152 ASTRONOMY FOR 1889, 1890. 



SUN. 



Rotation of the sun. — ^Ir. Crew, whose observations of the rotation 

 of the sun were noted in a previous summary, has made a new se- 

 ries of observations for the correction or confirmation of his concki- 

 sion that the angular velocity of rotation increased with an increase 

 of latitude. He still finds shorter rotation periods for the higher lati- 

 tudes, the mean value for the period at latitude 45° being 18 hours 

 shorter than at the equator, though owing to the smallness of this 

 amount and the uncertainty of the observations he is of theopinion that 

 " no certain variation of period with latitude has been detected with 

 the spectroscope." Attention is called ]u)wever to the wide differences 

 of the equatorial period as obtained by different methods, differences 

 which may be due to the fact that we are really dealing with different 

 strata of the sun, though here also much reliance must not be placed 

 upon the observations. 



Spectroscopic observations made by Duner for determining the rota- 

 tion time of the sun, confirm the slowing down of the time of rotation 

 with an increase of heliocentric latititude, quite contrary to the result 

 recently obtained by Wilsing. A i)eriod of 25.46 days is deduced lor 

 the sidereal rotation at the equator, and 38.54 days for that at latitude 

 74.SO. 



Diameter of the sun. — Dr. Auwers discusses, in the third memoir on the 

 diameter of the sun, communicated to the Berlin Academy', the observa- 

 tions at Greenwich by IMaskelyne and his assistants from 17G5 to 1810. 

 Curious differences of personal equation between different observers are 

 brought out. Instead of Maskelyne's observations giving progressively 

 smaller values of the sun's diameter during his whole observing life, as 

 has hitherto been supposed. Dr. Auwers's very exhaustive discussion 

 indicates that after the first two years (which gave a very large value) 

 the observed diameter remained nearly constant for the period 1767- 

 1772, then during the years 1772-1790 the diameter was continually 

 decreasing, lastly from 1790-1810 the observations gave a diameter con- 

 tinually increasing. The minimum value in 1790 was 31' 5S'Mo — about 

 1" smaller than the value obtained from modern heliometer measures. 



jS2ioerer\s researches on. .v»w spots.— Professor Spoerer, who has devoted 

 much attention not only to the cuirent state of the solar activity, but 

 also to the early records of sun spots, published early in 1889 two im- 

 portant papers on the results of his researches in the latter field. The 

 two papers are entitled respectively, Ueber die Periodicitlit der ISonncn- 

 Jfecken seit dem Jahre 1618, communicated to the Eoyal Leopold-Caro- 

 line Academy, and Sur les dijferene^'s que presentent Vhemisphere nord et 

 Vhemisphere sud du Soleil, appearing in the number of Bulletin Astrono- 

 mifjue for February, 1889. The conclusions arrived at in these two 

 papers may be summarized under the three following heads : 



First: These earlier observations afford us many examples of the 

 operation of the "law of zones;" that is to say, a little before a mini- 



