10 



o. A. Åkesson 



Up to the year 1863, when Cabrington published his. classic work, the prevail- 

 ing idea was that the sun rotated as a rigid body, thus with the same angular 

 velocity at all latitudes. It is true, Scheiner and also some other investigators 

 after him had observed that spots at different heliographic latitudes moved with 

 different velocities, but it was Carrington that first succeeded in proving that the 

 sun's angular velocity diminished as the latitude increased. This fact Carrington 

 expresses in the following formula, where 4 is the daily rate of rotation, and ß the 

 solar latitude: 



4 = 865'— 165'sin^ß. 



Spöreb S from his first observations at Anclam 1861 — 1871, which have ever 

 since been constantly continued, has verified the result of Carrington. After 

 trying different expressions for the rotation of the sun, he accepts the following, 

 which he found to represent the observations in the best way: 



i = 512'. y + 347'. 9 cos ß. 



In an interesting paper of 1871, Zöllner ^, from certain theoretical assumptions, 

 comes to a law of rotation of the form of 



. 71/— iVsin-^ß 

 cos ß 



From the observations of Carrington, he gets 



^ _ 863'. 4 — 620' sin^ß 

 cos ß 



An extrapolation of these formulœ, outside the dominion of the observations, is 

 naturally not allowed. 



The most complete examination of the solar rotation, deduced from the motions 

 of the spots, is that made by Mr and Mrs Maunder * in 1905. As the material 

 employed by the Maundees is also the basis of the researches of the present paper, 

 I shall in the next chapter give a more complete account of these observations. In 

 the determination of the rotation-period, all such spots or groups of spots are excluded 

 as have not been observed for at least six consecutive days, whereas the mean of the 

 positions of the group on the first three days was taken as the first position of the 

 group; the mean of the positions on the last three days as the second position. From 

 these two positions the daily drift in longitude, referred to a system of co-ordinates, 

 rotating in 25,38 days, was computed. From the drifts of the spots, referred to this 

 system of co-ordinates, the mean rotation-period at different latitudes was obtained. 



' G. Sporer: Beobachtungen der Sonnenfleckeii zu Anclam. Publikationen der Astronomischen 

 Gesellschaft XIII. 1874. 



^ F- Zöllner: Über das Rotations-Gesetz der Sonne und der grossen Planeten. Astronomische 

 Nachrichten 1872. N:o 1849-1852. 



' W. Maunder and A. S. D. Maunder: The solar rotation period from Greenwich Sunspot 

 Measures 1879—1901. Monthly Notices LXV. 



