POOR, THE FIGURE OF THE SUN 397 



Observatory during a period of neariy thirteen years, from 1890 to 1902, 

 and it furnishes the best and most precise series of measures of the sun's 

 diameter which have yet been made. 



When the Repsold heliometer was mounted in Gottingen, Schur de- 

 termined to investigate this subject thoroughly, and to make with that 

 instrument a complete and uniform series of measures which should extend 

 over the whole of a sun-spot period. In carrying out this programme, 

 every conceivable precaution was taken to exclude systematic errors: in 

 fact, two complete, parallel and independent series of observations were 

 made, one by Schur and one by Ambronn. Whenever possible, each 

 observer obtained a series of four measures each week, two of the polar and 

 two of the equatorial diameter. All the necessary instrumental constants 

 for the reduction of these observations were obtained by each observer 

 independently of the other. But the same methods and the same formulas 

 of reduction were used in the two series; so that these series are directly 

 comparable. The series of Schur extends from 1890 to the beginning of 

 1901; that of Ambronn, from 1890 to the end of 1902; both series thus 

 covering an entire sun-spot period. 



In reducing and discussing this great number of observations, Ambronn 

 investigates the questions of the figure and of the variability of the sun 

 separately. A brief resume of his methods of investigating each of these 

 points is given below, together with the conclusions he reaches in regard 

 to these important questions. 



1. The Figure of the Sun. — On each day of observation the polar and 

 equatorial diameters were both measured t\Nnce, so that the research fur- 

 nishes a great mass of data regarding the shape of the sun. The values of 

 the differences between the diameters, in the sense polar minus equatorial, 

 are tabulated and given by Ambronn. From these are formed the mean 

 values of this difference for each year; and from these yearly means, the 

 mean value for the entire series of observations. 



The yearly means as given by Ambronn in Appendix IV, and also on 

 page 44 of his memoir, contain typographical errors which were later 

 corrected by the author.^ The corrected table is as follows : 



1 Astronomische Nachrichten, Vol. CLXXI, No. 4086. May, 1906. 



