After having studied the structure and the movement of the spots, one is naturally led to ask if their apparitions at different periods are subject to any general law. This question is one which has much engaged the attention of modern astronomers. The older observers noticed that the number of the spots visible differed in different years. There were said to have been periods when months and even years passed away without any spots being observed. Even allowing that this statement, so far as “years” are concerned, might be exaggerated, and that the absence of spots was due to the want of sufficient care in making the observations, and especially to the want of efficient instruments, it is none the less true that the number of the spots is extremely variable, and that there have been epochs when they were very scarce.
Sir W. Herschel was the first who devoted himself to the question of seeking to establish a relation between the variation of the spots and terrestrial meteorology. For the want of any better object, he compared the annual number of the spots with the price of wheat; but it is easy to see that nothing could result from such a comparison. Without doubt the meteorological phenomena of the globe must depend to some extent on solar changes: but the term of comparison selected by Herschel had no direct bearing on the state of the Sun.
In our time this question has been investigated to its very foundation by Wolf, Director for many years at the Observatory of Zurich. It is to his zeal that we owe a very interesting assemblage of old observations which were buried in archives and chronicles. It was he who endeavoured to reduce them into a systematic form, so as to supply as far as possible the numerous gaps which exist in the different series.
The two most attentive observers at the period when the spots were discovered were Marriott at Oxford and Scheiner at Ingoldstadt, but Scheiner himself has informed us that he did not note down all the spots which he saw; he only recorded those which were likely to assist him in his special task of determining the period of the Sun’s rotation. Several observers after him made isolated series of observations; but some of these have been lost and the others show important gaps. J. G. Staudacher, at Nuremburg, observed the Sun with great perseverance during fifty years from 1749 to 1799. Before him the Cassinis, Maraldi, and others were engaged in the same sort of work, but only in an indirect way: that is to say, they contented themselves, whilst making meridional observations of the Sun, with noting anything in the way of spots which they deemed important. Zucconi and Flaugergues also left behind them a good collection of observations which Wolf utilised, rendering them comparable one with another by applying suitable corrections. The great difficulty herein arises from the fact that the observers were not provided with instruments of equal power; one man, armed with a better telescope than his contemporaries, naturally observed and recorded spots which would escape the others. The numbers entered in their registers are therefore not comparable _inter se_. Wolf endeavoured to replace these numbers by others which would represent the spots which might have been seen if the observers had all employed telescopes of a given kind and power. The result of his efforts in this direction is an almost continuous series of Sun-spot records from an epoch sufficiently remote, up to the time when this branch of science was taken up with the vigour of modern scientific methods.