Background
David Gill was born on June 12, 1843, in Aberdeen, Scotland. He was the eldest surviving son of David Gill, a well-established watch- and clockmaker in the city of Aberdeen, and his wife Margaret Mitchell.
Dollar Academy, Dollar FK14 7DU, United Kingdom
David was educated at Dollar Academy.
United Kingdom
David Gill
Marischal College, Broad St, Aberdeen AB10 1AB, United Kingdom
Gill studied for two years at Marischal College of the University of Aberdeen.
David Gill was born on June 12, 1843, in Aberdeen, Scotland. He was the eldest surviving son of David Gill, a well-established watch- and clockmaker in the city of Aberdeen, and his wife Margaret Mitchell.
David Gill's father intended that David should follow him in the family business and educated him accordingly. He was educated first at Bellevue Academy in Aberdeen then at Dollar Academy. After two years at Marischal College of the University of Aberdeen, where he attended classes conducted by James Clerk Maxwell, Gill was sent for another two years to learn the fundamentals of clockmaking in Switzerland, Coventry, and Clerkenwell. There is no doubt that this somewhat informal training was extremely valuable to him in his later career, both for the experience and knowledge of fine mechanisms that he acquired and for the mastery of the French language and of business methods.
Gill duly succeeded his father and ran the business for ten years. His first astronomical project was undertaken soon after his return to Aberdeen. This was the provision of a reliable time service for Aberdeen based on astronomical observations made with a portable transit instrument set up in a small observatory at King’s College.
Spurred on by the success of his time service and his delight in making precise observations, he soon acquired an instrument that made possible a wider range of astronomical work - a twelve-inch reflector. His main objective in its use was the measurement of the parallaxes, or distances, of stars, with the micrometric method that Struve had employed to find the distance of Vega. This was, at the time, one of the most interesting and difficult problems of practical astronomy, and it is intriguing to imagine what Gill would have made of it. He never completed it with this instrument, however, because at about this time he received an invitation to become private astronomer to Lord Lindsay, an invitation which he accepted with alacrity although it involved giving up his business and consequently entailed a heavy financial sacrifice. When Gill later returned to the measurement of stellar parallaxes, it was with a heliometer, a far more powerful instrument for the purpose.
Lord Lindsay was building an observatory at Dun Echt, about a dozen miles from Aberdeen, and it was Gill’s job to help with the planning and to supervise the building. This was no small task as Lord Lindsay desired to have an observatory second to none and furnished with the best instruments available. The Dun Echt observatory later became the nucleus of the Royal Observatory on Blackford Hill, Edinburgh, to which it was removed in 1894. The work of fitting out the observatory gave Gill the opportunity of meeting and becoming friendly with most of the leading European astronomers and instrument makers and of acquiring much firsthand experience of practical details that was to be invaluable to him in his later work at the Cape observatory. It was also at Dun Echt that he first encountered and mastered a heliometer, which was to become his own special instrument. Heliometers, although potentially very accurate, require great dexterity of hand and eye to operate, skills that Gill had highly developed through astronomical observation, shooting, and watchmaking.
Gill remained at Dun Echt from 1872 to 1876, during which time he went on an expedition to Mauritius, where, with Lord Lindsay and others, he observed the 1874 transit of Venus. The object of the many transits of Venus expeditions of that year was to determine the distance of the sun and the associated constants, which must be accurate if the data computed in nautical almanacs are to be reliable. The method used was that originally proposed by Halley - combining the observed times of transit of Venus across the face of the sun as observed from a number of places as widely scattered across the earth as possible. As the last such transit had occurred in 1769, that of 1874 was eagerly anticipated, and several nations prepared elaborate expeditions to observe it. While on Mauritius, Gill used the Dun Echt heliometer to observe a near approach of the minor planet Juno and was able from relatively few observations to deduce a value of the solar parallax that was fully as reliable as that deduced from all the elaborate transit-of-Venus expeditions put together. The method he used was to measure the parallactic displacement of Juno resulting from the diurnal movement of the place of observation between the early evening and the late morning. Such observations are most effective when the place of observation is close to the equator and the object observed is close to opposition.
This method of determining the solar parallax and the related astronomical constants was clearly worth pursuing. Thus, in 1877 Gill and his wife went on a private expedition sponsored by the Royal Astronomical Society to Ascension Island, where he spent six months observing a near approach of Mars. The instrument he used was the same one that he had used in Mauritius - the four-inch heliometer lent to him by Lord Lindsay for the purpose. Gill was living in London, working up the results of this expedition, when in 1879 he was appointed Her Majesty’s astronomer at the Cape of Good Hope. Before sailing for the Cape he toured the major European observatories, renewing his acquaintance with most of the leading astronomers and laying the foundations for future cooperative schemes, particularly those to determine the solar parallax.
The Royal Observatory at Cape Town had been founded in 1820 and was intended to make observations in the southern hemisphere strictly comparable with those made at Greenwich. For this purpose, it had been supplied with similar instruments, but by 1879 they were in a very poor state of repair and largely obsolete. Moreover, many observations had not been fully reduced or published.
Gill’s first work at the Cape was to clear away these arrears of reduction and publication and to recondition the various instruments. He paid particular attention to the Airy transit circle, the twin of that on longitude zero, which had been installed at the Cape in 1855 and with which the meridian observations - the main work of the observatory - were made. This instrument was not reversible and therefore, in Gill’s opinion, was not really suitable for the determination of fundamental star positions. Nevertheless, Gill improved it as much as he could and kept it in active use until near the end of his term of office, when it was replaced by a fine reversible transit circle constructed by Troughton and Simms to Gill’s own design. In 1900 this design was revolutionary, but it was so good that it has provided the pattern for most of the transit circles that have since been made.
Gill closely supervised observations with the transit circle but did most of his personal observing on the heliometers. In cooperation with a number of northern observatories, he determined the solar parallax by systematic observations of three minor planets, Iris, Victoria, and Sappho. The value he deduced was used in the computation of all almanacs until 1968, when it was replaced by 8.794 arc seconds derived by radar echo methods and by observations of Mariner space probes. Gill also used the heliometers to measure the distances of a score of the brighter and nearer southern stars and obtained results of which the accuracy was later confirmed by photographic observation.
Photographs of the bright comet of 1882 drew Gill’s attention to the possibility of accurately charting and measuring star positions by means of photography. The immediate outcome was the Cape Photographic Durchmusterung, which gives the approximate positions and brightness of nearly half a million southern stars and which was the first major astronomical work to be carried out photographically.
While work on the Durchmusterung was still in progress, Gill became involved with the Paris astronomers, to whom he had sent copies of the 1882 comet photographs, in the initiation of the Carte du del astrographic project. A much larger and more ambitious undertaking, it aimed at preparing a photographic chart of the whole heavens showing stars to the fourteenth magnitude and a catalog giving precise positions for all stars to the eleventh magnitude - that is, for over two million stars. This vast project was divided among a dozen observatories but was too big for many of them; thus the catalog for the whole sky was not completed until 1961. The Cape observatory undertook a major section of this work, and a suitable telescope, the astrographic refractor, was acquired and made ready for use by 1892; all the necessary plates had been obtained by 1900.
Apart from his astronomical work, Gill acted as the organizer of geodetic and boundary surveys throughout southern Africa as well as of projects to determine the longitude and latitude of its various ports. His most ambitious project was for a triangulation of the thirtieth meridian of east longitude from South Africa to Norway, a total arc of 105°, the longest observable meridian in the world. Gill made himself responsible for the southern end of this arc, the survey of which as a whole was not completed until after the end of World War II.
Gill retired from the Cape at age 63, two years before he need have done. On his arrival in 1879, he had found a small, rather run-down, dispirited institution; when he left in 1906 the Cape observatory was generally recognized as one of the best equipped in the world with a large, young staff fully engaged in important astronomical projects. On his retirement, Gill and his wife went to live in London, where they were able to keep in close touch with astronomers and scientists from all over the world.
Gill was elected a fellow of the Royal Society in 1883. He was also a member of the Royal Astronomical Society and the Royal Swedish Academy of Sciences.
Gill married Isobel Black in 1870.