Scottish astronomer. He pioneered the use of photography to catalogue stars and also made much use of a heliometer, determining the solar parallax and measuring the distances of 20 of the brighter and nearer southern stars.
In 1882 Gill realized it should be possible to chart and measure star positions by photography. He initiated a vast project, with the help of other observatories, to produce the Cape Durchmusterung, which gives the positions and brightness of more than 450,000 southern stars. Gill also served on the council for the International Astrographic Chart and Catalogue, which was to give precise positions for all stars to the 11th magnitude. It was not completed until 1961, although all the photographs had been taken by 1900. He also organized the large-scale surveys of southern Africa and initiated the triangulation of the meridian arc that stretches from South Africa to North Cape in Norway. He was made a KCB in 1900.
Gill was born in Aberdeen and studied there at Marischal College; he then went to Switzerland to learn clockmaking. A keen amateur astronomer, he gave up his inherited clockmaking business in 1872 to become director of Lord Lindsay's private observatory at Dun Echt, near Aberdeen, where he became a heliometer observer without equal. He was astronomer at the observatory at the Cape of Good Hope, South Africa 1879–1906. With Lindsay, Gill went on an expedition to Mauritius 1872–78 in order to measure the distance of the Sun and other related constants, particularly during the 1874 transit of Venus. He also went on an expedition to Ascension in 1877 during the very close approach of Mars. In both cases his object was to determine the solar parallax, a problem he tackled more thoroughly at the Cape of Good Hope.