The Cape observatory had been founded in 1822 by the lords commissioners of the Admiralty for observational work for the special benefit of navigators. This ‘fundamental’ astronomy was the main, and almost the sole, work of the observatory when Gill was appointed. It was continued vigorously under his direction. Many thousands of stars visible at the Cape were catalogued, and the observations of one of his predecessors, Sir Thomas Maclear [q.v.], were reduced and published. Gill found a congenial task in detecting and eliminating the causes of error, especially those of systematic character, and in improving methods of observation generally. He early came to the conclusion that a new instrument was required, but it was many years before he was enabled to carry out his wishes. In 1897 the necessary expenditure was sanctioned, and his experience and engineering skill were brought to bear on the design of a transit circle which contained many new features and proved a great success.
With the assistance of W. L. Elkin, a young American astronomer, Gill next undertook the arduous task of measuring with the 4-inch heliometer (which he had now bought from Lord Lindsay) the distances of some of the brighter southern stars, including Canopus and Sirius. Measurements of great refinement continued night after night, both after sunset and before sunrise, and results of surprising accuracy were obtained with this small instrument. But a larger one was required, and in 1887 the Admiralty sanctioned the purchase of a heliometer with a 7-inch object glass from Messrs. Repsold. With this telescope Gill, assisted by W. de Sitter, a Dutch astronomer, measured the distances of no less than twenty-two stars with an accuracy which marked an era in the determination of stellar distances. This precision was due in part to Gill’s great personal skill as an observer, and in part to the admirable design of the new instrument. Gill took the first opportunity of using the new heliometer for a redetermination of the sun’s distance. He found that the small planet Iris would approach near the earth in 1888, and the small planets Victoria and Sappho in 1889. Very elaborate observations were planned, in which he had the co-operation of Arthur Auwers at the Cape, Elkin and Asaph Hall, junior, at Yale, B. Peter at Leipzig, W. Schur at Göttingen, and E. Hartwig at Bamberg. Subsidiary observations were also made at twenty observatories, and very elaborate calculations made at the office of the Berliner Jahrbuch. The result of this fine piece of work was the determination of the sun’s distance correct to one part in a thousand. The details of the observations and calculations are published in two volumes of the Cape observatory Annals. The determination of the mass of Jupiter was a third important research carried out with the 7-inch heliometer. Here again Gill had the assistance of the young astronomers W. de Sitter and Bryan Cookson, whose enthusiasm he had kindled.
The application of photography to astronomy made great strides at the close of the nineteenth century, and Gill was one of the pioneers. His success in photographing Finlay’s comet in 1882, with a Dallmeyer lens of 11 inches focus attached to an equatorial, convinced him of the practicability of constructing star maps by photography. He obtained a larger lens of 6 inches aperture and 54 inches focus, and in February 1885 commenced a photographic survey of the southern heavens. In December of the same year he received from Professor J. C. Kapteyn, of Groningen, an offer to undertake the measurement of the plates and the derivation from them of the positions and magnitudes of the stars. Methods were discussed between them, measurements begun in 1886, and completed in 1898. The results, published in the Annals of the Cape observatory, show the positions and magnitudes of no less than 400,000 stars. This great survey has proved of great value to astronomers, and formed the basis of important investigations on the distribution of the stars. Gill’s successful photography of Finlay’s comet also contributed to the adoption of photographic charting at the Paris observatory, where, thanks to the constructive skill of the brothers Henry, the administrative ability of Admiral Mouchez, the director, and the persistence and energy of Gill, an international scheme for cataloguing and charting the heavens by photography on a large scale was launched in 1887. Naturally the Cape observatory took a share in the work, and in addition the whole scheme was largely guided by Gill’s views.
Gill’s power of getting a comprehensive scheme carried through is well illustrated in the geodetic survey of South Africa. But for him the different states would probably have been content with small, local surveys. He outlined a system of principal triangulation for Cape Colony, Natal, the Orange Free State, and the
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