PAST CHANGES IN SUN-SPOTS 553 



THE SCARCITY OF EARLY SUN-SPOT DATA 



As is already clear to the reader, I am strongly inclined to believe that 

 Pettersson has discovered a genuine connection between sun-spots and the 

 peculiar climatic conditions of Europe in the fourteenth century. Never- 

 theless it is important that we should clearly recognize the difficulties in 

 the way of any exact knowledge as to the sun-spots of past times. At a 

 period as remote as the fourteenth century the difficulty is great. When 

 still earlier periods are considered, the difficulty increases in geometrical 

 ratio ; so that we can scarcely hope ever to have any exact knowledge 

 based on actual observation. Previous to the invention of the telescope 

 it was almost impossible to form any accurate idea of the activity of the 

 sun's surface. Occasionally spots of unusual size might be visible to the 

 naked eye. This, however, in itself is not a proof that the sun possessed 

 an unusual degree of spottedness. Often a large spot occurs at a time 

 when the general surface shows a minimum amount of disturbance. An- 

 other reason for laying little weight on the sun-spot records previous to 

 1600 is that there was no one whose duty it was to record the occurrence 

 of spots. The fact that a certain observer who was interested in the mat- 

 ter recorded a large number may lead us to think that his particular 

 period was marked by intense solar activity. This may merely mean, 

 however, that at other periods of much greater activity there happened 

 to be no one who was interested enough to record the matter.*^ 



MAJOR SUN-SPOT CYCLES OF THE PRESENT TIME 



In view of the difficulty of obtaining exact knowledge of sun-spots more 

 than two or three hundred years ago, let us examine the matter in another 

 way. Let us look at the sun-spot curve since 1749, the date when accu- 

 rate figures first become available, and let .us see what it suggests as to the 

 laws of variability. The curve is given in figure 18, where the sun-spot 

 numbers for each year are plotted according to Wolfer's tables. Dotted 

 lines have been added connecting the various maxima and also the min- 

 ima. So much is said about the 11-year period of the solar spots that we 

 are apt to think that this is their most important variation. Next after 

 this in importance in our ordinary thought of the matter is the 35-year 

 cycle of Bruckner. A glance at the curve before us, however, shows that 

 a still larger cycle is quite as important. Beginning with 1750, we see 

 that during the first year of maxima the sun-spot number was about 83.4. 



*^ Our knowledge of the whole matter is largely due to Wolf. His observations, to- 

 gether with later information as to both sun-spots and the related phenomena of polar 

 lights, are summed up by H. Fritz in the Zurich Viertel.iahrschrift, vol. 38, 1893, pp. 

 77-107. An abstract of this article is given in Annalen der Physik und Chemie, Beib- 

 latter, v. 17, 1893, p. 930. 



