Sir David Gill (12 June 1843 – 24 January 1914) was a Scottish astronomer who spent most of his career as H.M. Astronomer at the Cape of Good Hope. He was born in Aberdeen, trained as a watch and clock-maker, and spent ten years in the family business, which he took over from his father. He became a noted amateur astronomer, and was invited by Lord Lindsay to manage his private observatory, which he accepted, selling the family busisness. He took part in astronomical expeditions to Mauritius and Ascension Island, before being appointed to the Cape in 1879. He was particularly noted for his observations with the heliometer, using this instrument for measurement of parallax in order to determine distances, both from the earth to the sun (solar parallax) and from our solar system to other stars (stellar parallax). At the Cape, he developed the observatory, making it a world-class institution. He was a pioneer of astrophotography, and used photographic methods to create a star atlas of the southern hemisphere, and also supported and took part in the world-wide Carte du Ciel star-mapping project. He was a meticulous observer, taking great trouble to identify and eliminate systematic sources of error. He was a proponent of international cooperation, supporting international projects and collaborating with many of the other leading astronomers of his day. He helped initiate a geodetic survey of Southern Africa which eventually connected with North cape to provide the longest meridian arc in the world, providing a basis for cartography, navigation and astronomical observations.
David Gill was born at 48 Skene Terrace in Aberdeen the son of David Gill, watchmaker and his wife Margaret Mitchell. He was educated first at Bellevue Academy in Aberdeen then at Dollar Academy. He was sent to Dollar Academy in 1857, at the age of fourteen, and boarded with Dr. Lindsay. Gill described Lindsay as an inspiring influence, whose teaching "filled me with the love of mathematics, physics and chemistry". He spent two years at Marischal College, Aberdeen where his teachers included James Clerk Maxwell. Another important influence at university was Prof David Thomson. In 1863 they jointly repaired the university clock and both set up a new mechanical telescope at the Cromwell Tower Observatory. This was his introduction to astronomy.
In 1860, Gill's father, who was 71 years old, insisted that David should join the family business, with a view to taking it over in due course. Gill, who had been thinking of a career in science, "very unwillingly yielded" to this request. He spent the years 1861-1862 learning the watchmaker's trade, travelling to the great centres for the manufacture of clocks and watches in Switzerland, and then in Coventry and Clerkenwell. In his later life as an astronomer, he greatly valued the technical skills he had learnt at this time In 1863 he became a member of the British Horological Institute and in the same year was made a junior partner in the family firm, which became known as David Gill & Son.
Also in 1863 Gill became aware that Aberdeen did not have an accurate time standard. Edinburgh used a time ball on top of Nelson's Monument in Edinburgh to give a time signal to the ships at Edinburgh's port of Leith. By 1861, this visual signal was augmented by the One O'Clock Gun at Edinburgh Castle. Gill obtained an introduction to Charles Piazzi Smyth, who had introduced these systems, from David Thomson, Professor at the University of Aberdeen. In Edinburgh, Piazzi Smyth showed him not just the timing system but all the instruments in the observatory, and how they were used. This was his introduction to astromomy. Back in Edinburgh, Gill and Thomson repaired the university clock and re-installed an old transit instrument in King's College, Old Aberdeen. This was used to establish sidereal time to calibrate a local clock. This was modified to send time signals which were used to control the main university clock.
In 1866 or 1867 he bought his own telescope, and set up a small observatory in his father's garden. The telescope was a silver on glass reflector with 12 inch aperture and 10 foot focal length. He designed the mounting and had it built by a local shipbuilder. He made the driving clock himself. With this telescope he carried out micrmetric measurements of double stars and for observations of nebulae. He also used this telescope for photography, taking one of the earliest quality photographs of the moon in 1869.
In 1869, Gill's father handed over full control of the business to his son, which gave him much more financial security. The next year he married Isobel Black, who he had first met five years earlier in her home village of Foveran.
Gill had acquired a considerable local reputation with his astronomical work, and this had come to the notice of Lord Lindsay, who had been interested in astronomy from an early age. He first met Gill when he asked to see his photographs of the moon, and the two became close friends. In 1872, Gill received a lettter from Lindsay's father, the Earl of Crawford offering him a position in charge of the observatory they were planning to establish at Dun Echt, near Aberdeen. Gill seized this opportunity to devote his time solely to science, accepted the offer, and sold the family business. He then began work on supervising building work and equipping the observatory. The design and equipment of the observatory was influenced by the description of the Pulkovo Observatory in Russia, described by its first director F.G.W. Struve.
In 1873 Gill travelled via Copenhagen and Stockholm to St Petersburg and then to Pulkovo, where Otto Struve, son of F.G.W. Struve, showed him the observatory. He then travelled with Struve to Hamburg for the Astronomical Society meeting, where he first met Simon Newcomb, Arthur Auwers and Friedrich Winnecke, who would become valued collaborators. Back in Scotland, as well as his work on the new observatory, which included overseeing the construction of a house for himself and Isobel, Gill was involved in planning and equipping an expedition to Mauritius to observe the transit of Venus, which would occur on 9 December 1874, to determine the astronomical unit, (AU, the distance of the earth from the sun). This was a private expedition, funded by Lord Lindsay, and independent of the national expeditions, though co-operating with them in a number of ways.
It was necessary to establish the exact location of the observation stations in order to analyse the results of the transit observations. The most accurate method for longitude measurement at the time used telegraphic signals for time determination. Mauritius had no telegraph link, so Gill organised a chain of telegraphic longitude determinations to Aden, with extension to Mauritius by chronometer. Fifty chronometers were used, which Gill transported from Liverpool, where they had been calibrated, by rail to Southampton, P&O steamer to Aden via the Suez Canal, then on another steamer to Mauritius, arriving on 4 August, after a journey of seven weeks. Observations were made on both outward and return journeys, and positions determined for a number of locations in the Indian Ocean. In the Seychelles, Gill was helped by Captain William Wharton in HMS Shearwater who was supporting the British transit expedition, and beginning his survey of east Africa. Wharton would become a close friend and ally. Within Mauritius, there were three observation stations, Pamplemousses (official British expedition), Solitude (German), and Belmont (Lindsay). Longitude determinations between the three stations were carried out using field telegraph, chronometers, and timings of rockets. On the return journey Dr Löw of the German expedition travelled to Suez, and collaborated in observations and exchange of telegraph signals with Gill in Aden. Arthur Auwers in Berlin assisted in the determination between Malta and Berlin, providing a second link to Suez via Alexandtia.
As well as carrying out observations of the transit, Lindsay's expedition had an additional objective, determination of the AU by diurnal parallax observations on the minor planet Juno. As the earth rotates, the apparent position of a nearby object, in this cae Juno, changes relative to the much more distant stars. By measuring this change between evening to dawn, the distance to Juno can be determined, and as the relative dimensions of the orbits in the solar system were already known, the AU could be calculated. The measurements were made with a heliometer, a specialised telescope which used an adjustable split image to assess angular distances. Heliometers had been used to measure the diameter of the sun (hence the name) and the separation between double stars, but its use for measuring parallax was an innovation, and was important because it eliminated the difficulties of accurate timing of the ingress and egress of Venus in transit, due to effects of Venus's atmosphere, and because it did not depend on rare events. Transits of Venus occur in pairs with intervals of over a century between one pair and the next. Parallax observations required the target to be at opposition, that is at its closest to earth, which is much more common. Juno is at opposition about every 16 months, and there are other asteroids that can be used, as well as the planet Mars. Another advantage of diurnal parallax measurement is that it does not require observers thouasands of miles apart. As Isobel Gill remarked, a single observer and his observatory can be carried by the rotation of the earth 6,000 or 7,000 miles between the times of his morning and evening observations.