Annual Address to the Members. Ixiii 



The new heliometer has also been appHed in extensive operations 

 for determining the great fundamental unit of astronomy — the Solar 

 Parallax. 



In the presidential address which I had the honour to deliver on 

 the 30th of July, 1880, I dealt with the various methods available for 

 this determination, and that which was indicated as in my opinion 

 the best was the observation of minor planets by means of the 

 heliometer. In the year 1888 there was a very favourable opposition 

 of the minor planet Iris, and in 1889 very favourable oppositions of 

 Victoria and Sappho ; and the heliometer observers at Yale College 

 (New Haven, U.S.), Gottingen, Leipzig, Bamburg and Oxford 

 promised co-operation. 



Plans of observation were prepared and circulated, with the result 

 that all the above-named heliometer observatories co-operated. Dr. 

 Auwers, of BerliD, came to the Cape in 1889 and assisted in the 

 observation of Victoria, and 22 meridian observatories shared in the 

 determination of the places of the comparison stars. The year 1890 

 was devoted to the heliometer triangulation of the comparison stars. 



Dr. Auwers discussed the 9620 meridian observations of the 

 comparison stars, and the 760 meridian observations of the planets • 

 Dr. Elkin discussed the heliometer observations of Iris for parallax, 

 those of Victoria and Sappho for the same object were discussed and 

 the whole combined by myself. 



The very exact places of the comparison stars obtained by a 

 combination of the meridian and heliometer observations, enabled me 

 also to derive a very valuable determination of the mass of the moon 

 from the observations of Victoria. The details of the whole are 

 contained in Vols. vi. and vii. of the Annals of the Cape 

 Observatory, and the results are : — 



"Value of the solar parallax from observations of — Probable 



error. 



Victoria 8"-8013 + 0"-0061 



Sappho -7981+ -0114 • 



Iris -8120+ -0090 



The finally adopted results were : — - 



Value of the solar parallax 8"-802, probable error + 0"*005 

 Eatio of the mass of the moon to that of the earth 1 : 81*702 + 0-094 



and these results have been adopted for use in the national ephemeris 

 by the international committee which met at Paris in 1896. 



