386 ANNALS NEW YORK ACADEMY OF SCIENCES 



Part I. Historical. 



INTRODUCTORY NOTE. 



Questions as to the exact size and shape of the sun are of great astro- 

 nomical interest and have been made the subjects of many researches 

 during the last hundred years. Many of these investigations, involving 

 long series of observations, show a distinct ellipticity of the sun and indi- 

 cate a possible variation of its diameter, yet, after a thorough re-discus- 

 sion of these observations, Auwers concludes that all such indications are 

 illusory and that the sun is sensibly a sphere of constant diameter. As a 

 result of an independent discussion, Newcomb confirms this conclusion 

 of Auwers, and traces the supposed observed variations to changes in ter- 

 restrial temperature and to fluctuations in the haziness and cloudiness of 

 the earth's atmosphere. As these investigations and discussions appeared 

 in various journals and publications of scientific societies, it may not be 

 without value to preface the present investigation with a resume of the more 

 important papers, and to show exactly upon what grounds Auwers and 

 Newcomb base their conclusions. 



Measures of the sun's diameter which have heretofore been used are of 

 two classes; 1st, Those made with a meridian circle or transit instrument, 

 and, 2d, Those made with a heliometer. The papers and discussions re- 

 lating to these two classes will be taken up separately, although this will 

 interfere with the chronological order in which the papers actually ap- 

 peared, 



MERIDIAN OBSERVATIONS. 



Von Lindenau. — The first important investigation was that of Von 

 I^indenau, which appeared in Zach's "Monatliche Correspondenz" for 

 June, 1809. From observations made at Seeberg, in 1808-09, with a 

 transit instrument, he found a periodic variation in the sun's diameter. 

 In order to test this suspected variation, he discussed the Greenwich me- 

 ridian observations made in the years 1750-55 and 1765-86. These ob- 

 servations apparently confirmed the results obtained from those made at 

 Seeberg, and Lindenau concluded that the sun is an ellipsoid, rotating 

 about its longer axis. His calculations made the polar radius exceed the 

 equatorial by from 4" to 6", or, what is the same thing, he found an equa- 

 torial compression of ~r to ~-^. These results, however, were criticised 



