rKKSIIlENTIAL ADDRESS SECTION A. 35 



EECEXT PRUGEESS IN ASTROXOMY. 



By H. E. Wood, M.Sc, F.R.A.S., F.R.Met.S. 



Union Ohservatorjj, Jolianneshur-fi . 



Presidential Address to Section A, delivered Juhj 14, 1920. 



I should like to commence my address by referring to 

 tlie fact that the present year marks a definite epoch in 

 the history of astronomy in South Africa. It is exactly 

 one hmidred years ago since the foundation of the Royal 

 Observatory at the Cape of Good Hope. The Commissioners 

 appointed by Act of Parliament " for more effectually 

 discovering the longitude at sea " first discussed the question 

 of the establishment of an Observatory at the Cape on 

 February 3, 1820. There was then no jiermanent astronomical 

 Observatory at the Cape, although the Abbe Lacaille had 

 spent two years at Capetown (1751-1753) and had made a 

 catalogue of southern stars. A bronze tablet erected by the 

 South African Philosophical Society — now the Royal Society 

 of South Africa — marks the site of Lacaille 's Observatory 

 in Strand Street. 



On October 20, 1820, by an Order of His Supreme Majesty 

 in Council, the Royal Observatory at the Cape was definitely 

 established, and the first of His Majesty's Astronomers at 

 the Cape — the Rev. Fearon Fallows — was appointed on 

 October 26, 1820. There was considerable delay in erecting 

 the necessary buildings, and it was not until the beg'inning' 

 of 1829 that the Royal Observatory was an accomplished 

 fact. Since its inception the aim of this institution has been 

 constant — to be the standard Observatory of the Southern 

 Hemisphere, and to be for that hemisphere what Greenwich 

 Observatory is for the Northern Hemisphere. 



The state of South Africa one hundred years ago was 

 very different from its state at the present time ; equally so 

 is there a very wide diff'erence between the state of astronomy 

 then and now. The distance of not a single star was known 

 then, and it stands to the credit of the second of His Majesty's 

 Astronomers at the Cape, Thomas Henderson, that he was 

 the first to measure successfully the distance of a star — the 

 star Alpha Centauri — although, partly on account of the 

 remoteness of the Cape from Europe in those days, he was 

 not the first to publish the distance of a star, being slightly 

 anticipated by the European astronomers Bessel and Struve. 

 At the present time the distances of many stars are known. 

 It was the chief work of Sir David Gill to measure the 

 distance of the Sun, which is the unit distance to which the 

 distances of the stars are referred, and also to measure 

 the distances of the brighter southern stars. 



The great practical problem of astronomy is the measure- 

 ment of the distance of the stars. A knowledge of stellar 

 distances is required in all investigations into the structure 



