Sir Harold Spencer Jones KBE FRS FRSE PRAS (29 March 1890 – 3 November 1960) was an English astronomer. He became renowned as an authority on positional astronomy and served as the tenth Astronomer Royal for 23 years. Although born "Jones", his surname became "Spencer Jones".
Early life Harold Spencer Jones was born in Kensington, London, on 29 March 1890. His father, Henry Charles Jones, was an accountant and his mother, Sarah Ryland, had earlier worked as a school teacher. He was educated at Latymer Upper School, in Hammersmith, West London, from where he obtained a scholarship to Jesus College, Cambridge. He graduated there in 1911, and was awarded a postgraduate studentship. He subsequently became a Fellow of the college.
Chief Assistant at the Royal Observatory, Greenwich, 1913–1923 In 1913 he was appointed Chief Assistant at the Royal Observatory, Greenwich, filling a vacancy created by the departure of Arthur Eddington to become Plumian Professor of Astronomy at Cambridge. In December 1913 Spencer Jones was elected to the Royal Astronomical Society. Spencer Jones's astronomical work extended over a range of subjects. He specialised in positional astronomy, particularly the motion and orientation of the Earth in space. He also studied the motions of stars. He travelled to Minsk in Eastern Europe in 1914 to observe a total solar eclipse, departing during peacetime but returning after the start of the First World War. His activities at the observatory were disrupted by the war, when he worked temporarily for the Ministry of Munitions, particularly on optics. In 1918 Spencer Jones married Gladys Mary Owers. He resumed his astronomical work after the war, including studying the positions of stars, the rotation of the Earth, and the brightnesses of stars. During this period he wrote his textbook General Astronomy. On 30 March 1921 Spencer Jones joined the British Astronomical Association. In 1922 he travelled to Christmas Island in an attempt to observe a total solar eclipse, intending to verify the deflection of the light of stars by the Sun that had been seen during a 1919 eclipse, but cloud defeated the attempts.
His Majesty's Astronomer at the Cape of Good Hope, 1923–1933 The astronomer in charge of the Royal Observatory at the Cape of Good Hope, Sydney Hough, died in 1923, and Spencer Jones was appointed Hough's successor as His Majesty's Astronomer at the Cape of Good Hope. Spencer Jones and his wife sailed to South Africa, arriving in December 1923. During his nine years at the observatory, Spencer Jones set about renewing its administration and scientific work. He made efforts to improve working conditions and morale at the observatory. He led efforts by the staff to measure the properties of large numbers of stars from photographic plates exposed on the observatory's telescopes. This work included measuring the positions of stars that had been studied years earlier to determine their proper motions (their very small movements across the sky relative to distant stars). The velocities of stars along the line-of-sight were measured from their spectra. The staff members measured brightnesses of 40000 stars from their images on photographs. They also determined the distances of stars from their parallaxes – the very small apparent annual motions as the Earth orbited the Sun. Many of these results were published as star catalogues. Spencer Jones's own research concentrated on the motions of the Earth and the Moon. He refined knowledge of the Moon's orbit using observations of occultations of stars. He obtained improved measurements of the distance of the Sun from the Earth using observations of the position of Mars in the sky through its parallax, and carried out a series of observations of the minor planet 433 Eros during its close approach in 1930–1931 for the same purpose. These Eros observations later gave the best measurement of the distance of the Sun then available. He was later awarded the Gold Medal of the Royal Astronomical Society and the Royal Medal of the Royal Society for this work.
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