Sir John Herschel, Sir David Gill 167 



astronomy, he became acquainted with, and ultimately 

 engaged as private assistant by, Lord Lindsay, an enthusiastic 

 amateur astronomer, then about to erect a private observatory 

 at Dunecht. He accompanied Lord Lindsay in his expedition 

 to Mauritius, undertaken for the purpose of observing the 

 transit of Venus in 1874. This rare event, as previously 

 explained in connexion with its first observation by J. 

 Horrocks, serves to determine the distance between the 

 earth and the sun, but alternative methods promising more 

 accurate results had already been suggested. The relative 

 distances of the different planets from the sun being known 

 by their times of revolution, we may substitute the measure- 

 ment of the distance of any one planet which is in a suitable 

 position for the direct determination of the solar distance. 

 Certain planets occasionally approach the earth sufficiently 

 near to apply this method. As the earth turns round its 

 axis, the observer's point of view is sufficiently altered 

 between a morning and evening observation to show a 

 measurable shift in the position of a planet as compared 

 with that of the surrounding stars. While at Mauritius 

 Gill found that one of the minor planets, Juno, happened 

 to be suitably placed to test the method, and he obtained 

 most encouraging results. A good opportunity of pursuing 

 the investigation presented itself in 1877, when the situ- 

 ation of the planet Mars was exceptionally favourable for 

 the purpose. Gill left the service of Lord Lindsay and 

 established himself on the island of Ascension. Though the 

 results obtained were good, Gill confirmed his conclusion 

 that the minor planets were better suited for accurate 

 measurements. He returned to the subject ten years later, 

 and a combination of observations of three minor planets, 

 made partly by Gill at the Cape and partly by other astronomers 

 whom he had interested in the work, has given us the best 

 determination of the solar parallax we possess. 



In 1879 Gill was appointed Astronomer Royal at the 

 Cape, and he directed the work of the observatory with 

 distinguished success until 1906. Unbounded perseverance, 

 unrivalled skill in observing, and an exceptional mechanical 

 knowledge which served him in the design of instruments 

 were combined in his person to a rare degree. A favourite 



