Introduction. 



Historical Summary. 



In this section I shall give a short account of the researches into the periodicity, 

 motion and distribution of the sun-spots. Consequently spectral-analytical investiga- 

 tions will not be discussed, however important and decisive these researches have 

 been for our ideas of the solar phenomena. In connection with the investigations 

 of Wolf T give the result of some numerical computations, which I have made 

 in order to determine the relation between the relative numbers and the sun-spot area. 



A study of the motion etc. of the sun spots was not possible until the 

 discovery of the telescope in the year 1608, though sun-spots had been observed 

 at different times long before ^. From the observations of Galilei, Scheiner 

 and Fabricius it soon became evident that the spots were formations on the sun, 

 and tliat their motion could be easily explained by the rotation of tlie sun about 

 an axis, inclining to that of the echptic. Scheiner found this inclination to 

 be 7 Vs", the longitude of the ascending node of the sun's equator on the ecliptic 

 6Û V2°, and the rotation period 25 days. These values deviate very little from values 

 found later. 



The Periodicity of the Sun-spots. On the whole, no systematic observa- 

 tions of the sun-spots were made during the seventeentli and eighteenth centuries. 

 The observations that were made, however, showed that the number of sun-spots 

 was subject to great variations. It was the Danish astronomer Christian Horrebow 

 who first noticed that these variations are periodic. Of his sun-spot counts, only 

 those for the years 1761 and 1764 — 1777 remain, although they were start- 

 ed as early as 1738, part of the material he had collected being destroyed during 

 the bombardment of Copenhagen in 1807 \ These observations of Horrebow being 

 of some interest, I borrow Table I from the paper of Thiele. In the diary of 

 Horrebow from 1775 — 1776 we find the following remark: »Obwohl sich aus den Beob- 



' J. Williams: Chinese observations of solar spots from A. D. 301 to 1205, extracted from 

 the encyclopaedia of Ma Twan Lin. Monthly Notices XXXIII. 



- Th. N. Thiele: De macularum Solio antiquioribus quibusdam observationibus Hafniee 

 institutis. Astrononnsche Nachrichten N:o 1193. 



