David Gill, Her Majesty's astronomer at the Royal Observatory, Cape of Good Hope, from 1879 to 1907, was the eldest son of David Gill, a watchmaker of Aberdeen, Scotland, and his wife Margaret Mitchell. In 1858 he entered Marischal College, University of Aberdeen, where he benefited from the teaching of, among others, the great physicist James Clerk Maxwell. Despite his interest in science, his father wanted him to join the family business. Hence he left the university in 1860 without graduating and spent the next year learning the art of clockmaking in France and England. Returning to Aberdeen in 1863 he joined his father's business. However, his ambition was to devote his time to science, and more specifically to astronomy. While planning to set up a small transit instrument for the accurate determination of local time in Aberdeen, he visited the Royal Observatory in Edinburgh where he met Charles P. Smyth*, who trained him in the methods for determining astronomical time and became a life-long friend. The planned time service for Aberdeen was duly introduced, with the assistance of the physicist Professor David Thomson, and the experience Gill gained contributed to his decision upon a career in astronomy. He became a member of the Royal Astronomical Society in 1867.
Gill's first scientific paper was his "Note on stars within the trapezium of the nebula in Orion", published in the Monthly Notices of the Royal Astronomical Society in 1867. In that year he erected a 300 mm reflector at his father's house and in May 1869 took photographs of the moon of exceptional quality. These came to the attention of Lord Lindsay (heir to the Earl of Crawford), who in December 1871 offered Gill the directorship of a private observatory he was planning to erect at his father's seat, Dun Echt. Gill, who had taken over his own father's business in 1869 and married Isobel ("Bella") Black in July 1870 (they had no children), accepted the offer with alacrity despite the reduced income it provided. The observatory was erected under Gill's supervision and equipped with instruments of high quality, including a 100 mm heliometer with which he became an expert observer.
In August 1873 he visited several observatories on the European continent and made some lifelong friends. The next year he and Lindsay travelled to Mauritius to observe the transit of Venus on 8 December 1874. Delays at sea of Lindsay's yacht, which carried most of the instruments, curtailed Gill's series of heliometer observations of the minor planet Juno, using the so-called diurnal method (measuring small changes in position from just after rising to just before setting to determine its diurnal parallax). He believed such observations would provide a more accurate measurement of the astronomical unit than transit observations. On the return trip Gill determined (by using a large number of chronometers) the longitudes of Reunion, Mahi (the capital of the Seychelles), Aden, Suez, Alexandria, and Malta. He also measured a one kilometer baseline for the Survey of Egypt and accurately measured some dimensions of the Great Pyramid. During the second half of 1876 he left Dun Echt and moved to London. With the support of the Astronomer Royal, Sir George B. Airy, and the Royal Astronomical Society he undertook an expedition to the island of Ascension, from June 1877 to January 1878, where he used Lindsay's heliometer and the diurnal method to observe a favourable opposition of the planet Mars and derive an accurate value for the astronomical unit. This value was widely adopted and earned him the gold medal of the Royal Astronomical Society in 1882 as well as a medal from the French Academy of Sciences.
In February 1879 Gill, who was by that time an eminent British astronomer, was appointed to succeed J.E. Stone* as HM Astronomer at the Cape of Good Hope. He first undertook a tour of various European observatories, where he made further useful personal contacts and formed an idea of what would be required to develop the Royal Observatory at the Cape into a leading institution. Arriving in May 1879 he found the observatory in a dilapidated state and, with the assistance of W.H. Finlay*, G. Maclear* and other staff, set about putting it in order. He bought Lindsay's heliometer with his own funds and added it to the observatory's instruments. From 1881 to 1883 he and W.L. Elkin* used it to measure the annual parallaxes of nine stars - the first substantial set of parallax determinations from the southern hemisphere. During 1897 to 1899, with the help of Finlay and a visit by W. de Sitter*, he extended the number of parallax determinations to 22. The importance and accuracy of his results were widely recognised.
In September 1882 the brightest comet to be observed during the 19th century appeared and Gill conceived the idea of photographing it with a camera mounted on a telescope, to compensate for the earth's rotation. Plates were taken with the help of a Cape Town photographer, E.H. Allis*, who supplied the camera. The plates showed not only the best images ever obtained of a comet, but also clear images of numerous stars, thus demonstrating the value of astronomical photography for charting the star sphere. This insight marked a turning point in the history of astronomy. Gill initiated a project to obtain a photographic record of the southern celestial hemisphere with a view to compiling a catalogue of the positions and magnitudes of southern stars. This project, known as the Cape Photographic Durchmusterung, commenced in April 1885. Most of the plates were taken by C. Ray Woods* and Henry Sawerthal* during the next four years, and were measured in the Netherlands under the supervision of professor J.C. Kapteyn*. Though Gill had to finance part of the work from his own pocket, the project was eventually completed with the publication of the results in three volumes of the Annals of the Cape Observatory in 1896-1900. The catalogue lists over 450 000 stars down to magnitude 9.5 between a declination of 18 degrees south and the celestial South Pole. However, Gill had an even more ambitious project in mind. He proposed that, through international collaboration, the whole sky should be similarly photographed with improved instruments and a star catalogue constructed. This project was organised at an Astrographic Congress in Paris during April 1887, with Gill as its president. He remained the principal organiser of the work until his retirement, and the Cape Observatory was an active participant from 1892 onward. It was the largest international astronomical project ever undertaken, leading many years later to the completion of the Carte du Ciel (a photographic atlas of stars down to magnitude 14) and the associated Astrographic Catalogue, listing the positions of stars down to the 12th magnitude.
The second transit of Venus during the 19th century occurred on 6 December 1882. Gill observed it at the observatory in Cape Town, but also discussed the event with Simon Newcombe*, leader of an American expedition which observed the transit at Wellington. From that time the two kept up a regular scientific and personal correspondence until Newcombe's death in 1909. Gill's view that a more accurate value of the solar parallax (and thus the astronomical unit) could be obtained by observing minor planets during close approaches to earth led him to plan observations of the minor planets Iris, Victoria and Sappho, which made particularly favourable approaches during 1888 and 1889. He visited Britain in 1884 and convinced the Admiralty to fund the construction of a larger heliometer for the purpose. The instrument was completed in 1887. During this visit he was awarded an honorary Doctor of Laws degree by the University of Edinburgh, having been similarly honoured by the University of Aberdeen earlier.
The observations of the minor planets, in which Elkin and several other European observers participated, were reduced at the Cape and yielded an astronomical unit of 149 469 000 km (corresponding to a horizontal equatorial solar parallax of 8.802 arc seconds) - an exceptionally accurate value that was internationally adopted for the next 45 years. The observations were furthermore so precise that an improved value of the mass of the moon and good estimates of the constant of nutation and the ellipticity of the earth could be derived from them. The planning, execution and publication of this huge project demonstrated Gill's unrivalled understanding of instruments and observations, his persistence and enthusiasm, and the leadership qualities that inspired all who participated.
Other valuable work with the heliometer included an extensive series of observations of Jupiter's major satellites by Gill and Finlay in 1891-1892, continued by a visitor, Bryan Cookson* in 1901-1902. This work led to an accurate value of the mass of Jupiter, relative to that of the sun. Again his ingenious methods of observation led to results of exceptional quality. As director of the observatory Gill also supervised an extensive programme of meridian observations, the reduction of these and earlier observations, and the publication of the resulting star catalogues for 1865, 1885, 1890 and 1900.
Gill was not a great theorist - his genius was as an organiser, observer and technologist. His expertise in instrument making was put to good use, for example, he designed a reversible transit circle which included many ingenious devices to permit errors to be measured and corrected. The instrument was constructed in England and installed at the Cape in 1901. Although it was brought into regular use only in 1905, it proved valuable in the design of later similar instruments. The observatory and its staff expanded markedly during his term of office and his extremely successful research programme was reported on in about 150 publications, attracted many visiting astronomers from overseas, and in 1897 led to the donation to the observatory of a 600 mm refractor and associated equipment for spectroscopic research, by Mr Frank McClean*. Gill's widespread activity and close association with all the great enterprises of observational astronomy of his time made him a leading figure in the international astronomical community.
Besides his immense contributions to astronomy Gill played a leading role in the planning and execution of the geodetic survey of southern Africa. Soon after his arrival at the Cape he proposed such a survey to the governor of the Cape Colony, Sir Bartle Frere, and arranged the cooperation of the government of Natal Colony in the venture. The field work was begun in 1883 by a party of Royal Engineers, commanded by Captain W.G. Morris*, and was concluded in September 1892. Gill acted as the project's scientific director and published the results in the first volume of the Report on the Geodetic survey of South Africa in 1896. After the Anglo-Boer War (1899-1902) he acted as scientific supervisor of the geodetic surveys of the Transvaal Colony and Orange River Colony. Under his supervision geodetic chains were measured throughout southern Africa. He dealt directly with seven different governments, appointed staff to execute the work, supervised the reduction of the observations, and published the result in the subsequent four volumes of the Report on the Geodetic Survey... (1901-1908). The results are a monument to his enterprise, dedication and ability and formed the essential basis for subsequent secondary triangulation, topographic and cadastral surveys and maps. The geodetic survey of the 30 degrees east meridian up to northern Zambia (eventually carried through from South Africa to Norway), was particularly important from a scientific point of view, as it helped to establish the precise shape of the earth.
Gill was a leading figure in organised science in South Africa. Shortly after his arrival he was elected president of the South African Philosophical Society for 1879-1881, and again for 1891-1893 and 1901-1903. He was furthermore the founding president of the South African Association for the Advancement of Science in 1902/3, and played the leading role in organising the joint meeting in South Africa of the British and South African Associations for the Advancement of Science in 1905. That year he and several other delegates to the joint meeting were awarded honorary Doctor of Science (DSc) degrees by the University of the Cape of Good Hope. Earlier he had served on the university's council for a few years (1881-1883). He was a member of the Geological Commission of the Cape of Good Hope, established in 1895 to organise a geological survey of the colony; was a member of the Cape of Good Hope Meteorological Commission from 1880 to 1905; served as a trustee of the South African Museum from 1880 to 1906, and as a trustee of the South African Public Library (now the Cape Town Campus of the National Library of South Africa). He was made a Knight Commander of the Bath (KCB) in 1900.
Gill was an exceptional person and scientist. His forceful personality, enthusiasm, integrity, charm and wit worked wonders in the achievement of his goals and his energy and drive knew no bounds. However, failing health led to his return to England in October 1906, followed by his retirement in February 1907. Settling in London he continued to lead an extremely active scientific life. For example, he served on the council of the Royal Society of London (which had elected him a Fellow in 1883), as president of the British Association for 1907, and as president of the Royal Astronomical Society in 1909-1910. In February 1907 he was elected as the British member of the Bureau International des Poids et Mesures (International Bureau of Weights and Measures) and actively participated in the meetings of its Committee every two years from 1907 to 1913. He gave a number of popular lectures, started a fairly profitable business as a consulting astronomical engineer, and received many distinctions and awards. His final major achievement was the completion of a book, History and description of the Royal Observatory, Cape of Good Hope, which was published in 1913.