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 onthe 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 maximum 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 facule 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 19137), 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. 
