396 ANNALS NEW YORK ACADEMY OF SCIENCES 



These observations were made along different diameters of the sun; 

 some were made on the equatorial diameter; some, at right angles to the 

 equator; and others, at various position angles. In all, nineteen sets of 

 observations were made which directly gave the difference between the 

 equatorial and polar diameters, and twenty-seven sets in which inclined 

 pairs of diameters were measured. From these, Auwers found that the 

 polar diameter exceeds the equatorial by 



P.-E. 



In the 19 sets +0''.026 



In the 27 sets +0".053 



And from the weighted mean of the whole series, he found that 



P.-E.= +0".038 ± 0".023. 



This apparent anomaly in the shape of the sun was explained by Auwers 

 as being due to the tendency on the part of an observer to measure vertical 

 diameters greater than horizontal diameters, and he concluded therefore 

 that the sun is sensibly a sphere. 



The mean diameter of the sun as determined from these observations 

 does not show any variation with the time. The observations were made 

 practically around the whole circumference of the solar disk, and mostly 

 fall into two great groups, the first containing those made in 1873-75, and 

 the second those made in 1880-85. There were two short series of obser- 

 vations made in 1876—77, not included in either group. The weighted 

 means of the two great groups are practically identical, the mean of the 

 second group exceeding that of the first by only 



-f0".04, 



a quantity less than half the probable error of the mean of the entire series 

 as given by Auwers 



± O^.IO. 



Observations of Schur and Ambronn. — In 1905 Ambronn pub- 

 lished, under the title "Die Messungen des Sonnen-durchmessers," ^ a 

 most exhaustive and valuable research upon the shape and size of the sun. 

 This paper embodies the results of the solar investigations of Schur and 

 Ambronn, made with the six-inch Repsold heliometer of the Gottingen 



1 Astronomische Mittheilungen der k. Sternwarte zu Gottingen, Theil 7, 1905. 



