$ 29 3 1 Planetary Observations : Sun-spots 385 



moon round the earth and of Japetus round Saturn 

 (chapter xn., 267) could be easily explained as the 

 result of tidal action at some past time when the planets 

 were to a great extent fluid- 



298. Telescopic study of the surface of the sun during 

 the century has resulted in an immense accumulation of 

 detailed knowledge of peculiarities of the various markings 

 on the surface. The most interesting results of a general 

 nature are connected with the distribution and periodicity 

 of sun-spots. The earliest telescopists had noticed that the 

 number of spots visible on the sun varied from time to time, 

 but no law of variation was established till 1851, when Hein- 

 rich Schwabe of Dessau (1789-1 8 75) published in Humboldt's 

 Cosmos the results of observations of sun-spots carried out 

 during the preceding quarter of a century, shewing that the 

 number of spots visible increased and decreased in a 

 tolerably regular way in a period of about ten years. 



Earlier records and later observations have confirmed 

 the general result, the period being now estimated as 

 slightly over n years on the average, though subject to 

 considerable fluctuations. A year later (1852) three inde- 

 pendent investigators, Sir Edward Sabine (1788-1883) in 

 England, Rudolf Wolf (1816-1893) and Alfred Gautier 

 (1793-1881) in Switzerland, called attention to the remark- 

 able similarity between the periodic variations of sun-spots 

 and of various magnetic disturbances on the earth. Not 

 only is the period the same, but it almost invariably happens 

 that when spots are most numerous on the sun magnetic 

 disturbances are most noticeable on the earth, and that 

 similarly the times of scarcity of the two sets of phenomena 

 coincide. This wholly unexpected and hitherto quite un- 

 explained relationship has been confirmed by the occurrence 

 on several occasions of decided magnetic disturbances 

 simultaneously with rapid changes on the surface of the sun. 



A long series of observations of the position of spots on 

 the sun undertaken by Richard Christopher Carrington 

 (1826-1875) led to the first clear recognition of the differ- 

 ence in the rate of rotation of the different parts of the 

 surface of the sun, the period of rotation being fixed (1859) 

 at about 25 days at the equator, and two and a half days 

 longer half-way between the equator and the poles ; while 



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