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JOHN JACKSON was born on 1887 February 11, in Paisley. At the age of 16 he entered Glasgow University. At that time his principal interest was chemistry but he was quickly attracted to mathematics and when he graduated M.A. In 1907 he was top of his year in mathematics. The following year he took the B.Sc. degree with special distinctions in mathematics, natural philosophy, astronomy and chemistry. Many of the students taking honours in mathematics attended the astronomy classes conducted by Professor Ludwig Becker. Under Becker, Jackson received a thorough grounding in spherical and dynamical astronomy, learned the adjustment and use of the principal astronomical instruments, and calculated the ephemeris of a minor planet from the elements of its orbit. The teaching of Professor Becker turned his thoughts toward an astronomical career. He had been appointed Ewing Fellow in 1907 but as there seemed no hope of obtaining an appointment from Glasgow, he decided to go to Cambridge if possible. He was awarded a major scholarship at Trinity College in 1909. In his second year he was given the Sheepshanks Exhibition. He obtained a first class in the Mathematical Tripos, Part I, in 1910 and in Part II (with distinction in advanced subjects) in 1912, being then awarded the Tyson Gold Medal in Astronomy. He was awarded a Smith's Prize in 1914.
Jackson had been appointed a McKinnon Research Student of the Royal Society in 1913, taking as the subject of his research the motion of the eighth satellite of Jupiter, which had been discovered by Melotte at Greenwich in 1908. His first published papers were concerned with retrograde satellite orbits and with the discordances between the observed and predicted positions of Jupiter VIII. In 1914 he was appointed Chief Assistant at the Royal Observatory, Greenwich, and resigned the McKinnon Studentship. At that time the staff of the Observatory was depleted, most of the younger members being absent on war service. Jackson undertook a large amount of routine observing, particularly with the Airy Transit Circle. Throughout his astronomical career he continued to take pleasure in observing. At about this time Jackson collaborated with F. J. M. Stratton in editing Vol. V of the Scientific Papers of Sir George Darwin, published by the Cambridge University Press in 1916
In 1917 he was granted a commission in the Royal Engineers, with a view to taking part in sound ranging, for which he attended courses at Chatham and Southampton. The critical position of the Allied armies in the spring of 1918 resulted in his being posted to the Fourth Army in France as a Trigonometric Survey Officer, for which his knowledge and experience were well suited.
In February 1919, Jackson returned from France to Greenwich and, once settled down to systematic work, Sir Frank Dyson entrusted him with the preparation for publication of the observations of double stars that had been made at Greenwich with the 28-inch refractor in the years 1893–1919. This led to a lifelong interest in double stars. He determined the orbits of a number of binary systems and, in collaboration with H.H. Turner, derived the hypothetical parallaxes of 556 double stars from the observed orbital motion, assuming the mass of the system to be known. These parallaxes were used to calculate the cross-linear velocities from the proper motions and to derive the speed and direction of the solar motion. For 15 years, beginning in 1919, Jackson contributed the double-star notes to the Society's Council Reports.
When Jackson went to Greenwich, the standard clock was a Dent regulator. Wireless time signals had made possible the intercomparison of time as determined at Greenwich, Paris, and Washington and had shown the need for a standard clock with a more uniform rate. In November 1924, Sir Frank Dyson obtained a Shortt Free Pendulum Clock, and it at once became apparent that this clock gave a much better performance than any previous clock. In conjunction with W. Bowyer, who was in charge of the time service at Greenwich, Jackson studied the performance of the clock. The dependence of its rate on temperature and pressure was determined. He showed that the optimum pressure at which the free pendulum should be maintained was 18 mm, as the change in the arc of vibration caused by a change in pressure then resulted in a change in rate that was almost exactly compensated by the change in rate resulting from the change in air resistance. He believed that most of the irregularities in the performance of the clock could be explained by the fact that real time, which is affected by nutation is not uniform. This was the first time that a clock was found to be more uniform than sidereal time. It led to the introduction of mean sidereal time, freed from the effect of nutation, in timekeeping. Jackson investigated the possibility that it might be possible to use free pendulum clocks to investigate the irregularities in the Earth's rate of rotation but concluded that the clocks were not sufficiently accurate for this purpose.
An extensive piece of work undertaken by Jackson at Greenwich in collaboration with H. Knox-Shaw at the Radcliffe Observatory, Oxford, was the reduction of the observations made during the years 1774–1798 at the Radcliffe Observatory by Thomas Hornsby, the Savilian Professor of Astronomy and the first Radcliffe Observer Hornsby's observations were known to be of high quality for their era, but Hornsby had been so assiduous in observing that he could find no time for the reduction of the observations, of which no use had consequently been made. The purpose of the reduction was to provide a check on the accuracy of the fundamental star system in use in the 20th century. The ephemerides in the Nautical Almanacs of the era were not of the required accuracy. Jackson undertook the task of computing, from Newcomb's Tables of the Sun, the positions of the Sun for every Greenwich Mean Noon and every Oxford Apparent Noon for the years 1774-1798, and the apparent places of the stars from the best tables available. Knox-Shaw, assisted by W. H. Robinson, used the observations to determine the errors of the instrument, apply the necessary corrections, and deduce the apparent places of the objects observed. The positions of the planets from Venus to Saturn were calculated by A. C. D. Crommelin and B. F. Bawtree. The investigation proved that the motion of the equinox adopted by Newcomb and the proper motions in the Boss Preliminary, General Catalogue were substantially correct. Jackson undertook the comparison of theory and observations for Mercury and verified the motion of the perihelion of Mercury adopted by Newcomb, which is in accordance with the motion required by generalized relativity theory.
In 1933, Jackson was appointed H. M. Astronomer at the Cape, in succession to H. Spencer Jones, who had succeeded Sir Frank Dyson at Greenwich. The main programs of work at the Observatory had been laid down. The Cape Astrographic Zones -40° to -52° had been rephotographed, and the plates measured against the original plates for the derivation of proper motions. The results were published by Jackson in two volumes giving the positions and proper motions of some 41,000 stars The introductions to these two volumes contained a detailed discussion of the data and the distribution of the stars. The results were summarized in a paper in M.N., 98, 491, 1938. An extensive program of stellar parallax determinations had been commenced in 1926, in collaboration with the Yale Southern Station in Johannesburg. At that time, there were few reliable determinations of parallaxes of southern stars. The Yale program included the brighter stars of the southern sky; the Cape program included mainly stars with appreciable proper motions. Some 400 parallaxes had been published before Jackson was appointed to the Cape. He played a large part in this program, particularly during World War II when the Observatory staff was much reduced. He took over the selection of the stars for observations and took the first few plates of each star, so as to determine the exposure time required to photograph suitable companion stars and the appropriate opening of the occulting shutter for the parallax star. He took a large share in the observations, obtaining as many as a thousand plates a year, and in the measurements and reductions. Between 1935 and 1950, four volumes of the Cape Annals were published, giving details of the measurements and of the derived parallaxes of about 1,600 stars.
Shortly before Jackson was appointed to the Cape, the determination of the positions of stars south of 30°, by means of photography with a wide-angle lens, had been commenced, and the first zone, -30° to -35°, had been photographed. The work of observation continued all the time he was at the Cape, and by the time he retired in 1950, all the zones down to -80° (except the Cape Astrographic Zones -40° to -52°) had been photographed, the positions of the comparison stars observed with the transit circle, the measurement of the plates completed to about -68°, and the results for two zones prepared for publication The plans included the determination of the magnitudes of the stars, and for this purpose a Schilt photometer had been obtained. This part of the program had not been started when Jackson arrived at the Cape. At the appointment of R. H. Stoy as Chief Assistant, the photometry program was entrusted to him, and as a result, the photometry of southern stars, which had been in a very unsatisfactory state, has now been established on firm foundations.
During Jackson's tenure as Chief Assistant at Greenwich, he took part in three expeditions to observe total eclipses of the Sun. The eclipse of June 29, 1927, was observed at Giggleswick, Yorkshire. The weather conditions for this eclipse, the first since Halley's time in which the track of totality had crossed England, were generally unfavorable, but at Giggleswick the sky was clear; totality was short and the Sun was very low, but photographs of the corona were obtained Jackson was in charge of expeditions to observe the total eclipses of 1929 May 9 at Alor Star, Malaya, when observations were spoilt by thick clouds, and of 1932 August 31 at Parent, Quebec, when clouds prevented spectroscopic observations with a slit spectroscope. Plans had been prepared for an expedition from the Royal Observatory, Greenwich, to Calvinia, Cape Province, to observe the total eclipse of 1940 October 1, the program being the determination of the displacement of stars in the vicinity of the Sun predicted by Einstein. The outbreak of World War II made it impossible to send a party from Greenwich, but the equipment was sent to the Cape and used by Jackson. The eclipse was observed in a clear sky, but unfortunately the definition proved to be very poor; the images of the stars were consequently too diffuse to permit a satisfactory determination of the displacement.
Jackson was elected a Fellow of the Society in 1913. He served as Secretary from 1923 to 1929, and as President from 1953 to 1955. He was awarded the Society's Gold Medal in 1952. He was elected a Fellow of the Royal Society in 1938. On his retirement from the Cape in 1950, he was made a Companion of the Order of the British Empire. He then returned to England and made his home in Ewell, Surrey. He died on December 9, 1958, after a brief illness. In 1920, he married Mary Beatrice Marshall, who survives him. They had one son who died shortly after birth.
H. Spencer Jones
Jackson had been appointed a McKinnon Research Student of the Royal Society in 1913, taking as the subject of his research the motion of the eighth satellite of Jupiter, which had been discovered by Melotte at Greenwich in 1908. His first published papers were concerned with retrograde satellite orbits and with the discordances between the observed and predicted positions of Jupiter VIII. In 1914 he was appointed Chief Assistant at the Royal Observatory, Greenwich, and resigned the McKinnon Studentship. At that time the staff of the Observatory was depleted, most of the younger members being absent on war service. Jackson undertook a large amount of routine observing, particularly with the Airy Transit Circle. Throughout his astronomical career he continued to take pleasure in observing. At about this time Jackson collaborated with F. J. M. Stratton in editing Vol. V of the Scientific Papers of Sir George Darwin, published by the Cambridge University Press in 1916
In 1917 he was granted a commission in the Royal Engineers, with a view to taking part in sound ranging, for which he attended courses at Chatham and Southampton. The critical position of the Allied armies in the spring of 1918 resulted in his being posted to the Fourth Army in France as a Trigonometric Survey Officer, for which his knowledge and experience were well suited.
In February 1919, Jackson returned from France to Greenwich and, once settled down to systematic work, Sir Frank Dyson entrusted him with the preparation for publication of the observations of double stars that had been made at Greenwich with the 28-inch refractor in the years 1893–1919. This led to a lifelong interest in double stars. He determined the orbits of a number of binary systems and, in collaboration with H.H. Turner, derived the hypothetical parallaxes of 556 double stars from the observed orbital motion, assuming the mass of the system to be known. These parallaxes were used to calculate the cross-linear velocities from the proper motions and to derive the speed and direction of the solar motion. For 15 years, beginning in 1919, Jackson contributed the double-star notes to the Society's Council Reports.
When Jackson went to Greenwich, the standard clock was a Dent regulator. Wireless time signals had made possible the intercomparison of time as determined at Greenwich, Paris, and Washington and had shown the need for a standard clock with a more uniform rate. In November 1924, Sir Frank Dyson obtained a Shortt Free Pendulum Clock, and it at once became apparent that this clock gave a much better performance than any previous clock. In conjunction with W. Bowyer, who was in charge of the time service at Greenwich, Jackson studied the performance of the clock. The dependence of its rate on temperature and pressure was determined. He showed that the optimum pressure at which the free pendulum should be maintained was 18 mm, as the change in the arc of vibration caused by a change in pressure then resulted in a change in rate that was almost exactly compensated by the change in rate resulting from the change in air resistance. He believed that most of the irregularities in the performance of the clock could be explained by the fact that real time, which is affected by nutation is not uniform. This was the first time that a clock was found to be more uniform than sidereal time. It led to the introduction of mean sidereal time, freed from the effect of nutation, in timekeeping. Jackson investigated the possibility that it might be possible to use free pendulum clocks to investigate the irregularities in the Earth's rate of rotation but concluded that the clocks were not sufficiently accurate for this purpose.
An extensive piece of work undertaken by Jackson at Greenwich in collaboration with H. Knox-Shaw at the Radcliffe Observatory, Oxford, was the reduction of the observations made during the years 1774–1798 at the Radcliffe Observatory by Thomas Hornsby, the Savilian Professor of Astronomy and the first Radcliffe Observer Hornsby's observations were known to be of high quality for their era, but Hornsby had been so assiduous in observing that he could find no time for the reduction of the observations, of which no use had consequently been made. The purpose of the reduction was to provide a check on the accuracy of the fundamental star system in use in the 20th century. The ephemerides in the Nautical Almanacs of the era were not of the required accuracy. Jackson undertook the task of computing, from Newcomb's Tables of the Sun, the positions of the Sun for every Greenwich Mean Noon and every Oxford Apparent Noon for the years 1774-1798, and the apparent places of the stars from the best tables available. Knox-Shaw, assisted by W. H. Robinson, used the observations to determine the errors of the instrument, apply the necessary corrections, and deduce the apparent places of the objects observed. The positions of the planets from Venus to Saturn were calculated by A. C. D. Crommelin and B. F. Bawtree. The investigation proved that the motion of the equinox adopted by Newcomb and the proper motions in the Boss Preliminary, General Catalogue were substantially correct. Jackson undertook the comparison of theory and observations for Mercury and verified the motion of the perihelion of Mercury adopted by Newcomb, which is in accordance with the motion required by generalized relativity theory.
In 1933, Jackson was appointed H. M. Astronomer at the Cape, in succession to H. Spencer Jones, who had succeeded Sir Frank Dyson at Greenwich. The main programs of work at the Observatory had been laid down. The Cape Astrographic Zones -40° to -52° had been rephotographed, and the plates measured against the original plates for the derivation of proper motions. The results were published by Jackson in two volumes giving the positions and proper motions of some 41,000 stars The introductions to these two volumes contained a detailed discussion of the data and the distribution of the stars. The results were summarized in a paper in M.N., 98, 491, 1938. An extensive program of stellar parallax determinations had been commenced in 1926, in collaboration with the Yale Southern Station in Johannesburg. At that time, there were few reliable determinations of parallaxes of southern stars. The Yale program included the brighter stars of the southern sky; the Cape program included mainly stars with appreciable proper motions. Some 400 parallaxes had been published before Jackson was appointed to the Cape. He played a large part in this program, particularly during World War II when the Observatory staff was much reduced. He took over the selection of the stars for observations and took the first few plates of each star, so as to determine the exposure time required to photograph suitable companion stars and the appropriate opening of the occulting shutter for the parallax star. He took a large share in the observations, obtaining as many as a thousand plates a year, and in the measurements and reductions. Between 1935 and 1950, four volumes of the Cape Annals were published, giving details of the measurements and of the derived parallaxes of about 1,600 stars.
Shortly before Jackson was appointed to the Cape, the determination of the positions of stars south of 30°, by means of photography with a wide-angle lens, had been commenced, and the first zone, -30° to -35°, had been photographed. The work of observation continued all the time he was at the Cape, and by the time he retired in 1950, all the zones down to -80° (except the Cape Astrographic Zones -40° to -52°) had been photographed, the positions of the comparison stars observed with the transit circle, the measurement of the plates completed to about -68°, and the results for two zones prepared for publication The plans included the determination of the magnitudes of the stars, and for this purpose a Schilt photometer had been obtained. This part of the program had not been started when Jackson arrived at the Cape. At the appointment of R. H. Stoy as Chief Assistant, the photometry program was entrusted to him, and as a result, the photometry of southern stars, which had been in a very unsatisfactory state, has now been established on firm foundations.
During Jackson's tenure as Chief Assistant at Greenwich, he took part in three expeditions to observe total eclipses of the Sun. The eclipse of June 29, 1927, was observed at Giggleswick, Yorkshire. The weather conditions for this eclipse, the first since Halley's time in which the track of totality had crossed England, were generally unfavorable, but at Giggleswick the sky was clear; totality was short and the Sun was very low, but photographs of the corona were obtained Jackson was in charge of expeditions to observe the total eclipses of 1929 May 9 at Alor Star, Malaya, when observations were spoilt by thick clouds, and of 1932 August 31 at Parent, Quebec, when clouds prevented spectroscopic observations with a slit spectroscope. Plans had been prepared for an expedition from the Royal Observatory, Greenwich, to Calvinia, Cape Province, to observe the total eclipse of 1940 October 1, the program being the determination of the displacement of stars in the vicinity of the Sun predicted by Einstein. The outbreak of World War II made it impossible to send a party from Greenwich, but the equipment was sent to the Cape and used by Jackson. The eclipse was observed in a clear sky, but unfortunately the definition proved to be very poor; the images of the stars were consequently too diffuse to permit a satisfactory determination of the displacement.
Jackson was elected a Fellow of the Society in 1913. He served as Secretary from 1923 to 1929, and as President from 1953 to 1955. He was awarded the Society's Gold Medal in 1952. He was elected a Fellow of the Royal Society in 1938. On his retirement from the Cape in 1950, he was made a Companion of the Order of the British Empire. He then returned to England and made his home in Ewell, Surrey. He died on December 9, 1958, after a brief illness. In 1920, he married Mary Beatrice Marshall, who survives him. They had one son who died shortly after birth.
H. Spencer Jones
John Jackson's obituary appeared in Journal of the Royal Astronomical Society 119:4 (1959), 345-348.