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Spencer Jones, Harold

SPENCER JONES, HAROLD

(b. Kensington, London, England, 29 March 1890; d. Greenwich, England, 3 November 1960)

astronomy.

Spencer Jones was the third child of Henry Charles Jones, an accountant. His early interest in mathematics was fostered at Hammersmith Grammar School, from which he won a scholarship to Jesus College, Cambridge. Thereafter his career followed, with minor modifications, the course usual for men of his abilities and background.

Spencer Jones’s scholarly career at Cambridge culminated in his election to a research fellowship of his college in 1913. That year the astronomer royal, F. W. Dyson—following the established pattern for recruiting chief assistants at the Royal Observatory —appointed Spencer Jones to Greenwich as replacement for A. S. Eddington, who returned to Cambridge as Plumian professor; he remained there until 1923, when he was appointed H.M. Astronomer at the Cape of Good Hope. Spencer Jones spent the next decade in South Africa, from which he returned early in 1933 to become astronomer royal—an office he held until his retirement at the end of 1955.

During most of his life Spencer Jones held high positions in the scientific civil service of his country and a number of honorary posts in international professional bodies, including the presidency of the International Astronomical Union in 1944–1948 (to which he succeeded following Eddington’s death) and the secretary-generalship of the International Council of Scientific Unions in 1955–1958. In Britain he received most of the national honors due a man of his station (including knighthood in 1943) and for most of his life was active in the Royal Astronomical Society, of which he was president from 1937 to 1939.

Spencer Jones’s scientific interests were connected mainly with the tasks of the observatories with which he was associated: primarily the problems of positional and fundamental astronomy. His outstanding personal contributions were a study of the speed of rotation of the earth, and one of the solar parallax. In his epochal paper, “The Rotation of the Earth and the Secular Acceleration of the Sun, Moon, and Planets” (1939) he proved—qualitatively, but quite definitely—that the fluctuations in the observed longitudes of these celestial bodies can be attributed not to any peculiarities of their motions but, rather, to fluctuations in the angular velocity of rotation of the earth.

Spencer Jones devoted several years to a new determination of the mean distance of the earth from the sun through measurements of the parallactic displacement of the asteroid Eros during its favorable opposition in 1930–1931. Still at the Cape of Good Hope, he contributed more than 1,200 photographic observations to this program and later, by international agreement, was entrusted with the reductions of all observations of Eros made in 1930–1931.

The principal result of this work (1941) disclosed that the value of the solar parallax was equal to 8.7904ʺ ± 0.0010ʺ—a considerable improvement over previous results. This figure did not, however, remain unchallenged for long; in 1950 the German astronomer E. K. Rabe, then working in the United States, found from a reduction of all observations made between 1926 and 1945—a feat facilitated by the advent of automatic computers—that a more accurate value was 8.7984ʺ ±0.0004ʺ, significantly larger than the value deduced by Spencer Jones.

A postscript to the age-long quest for determination of the solar parallax was added after 1961 by a completely different technique, based on the direct measurements by radar of the distance to Venus. The most recent (1967) value of the parallax—8.79410ʺ ±0.0001ʺ—rendering the semimajor axis of the terrestrial orbit close to 149,597,890 kilometers (with an uncertainty of a few units of the penultimate digit), is more than ten times as accurate as Spencer Jones’s result; and its remaining error hinges, in fact, on limitations of the present knowledge of the velocity of light.

All considered, Spencer Jones’s principal original contributions suffered—as did those of many others—from the fact that they were concerned with problems eminently suitable for treatment by automatic computing machinery, but were carried out ten years or so before its advent.

BIBLIOGRAPHY

A complete bibliography of papers published (alone, or jointly) by H. Spencer Jones since 1913 includes 59 separate entries; their sequence terminated rather abruptly in 1945. Many of these papers dealt with subjects of more routine nature (reporting on work carried out at observatories under Spencer Jones’s direction). Some of those which should remain of permanent interest for the historian of science are “The Rotation of the Earth and Secular Accelerations of the Sun, Moon, and Planets,” in Monthly Notices of the Royal Astronomical Society, 99 (1939), 541; and “The Solar Parallax and the Mass of the Moon From Observations of Eros at the Opposition of 1931,” in Memoirs of the Royal Astronomical Society, 76, pt. 2 (1941), 11–66. See also his earlier papers on the motion and figure of the moon, in Monthly Notices of the Royal Astronomical Society, 97 (1937), 406; and his redetermination of the constant of nutation, ibid., 98 (1938, 440; and 99 (1939), 211.

ZdenĚk Kopal

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