David Gill (astronomer)

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Script error: No such module "Message box". Template:Short description Template:EngvarB Template:Use dmy dates Script error: No such module "infobox".Script error: No such module "Check for unknown parameters".Script error: No such module "Check for clobbered parameters".Template:Wikidata image Sir David Gill (12 June 1843 – 24 January 1914) was a Scottish astronomer who is known for measuring astronomical distances, for astrophotography and geodesy. He spent much of his career in South Africa.

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.[1] He was sent to Dollar Academy in 1857, at the age of fourteen, and boarded with Dr. Lindsay.[2]Template:Rp Gill described Lindsay as an inspiring influence, whose teaching "filled me with the love of mathematics, physics and chemistry".[3]Template:Rp He spent two years at Marischal College, Aberdeen where his teachers included James Clerk Maxwell.[4] 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.[5]

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.Template:R 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 timeTemplate:R In 1863 he became a member of the British Horological Institute[6] and in the same year was made a junior partner in the family firm, which becme known as David Gill & Son.Template:R

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.[5]Template:R

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.Template:R He also used this telescope for photography, taking one of the earliest quality photographs of the moon in 1869.Template:R

File:Isobel Black and David Gill davidgillmanastr00forbrich 0057 (cropped).jpg
Isobel Black and David Gill before their marriage

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.Template:R

Lord Lindsay and Dun Echt

Gill had acquired a considerable local reputation with his astronoomical 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.Template:R 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.Template:R[7]

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.Template:R As well as hid 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.[8]Template:Rp

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. 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.[9]Template:Rp

File:David Gill's heliometer, Provost Skene's House Museum, Aberdeen.jpg
David Gill's heliometer, Provost Skene's House Museum, Aberdeen

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.Template:R 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 the sun'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. Juno is at opposition about every 16 months, and there are other asteroids that can be used, as well as the planet Mars.

File:Lindsay 1874 Path of Juno.jpg
Path of Juno showing the stars to be used for measurement of parallax.[10]

Observation from a station close to the equator was desired, because the distance between morning and evening positions is greatest at the equator. There was an opposition scheduled for October 1874. However, while Gill was on Mauritius at that time, Lord Lindsay was sailing in his own boat, Venus around the Cape, with much of the expedition's equipment including the heliometer. Delayed by bad weather, he did not arrive in Mauritius until 2 November. Fortunately this was in time to make a series of parallax observations that gave a good result, and proved the practicality of the method. Most of the observation work in Mauritius was carried out by Gill, as Lindsay was in poor health after the voyage. Template:R

Gill returned to Dun Echt in mid-1875. His status and reputation as an astronomer had greatly increased due to his management of the expedition, his innovative work on solar parallax, and his cooperation with members of the various expeditions. He was frequently visited by colleagues, and this led to tension with Lady Crawford, Lord Lindsay's mother, who had not expected guests to arrive at their country estate with horses and carriages. Lindsay and Gill discussed the situation and it was mutually agreed to terminate Gill's contract at Dun Echt. The break was entirely amicable, and Gill and his wife remained at Dun Echt until mid-1876. The two men continued to cooperate and remained friends until Lindsay died the year before Gill. Gill spent much of his remaining time at Dun Echt calculating the results from the observations on the Mauritius expedition. Template:RTemplate:R

Ascension Island

File:Camp at Mars Bay, Ascension, 1877 davidgillmanastr00forbrich 0117.jpg
Camp at Mars Bay, Ascension Island, 1877
File:IMRAY(1884) p0261 ASCENSION.jpg
1884 Map of Ascension Island showing Georgetown, where the observatory was originally set up, and Mars Bay

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 13th 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.Template:R Isobel wrote an account of their time on Ascension.[11]

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. 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.Template:R. The Gills then sailed for South Africa, arriving there in June 1879.Template:R

Astronomer at the Cape

Over the following 27 years he was to refurbish the observatory completely, turning it into a first-rate institution. Gill was a meticulous observer and had a flair for getting the best out of his instruments. His solar parallax observations with a heliometer and his calculations of distances to the nearer stars, are testimony to his outstanding work. He recruited Robert Innes to the Cape Observatory.[12]

Gill had used the parallax of Mars to determine the distance to the Sun,[13] and also measured distances to the stars. He perfected the use of the heliometer. He was Her Majesty's Astronomer at the Royal Observatory at Cape of Good Hope from 1879–1906. He was a pioneer in the use of astrophotography, making the first photograph of the Great Comet of 1882, and one of the early proponents of the Carte du Ciel project.

File:Clerke 1902 Plate 2 Comet May 1901 cu31924012301598 0369.jpg
Great Comet of 1901 photographed at the Cape Observatory

The invention of dry plate photography by R.L. Maddox made Gill realise that the process could be used to create images of the stars and to more easily determine their relative positions and brightness. This led to a massive project in collaboration with the Dutch astronomer J.C. Kapteyn, and the compiling of an index of brightness and position for some half a million southern stars. The work was published as Cape Photographic Durchmusterung in 3 volumes between 1896–1900. Gill also played a leading role in the organising of the Carte du Ciel, an ambitious international venture aimed at mapping the entire sky. He initiated the idea of a geodetic survey along the 30th meridian east stretching from South Africa to Norway, resulting in the longest meridian arc yet measured on Earth (see arc measurement).

File:The grave of David Gill, St Machar's Cathedral.jpg
The grave of David and Isobel Gill, St. Machar's Cathedral, Aberdeen.

Gill's concern with measurement led to him becoming a member of the International Committee for Weights and Measures (CIPM) from 1907-1914.[14] As president of the British Association for the Advancement of Science, 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.[15]

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 before his death in 1914.

He is buried next to his wife, Isobel Sarah Gill, on the grounds of St. Machar's Cathedral, Aberdeen.[16][17] Their grave lies on the east outer wall of the church.

Selected writings

His writings include:

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  • "A Determination of the Solar Parallax from Observations of Mars at the Island of Ascension" (in the Memoirs of the Royal Astronomical Society, volumes xlvi and xlviii, 1881 and 1885). New International Encyclopedia
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Honours

Lectures

In 1909 he was invited to deliver the Royal Institution Christmas Lecture on Astronomy, Old and New.

Awards

Named after him

References

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External links

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Obituaries

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