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[John Feltham]

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Dyson, Sir  Frank Watson  (1868-1939), astronomer, was born at the Baptist manse in Measham, near Ashby-de-la-Zouch, Derbyshire, on 8 January 1868, the eldest child in a family of four sons and three daughters of Watson Dyson (1837-1904), a Baptist minister, and his wife, Frances (1839-1909), the daughter of James Dodwell, a farmer of Long Crendon, Buckinghamshire, and his wife, Deborah. After early schooling in Nottinghamshire, Dyson moved with his family to Yorkshire, where he attended Heath grammar school in Halifax. He won scholarships first to Bradford grammar school and then to Trinity College, Cambridge. He was second wrangler in the mathematical tripos in 1889. In 1891 he became first Smith's prizeman and was elected a fellow of his college, and the following year he was awarded an Isaac Newton studentship to carry out research in astronomy.

The International Astrographic Catalogue

In 1894 Dyson was offered by the astronomer royal, William Christie, the post of chief assistant at the Royal Greenwich Observatory. He accepted with alacrity, as this enabled him to marry Caroline (Carrie) Bisset (1867-1937), the daughter of Palomen Best MB JP of Louth, Lincolnshire. They had two sons and six daughters. Though he had little prior experience of instrumentation, he threw himself into the work of practical astronomy with considerable enthusiasm. Christie had committed the Greenwich observatory to an international project involving photography of specified regions of the sky, followed by measurement of the positions of the stars recorded. Dyson's first major task was to take charge of the Greenwich share in this International Astrographic Catalogue. By 1909 the Greenwich observatory was the only one of the participating institutions to have completed its allotted programme of observation, measurement, and publication. This work brought home to Dyson the need to improve knowledge of stellar motions. He therefore undertook with one of the assistants, William Grasset Thackeray, a new reduction of observations of more than 4000 circumpolar stars made between 1806 and 1819 by Stephen Groombridge. Combining the reduced data with new observations extended knowledge of stellar motions to considerably fainter stars than hitherto. In 1906 Arthur Stanley Eddington, who had just been appointed to Greenwich, used this new information to confirm the proposal by the Dutch astronomer Jacobus Cornelius Kapteyn that stars had preferred directions of motion in space.

In the same year Dyson moved to Edinburgh to take up the posts of astronomer royal for Scotland and regius professor of astronomy at the university. His most important work there continued to be on stellar positions and motions. He reached agreement with staff at Perth observatory in Australia to help with measuring and reducing the photographs of the southern sky they were taking for the International Astrographic Catalogue. He also countered criticisms of Kapteyn's work by showing that preferential directions of motion were exhibited by stars with large proper motions, where observational errors were less important.

Astronomer royal

On Christie's retirement in 1910, Dyson was appointed ninth astronomer royal. He is the only person to have been successively astronomer royal for Scotland and astronomer royal. As one of his first projects he turned again to the stars observed for the International Astrographic Catalogue, but now concentrated on the measurement of their parallax to try and determine stellar distances. By the time he retired some 500 parallaxes had been determined at Greenwich-an important addition to the existing total. He also initiated studies into determining stellar magnitudes and colours on a systematic basis, as well as continuing the tradition of meridian observations at Greenwich. However, his most important initiative related to his interest in solar eclipses.

In 1896, and again in 1898, Christie was away for several months on eclipse expeditions. This gave Dyson early experience in the running of a national observatory. The first solar eclipse Dyson observed was from Portugal in 1900, and the weather was fine. This began a remarkable run of six eclipse expeditions in which he was officially involved, where the conditions were good enough to make observations. Dyson developed a special interest in eclipse spectroscopy. At his first two eclipses he obtained some of the best observations of coronal lines up to that time. He fully realized the need for innovation in eclipse work. At the 1927 eclipse, visible from northern England, he helped organize the first use of colour film and of making photographs from an aircraft. The results of this continuing interest in eclipses were later summarized in a book published in 1937, Eclipses of the Sun and Moon, which was written in conjunction with Richard van der Riet Woolley, later eleventh astronomer royal.

In 1917 Dyson pointed out that Einstein's recently propounded general theory of relativity could be subjected to an observational test at the solar eclipse due in 1919. The theory predicted that stars seen near the eclipsed sun would appear to be shifted in position by a small, but measurable, amount. Dyson's proposal came at a difficult moment during the First World War, but the necessary preparations were made, and after the war the observations were obtained. When Dyson presented the results to a joint meeting of the Royal Society and the Royal Astronomical Society at the end of 1919, demonstrating that the data favoured Einstein's predictions, it was recognized both within science and outside that a major turning point in scientific thought had been reached.

Time measurement

The Greenwich observatory had always been involved in time measurement, and this became one of Dyson's particular concerns. Under his guidance the observatory introduced a Shortt free-pendulum clock for its time service from 1924 onwards. At about the same time he was approached by John Reith, who asked that the Greenwich observatory should provide time signals to the public via the British Broadcasting Corporation. It was Dyson who ultimately decided to adopt the system of six pips, with the sixth denoting the hour. This 'Greenwich time signal' was transmitted worldwide for the benefit of shipping from 1927. Dyson was for many years president of the British Horological Institute, which awarded him its gold medal in 1928. He was also twice master of the Clockmakers' Company.

When Dyson took over from Christie, he decided that it had become essential to update the observatory's magnetic equipment. A new building was erected to house the equipment, and Dyson, working in collaboration with his chief assistant, Sydney Chapman, overhauled the recording and publication of magnetic data. After the First World War the growth of electrification of suburban rail services made magnetic measurements at Greenwich increasingly difficult. Dyson therefore decided to move the magnetic equipment to Abinger, near Leith Hill, in Surrey. (Not long after Dyson's retirement it became necessary to move it again, this time to Devon.)

Dyson had to work hard to keep the Greenwich observatory running during the First World War, when many of the staff joined the armed forces. After the war he played a major role in reconstituting international scientific co-operation via the International Research Council and, especially, via the formation of the International Astronomical Union (of which he was president from 1928 to 1932). He was president of the Royal Astronomical Society from 1911 to 1913 and of the British Astronomical Association from 1916 to 1918. He played an important role in the former body by ensuring that members of the new discipline of geophysics continued to be members along with the astronomers. Dyson was elected to the Royal Society in 1901, and was awarded its royal medal in 1921. He was later awarded the Bruce gold medal of the Astronomical Society of the Pacific (1922) and the gold medal of the Royal Astronomical Society (1925). He received honorary degrees from a number of universities, including Cambridge and Edinburgh, and was foreign or corresponding member of various academies. He was knighted in 1915 and appointed KBE in 1926.

Dyson was due to retire in 1928 at the age of sixty, but he was asked to continue for another five years until 1933. In 1931 William Johnson Yapp, a wealthy manufacturer, offered to present the Greenwich observatory with a new telescope, in commemoration of Dyson's work. The 36 inch reflector came into use at Greenwich the year after Dyson retired.

Dyson was a man who combined charm with common sense. He retained his religious belief throughout his life, along with liberal views in politics. He died on board ship, on the Ascanius, on 25 May 1939, while returning from a visit to Australia, and was buried at sea on the same day.

A. J. Meadows 

Sources  M. Wilson, Ninth astronomer royal (1951) + A. S. Eddington, Obits. FRS, 3 (1939-41), 159-72 + A. J. Meadows, Greenwich observatory: the story of Britain's oldest scientific institution, 2: Recent history (1836-1975) (1975) + H. A. Bruck, The story of astronomy in Edinburgh from its beginnings until 1975 (1983) + W. H. McCrea, Royal Greenwich Observatory: an historical review (1975)
Archives CUL, corresp. and papers + RAS, letters to Royal Astronomical Society, London + Royal Observatory Library, Edinburgh, corresp. and papers | California Institute of Technology, Pasadena, archives, corresp. with G. E. Hale
Likenesses  W. Stoneman, photograph, 1919, NPG [see illus.] · photograph, repro. in Jones, Obits. FRS · photographs, repro. in Wilson, Ninth astronomer royal · photographs, RAS
Wealth at death  £4331 6s. 7d.: resworn probate, 27 July 1939, CGPLA Eng. & Wales


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