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.
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Early life and education
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.
Clocks and watches
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.
Lord Lindsay and Dun Echt
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.
Ascension Island
The Gills moved to London in 1876, and Gill began planning his next project, determination of solar parallax by observations of Mars, which would be in close opposition in 1877. Ascension Island was selected as the observation station. Gill obtained Linsday's agreement to the loan of the heliometer, and secured funding in part from the Royal Astronomical Society, and in part from subscriptions by colleagues including the Astronomer Royal, Sir George Airy. He travelled to Ascension with Isobel, arriving there on 13 July 1877. The island was a naval base, and its commander Captain Phillimore arranged accommodation and assistance in setting up the instruments. However although Ascension had been chosen in part for its faourable weather, the skies remaind overcast for several weeks. On the hope that the clouds might be local, Isobel Gill set off to the other side of the island with two guides, and found that the skies were clear. All the equipment was packed, and transported to the windward side of the island. The new location was named Mars Bay. Although time had been lost, they were able to get enough observations to calculate an accurate value, and work was completed by 9 November. Isobel wrote an account of their time on Ascension.
The Gills returned to England on 24 January 1878. Gill spent a considerable time calculating the results. The resulting value for the AU became the accepted standard for many years. On 6 April his father died. David Gill had travelled to Aberdeen on hearing of his father's ilnness, and spent a month handling the estate. In May, the Rev. Robert Main, Observer at the Radcliffe, Oxford, died. Gill applied for the vacant position, but it was awarded to Edward Stone, Her Majesty's Astronomer at the Cape Observatory. Gill then applied for the position at the Cape, and this time he was successful, rather to his surprise, as the other candidate was William Christie, Chief Assistant at Greenwich, who had the support of Airy. Before travelling to the Cape, Gill visited the observatories of Paris, Leiden, Groningen, Hamburg, Copenhagen, Helsingfors, Pulkowa, Strasbourg and Paris. At Strasbourg he met Willliam Elkin, one of Winnecke's senior students who was working on stellar parallax. The two became friends, and Gill invited Elkin to visit and work at the Cape when he finished his degree.. The Gills then sailed for South Africa, arriving there in June 1879.
Astronomer at the Cape
Over the following 27 years Gilll was to refurbish the Cape observatory completely, turning it into a first-rate institution.
His first concern was with the astronomical instruments in the observatory. The first of these was the transit circle, used for determnining the time and altitude of a star as it passed the local meridian. The instrument at the Cape was almost identical to the Airy circle at the Royal Observatory, Greenwich. Gill found that the adjustment screws and the bearings of the instrument were badly worn, and the lenses fogged and deteriorated. Some parts had to be sent back to England for repair or replacement, others could be restored locally. The 7-inch equatorial telescope had similar problems, and the photo-heliometer was considered unusable. Gill was able to purchase the 4-inch heliometer that he had used on Ascension Island from Lord Lindsay, in order to continue his parallax observations.
The heliometer arrived at the Cape in December 1880, and Elkin arrived the following month for a visit that would last until May 1883. The pair then set out on a programme to determine the distance of a number of nearby stars, using the annual parallax from the orbital motion of the earth. Forbes describes this as the "first systematic attack ever undertaken on star distances". Gill's predecessor at the Cape, Thomas Henderson had been one of the first to demonstrate measurable parallax of a star, Alpha Centauri. Henderson's results had the limitation, which he himself acknowledged, that they were derived from measurements of star positions that were not intended for parallax determination. Robert Main, commenting on Henderson's results in 1840, remarked: "There cannot exist a doubt that the necessity of using other more delicate means of observation for the discovery of an inequality of so trifling amount, as is proved to exist in the case of the brightest of the stars of both hemispheres, will now be sufficiently impressed upon all astronomers." Gill and Elkin used the heliometer, the most delicate means of observation available, to measure the angular distances of a target from reference stars. They carried out 14 series of observations on 9 stars. Each series included observations made over at least a year, and in some cases up to 2 years. Each series used a pair of reference stars on either side of the target star. Where more than one series was carried out on a particular target, a different pair of reference stars was used for each series. For Alpha Centauri, there were four series, two by Gill, two by Elkin. The parallax of Alpha Centauri was found to be 0.75" ± 0.01. This is in good agreement with the modern value of 0.743".
Retirement and death
On Gill's retirement in 1906, the couple moved to London, where Gill served for two years (1909–1911) as president of the Royal Astronomical Society. His concern with measurement led to him becoming a member of the International Committee for Weights and Measures (CIPM) from 1907-1914. As president of the British Association, he gave a presidential address in 1907 in which he advocated definitions based on fundamental physical properties, rather than on arbitrary standards such as a rod of metal with lines ruled upon it, to determine the yard or metre.
Gill died in 1914. He is buried next to his wife, Isobel Sarah Gill, who died in 1919, on the grounds of St. Machar's Cathedral, Aberdeen. Their grave lies on the east outer wall of the church.