrestrial Effects^ 



By Sir Harold Spencer Jones 



Astronomer Royal of Great Britain 



[With 2 plates] 



In the year 1610 Galileo, using the then newly invented telescope, 

 discovered that there were spots on the sun. He observed that they 

 moved from east to west across the face of the sun and that they 

 changed their shape and size from day to day, so establishing that 

 they were not planetary bodies seen in projection upon the face of 

 the sun. 



For more than 200 years little more was learned about sunspots. 

 In 1826 Schwabe, an apothecary of Dessau, commenced the systematic 

 observations of the sun which he continued for nearly 50 years, making 

 a record of the spots seen in his telescope on each day that the sun 

 was visible. He soon noticed that the appearance of the spots was 

 not accidental. In some years there were very few days of observa- 

 tion on which no spots were to be seen ; in some other years there were 

 many. In 1843 he announced that there was a periodicity in the 

 appearance of the spots and this was fully confirmed by his further 

 observations. 



It may be remarked that the discovery of sunspots and their perio- 

 dicity might have been made before the invention of the telescope, 

 if the sun had been observed systematically when dimmed by haze 

 near the horizon. Large spots are easily visible to the naked eye 

 and a record of the appearance of naked-eye spots is adequate to reveal 

 the periodicity. 



The average length of the sunspot period is about 11.2 years, but 

 there is a considerable range in the actual length of the cycle, which 

 may be as short as 8 years or as long as 13 years. The height of the 

 maximum can vary widely from one cycle to another ; the maximum 

 of the last cycle was the highest for more than a century. The rise 

 in sunspot frequency fi-om minimum to maximum is usually more 

 rapid than the subsequent fall from maximum to minimum. The last 



* Twenty-second James Arthur lecture, given under the auspices of the Smith- 

 sonian Institution on April 27, 1955, 



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