CORRELATION WITH SUNSPOTS. 81 



THE SUNSPOTS AND THEIR POSSIBLE CAUSES. 



If the sunspots are an index of some solar activity so far reaching as 

 to affect our climate and vegetation, it is well to note very briefly 

 their appearance and the suggested causes of their periodic character. 



Appearance At first view sunspots are small black areas appear- 

 ing from time to time on the sun. In actual size they vary from a few 

 hundred miles in diameter to more than a hundred thousand. Rarely 

 seen by the naked eye, the vast majority are only discovered through 

 the telescope; hence it was only after the invention of that instrument 

 that records of them were kept and their nature investigated. As Hale 

 (1908) has found, they are cooling places; they merely look black by 

 contrast with their more intensely bright background. His remark- 

 able photographs show that they often have a rotation about their own 

 center. They usually come in groups between latitude 5 and 25 in 

 each hemisphere of the sun and are almost continuously changing in 

 small details. Their life is usually less than one rotation of the sun. 



Schwabe in 1851 announced their periodic character with maxima 

 every 11 years. During sunspot rnaximum a small telescope will show 

 5 to 20 spots, but during the minimum one may search for weeks 

 without finding a spot that can be certainly recognized. Records of 

 the numbers of spots were specially collected by Wolf for many years 

 and later by Wolfer of Zurich. At the present day many observatories 

 are taking daily photographs of them. The term relative sunspot 

 number was invented to convey an idea of the average number of spots 

 visible at any one time under favorable circumstances. The number 

 actually counted receives a simple correction for unfavorable weather 

 or small telescope, so that the published numbers shall be as nearly 

 standard as possible. 



While the spot appears black and may possibly be sinking into the 

 sun, it is usually attended by intensely bright areas or faculse and even 

 by prominences which are often violently explosive, ejecting matter 

 hundreds of thousands of miles from the sun's surface. Thus the sun- 

 spot maximum indicates increased activity at the surface of the sun, 

 which, according to Abbot (1913 and 1913 2 ), is actually sending us 

 increased heat radiation. During the maximum the magnetic condi- 

 tion of the earth is profoundly affected, as evidenced by northern 

 lights, magnetic storms, earth currents, and variations of the earth's 

 magnetic constants. This relation to the earth's magnetism has been 

 recognized from the first discovery of the periodicity of sunspots. But 

 the effect of the change of solar radiation on climate and ordinary 

 weather elements is more obscure. General effects on climatic con- 

 ditions have been admitted as probable by Penck (1914), but in general 

 the great weight of opinion has been against a traceable effect of solar 

 activity on weather or climate. 



