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 appearing 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 remarkable 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 faculæ and even by prominences which are often violently explosive, ejecting matter hundreds of thousands of miles from the sun's surface. Thus the sunspot maximum indicates increased activity at the surface of the sun, which, according to Abbot (1913 and l9132), is actually sending us increased heat radiation. During the maximum the magnetic condition 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 conditions 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.