# Dictionary of National Biography, 1927 supplement/Gill, David

​GILL, Sir DAVID (1843–1914), astronomer, the eldest son of David Gill, a watchmaker with a well-established business in Aberdeen, by his wife, Margaret Mitchell, was born at 48 Skene Terrace, Aberdeen, 12 June 1843. His interest in physical science, first aroused by the teaching of Dr. Lindsay at the Dollar Academy, was later extended and intensified at Marischal College and the University, Aberdeen, under the inspiring influence of Clerk Maxwell [q.v.]. At his father’s desire he entered the business and for a time had complete charge of it. He mastered all the details, and to the end of his life kept a clock made with his own hands. His spare time, however, he devoted to physics and chemistry in a small laboratory which he set up at home.
Gill’s active interest in astronomy dates from 1863. It occurred to him that a time service similar to that established by Charles Piazzi Smyth [q.v.] in Edinburgh might be usefully installed in Aberdeen. An introduction to Piazzi Smyth followed, when he was shown the arrangements for the time-gun and ball as well ​as the instruments of the Edinburgh observatory. With the help of Professor David Thomson [q.v.] he re-established a disused ‘observatory’ of King’s College, Aberdeen. A small portable transit instrument was unearthed and mounted; a mean solar clock was added, with arrangements for control to within a fraction of a second of Greenwich time; to this contact springs were fitted, establishing electric control of the turret clock of King’s College and other clocks of the town. Gill next procured a small Dallmeyer refracting telescope with which he made observations of double stars. He also purchased from the Rev. Henry Cooper-Key a 12-inch speculum, the equatorial mounting for which was of his own design, and the driving-clock made by his own hands on the general plan of Airy’s chronograph at Greenwich. He said, in later life, that he never found a clock which worked better. With this instrument he observed double stars and nebulae and took some photographs of the moon.
In 1870 Gill married Isobel, daughter of John Black, a farmer of Linhead, Aberdeenshire, and shortly afterwards the opportunity came to him of devoting his time exclusively to scientific pursuits. This arose out of the friendship which he had formed with Lord Lindsay (afterwards twenty-sixth Earl of Crawford, q.v.), an amateur astronomer who was attracted by Gill’s enthusiasm and skill. Gill was offered in 1872 the charge of the private observatory erected by Lord Lindsay at Dunecht. He accepted at once, although it involved a considerable sacrifice of income. The years 1873–1874 were busily occupied in the equipment of this fine observatory. The instruments comprised a 15-inch refractor by Grubb, an 8-inch reversible transit circle by Troughton and Simms, Gill’s 12-inch speculum from Aberdeen, and a 4-inch heliometer by Repsold. Visits to many European observatories were made and Gill met the leading astronomers of the day. Preparations were next made for Lindsay and Gill to take part in the observations of the transit of Venus in 1874. It was thought desirable that the longitude of their station at Mauritius should be determined with all possible accuracy. As the electric telegraph only went as far as Aden, it was necessary to transport chronometers, of which no less than fifty were hired by Gill from the best makers. These had to be carried with great care, regularly wound, and compared. Gill took his chronometers to Greenwich, checked them, and then drove off with them cheerfully to his steamer in the docks, leaving Airy and the staff at Greenwich amazed at his temerity. By incessant watchfulness, however, the journey to Mauritius and back was made without mishap. The main interest in the expedition to Mauritius centres round the observations of the minor planet Juno, made with the heliometer for the determination of the solar parallax and thus of the sun’s distance from the earth. From observations on twelve evenings and eleven mornings a very good result was obtained by Gill, who was profoundly convinced of the possibilities of the heliometer for astronomical measurements of the highest precision. As is well known, the observations of the transit of Venus, carefully planned and executed at great expense of time and money by expeditions all over the world, gave a disappointing result. But the Mauritius expedition proved to be the inauguration of a successful method of determining the sun’s distance, a measurement of fundamental importance in astronomy.
While at Mauritius Gill was invited by General Stone, chief of the military staff of the khedive, to return via Egypt and measure a base-line for a projected survey of the country. With the assistance of the American astronomer, Professor James Watson, a base-line was laid down near the Sphinx. This was Gill’s first practical experience of geodetic work.
Gill left Dunecht in 1876. He obtained from Lord Lindsay the loan of the heliometer, and with funds obtained from the Royal Society and the Royal Astronomical Society made an expedition to the island of Ascension in order to measure the distance of Mars when it came exceptionally near the earth in the year 1877, and from the result to derive the sun’s distance. In this difficult and adventurous expedition he was accompanied by his wife, who published in 1878 an interesting account of their experiences—Six Months in Ascension: an Unscientific Account of a Scientific Expedition. The observatory was first set up at Garrison, but, owing to interference from clouds, Gill heroically moved it in the course of five days to a new site, ‘Mars Bay’, just in time to secure favourable observations. The expedition was crowned with success, the sun’s distance being determined with much greater accuracy than had previously been attained.
In 1879 Gill was appointed H.M. astronomer at the Cape of Good Hope. ​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 ​Transvaal, which was carried through for the Cape and Natal between 1883 and 1886, and for the Transvaal and Free State after the close of the South African War. He further saw that these operations might be made the starting-point of the still greater project, a chain of triangulation stretching the whole length of Africa approximately on the 80th meridian. This scheme he forwarded at every opportunity, and saw the chain carried to within seventy miles of Lake Tanganyika.
Gill, who had been created K.C.B. in 1900, left the Cape in 1907 after twenty-eight years of service. When he went there the observatory was comparatively small, and possessed but one instrument of much value. He left it well equipped with modern instruments, including the Victoria (24-inch) photographic refractor, the gift of Mr. Frank McClean, of Tunbridge Wells. The staff was increased to correspond with the larger equipment, so that the observatory is now qualified to carry out work of the highest order in many different directions. After his retirement Gill settled in London, and took an active share in its scientific activities. He was president of the British Association at the Leicester meeting in 1907. He was president of the Royal Astronomical Society 1910–1912, and succeeded Lord Cromer as president of the Research Defence Society. Much of his time was given to a history of the Cape observatory. Nothing gave him greater pleasure than to invite his astronomical friends to his house, especially if an occasion was provided by the visit of a distinguished foreign astronomer, to ‘have a talk with astronomers about astronomy’. His health was excellent till in December 1913 he was seized with pneumonia and passed away in London on 24 January 1914.
In his History and Description of the Royal Observatory, Cape of Good Hope (1913) Gill mentions the delight with which he read Struve’s History of the Pulkowa Observatory—‘the author had the true genius and spirit of the practical astronomer, a love of refined and precise methods of observation and the inventive and engineering capacity’. These words are as true of Gill as of Struve. The tedium of making similar observations night after night was counterbalanced by the pleasure of making them as accurately as possible. His enthusiasm communicated itself to his colleagues and assistants, and his kindness of heart made them devoted to him. His force of character enabled him to triumph over difficulties and carry out great projects. It was said of him at the Admiralty that if he wanted anything no one had any peace till he got it. He had a happy married life, tempered only by anxieties about his wife’s health. They had no children, but brought up three orphan sons of his brother. He took a lively interest in all that was going on around him, particularly in political and social matters in South Africa, and was well acquainted with many of the men who helped to shape its history.
Gill’s portrait was painted by George Henry in 1912 (Royal Academy Pictures, 1912).
[G. Forbes, David Gill, Man and Astronomer, 1916; Sir David Gill’s History of the Cape Observatory contains records of his astronomical work at the Cape; Proceedings of the Royal Society, vol. xci, A, 1915 (portrait); Monthly Notices of the Royal Astronomical Society, vol. lxxv; Astrophysical Journal, vol. xl.]
F. W. D.