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James Bradley

English Astronomer Royal (–)

For other people named James Bradley, see James Bradley (disambiguation).

James BradleyFRS (September – 13 July ) was an English astronomer and priest who served as the third Astronomer Royal from He is best known for two fundamental discoveries in astronomy, the aberration of light (–), and the nutation of the Earth's axis (–).

These two discoveries were called "the most brilliant and useful of the century" by Jean Baptiste Joseph Delambre, historian of astronomy, mathematical astronomer and director of the Paris Observatory. In his History of astronomy in the 18th century (), Delambre stated:[1]

"It is to these two discoveries by Bradley that we owe the exactness of modern astronomy.&#; This double service assures to their discoverer the most distinguished place (after Hipparchus and Kepler) above the greatest astronomers of all ages and all countries."

Biography

Bradley was born at Sherborne, near Cheltenham in Gloucestershire, to William Bradley and Jane Pound in September [a] His nephew John was also an astronomer.[4] After attending Westwood's Grammar School[5] at Northleach in Gloucestershire,[6] he entered Balliol College, Oxford, on 15 March , and took degrees of Bachelor of Arts and Master of Arts in and respectively.

His early observations were made at the rectory of Wanstead in Essex, under the tutelage of James Pound, his uncle and a skilled astronomer. Bradley was elected a fellow of the Royal Society on 6 November [7]

He took orders on becoming vicar of Bridstow in Herefordshire in the following year, and a small sinecure living in Wales was also procured for him by his friend Samuel Molyneux.

He resigned his ecclesiastical preferments in , when appointed to the Savilian chair of astronomy at Oxford, while as reader on experimental philosophy from to , he delivered 79 courses of lectures at the Ashmolean Museum.[7]

In , Bradley was appointed to succeed Edmond Halley as Astronomer Royal; his reputation enabled him to apply successfully for a set of instruments costing £1,; and with an 8-foot quadrant completed for him in by John Bird, he accumulated at Greenwich in ten years materials of inestimable value for the reform of astronomy.

A crown pension of £ a year was conferred upon him in [7]

Bradley retired in broken health, nine years later, to the Cotswold village of Chalford in Gloucestershire.[7] He had medical attention from a local doctor, and Daniel Lysons from Oxford. He died at Skiveralls House on 13 July He was buried in the churchyard of Holy Trinity church, Minchinhampton in Gloucestershire.[6][8] His wife died before him, in He was survived by one daughter.

Works

In , Bradley measured the angular diameter of Venus with a large aerial telescope with an objective focal length of &#;ft (65&#;m).[9]

Bradley's discovery of the aberration of light was made while attempting to detect stellar parallax.[10] Bradley worked with Samuel Molyneux until Molyneux's death in , trying to measure the parallax of Gamma Draconis.

This stellar parallax ought to have shown up, if it existed at all, as a small annual cyclical motion of the apparent position of the star. However, while Bradley and Molyneux did not find the expected apparent motion due to parallax, they found instead a different and unexplained annual cyclical motion.

Shortly after Molyneux's death, Bradley realised that this was caused by what is now known as the aberration of light.[b][10] The basis on which Bradley distinguished the annual motion actually observed from the expected motion due to parallax, was that its annual timetable was different.

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    • Calculation showed that if there had been any appreciable motion due to parallax, then the star should have reached its most southerly apparent position in December, and its most northerly apparent position in June. What Bradley found instead was an apparent motion that reached its most southerly point in March, and its most northerly point in September; and that could not be accounted for by parallax: the cause of a motion with the pattern actually seen was at first obscure.

    A story has often been told, probably apocryphally, that the solution to the problem eventually occurred to Bradley while he was in a sailing boat on the River Thames. He noticed that when the boat turned about, a small flag at the top of the mast (a telltale) changed its direction, even though the wind had not changed; the only thing that had changed was the direction and speed of the boat.

    Bradley worked out the consequences of supposing that the direction and speed of the earth in its orbit, combined with a consistent speed of light from the star, might cause the apparent changes of stellar position that he observed. He found that this fitted the observations well, and also gave an estimate for the speed of light, and showed that the stellar parallax, if any, with extremes in June and December, was far too small to measure at the precision available to Bradley.

    (The smallness of any parallax, compared with expectations, also showed that the stars must be many times more distant from the Earth than anybody had previously believed.)

    This discovery of what became known as the aberration of light was, for all realistic purposes, conclusive evidence for the movement of the Earth, and hence for the correctness of Aristarchus', Copernicus', and Kepler's theories; it was announced to the Royal Society in January [11] The theory of the aberration also gave Bradley a means to improve on the accuracy of the previous estimate of the speed of light, which had previously been estimated by the work of Ole Rømer and others.[10] Bradley's published value for the speed of light, expressed as a time of 8 minutes 12 seconds for light to travel the distance between the Earth and the Sun, was only about % too high, and was the first accurate determination of this fundamental physical constant.

    The earliest observations upon which the discovery of the aberration was founded were made at Molyneux's house on Kew Green, and were continued at the house of Bradley's uncle James Pound in Wanstead, Essex. After publication of his work on the aberration, Bradley continued to observe, to develop and check his second major discovery, the nutation of the Earth's axis, but he did not announce this in print until 14 February , when he had tested its reality by minute observations during an entire revolution ( years) of the moon's nodes.[12]

    The publication of Bradley's observations was delayed by disputes about their ownership; but they were finally issued by the Clarendon Press, Oxford, in two folio volumes (, ).

    The insight and industry of the German astronomer Friedrich Wilhelm Bessel were, however, needed for the development of their fundamental importance.[7][13]

    Notes

    References

    1. ^Histoire de l'astronomie au dix-huitième siècle, p.&#; (edited by Claude-Louis Mathieu, and published by Bachelier, Paris, ).

      See also pp. xvii and

    2. ^Hirshfeld, Alan ().

      Steve garvey Oxford: OUP, James Bay. James Avery Craftsman, Inc. For other people named James Bradley, see James Bradley disambiguation.

      "Bradley, James". In Hockey, Thomas; Trimble, Virginia; Williams, Thomas R. (eds.). Biographical Encyclopedia of Astronomers. New York: Springer Publishing. pp.&#;– doi/ ISBN&#;.

    3. ^Vickery, Bruce ().

    4. James bradley for president
    5. James bradley ca
    6. Sarah sun liew

      7. "James Bradley and others - fact checked!". Astronomical Society of Edinburgh. Retrieved 28 June

    7. ^Kennett, Carolyn (). "An Astronomer's Summer Outing to the Lizard, Cornwall, ". The Antiquarian Astronomer. 9. Society for the History of Astronomy: 4– BibcodeAntAsK.
    8. ^Rawlinson, Caroline ().

      "Royal Star Gazer Was a Pupil". History of Westwood's Grammar School. Westwood's Grammar School Group. Retrieved 24 October

    9. ^ abWilliams, Mari E.&#;W. "Bradley, James (bap. , d.

      Biography of james bradley By detecting and measuring this, he would be able to calculate the spheroidicity of the Earth, thus settling the dispute between Newtonians and followers of Jacques Cassini about whether the Earth was flattened or elongated at the poles. Gamma Draconis was specifically chosen since it crosses the meridian almost overhead, thus allowing observers to ignore any problems caused by refraction of light through the earth's atmosphere. Further Reading There is a thorough biography of Bradley in the preface to Stephen Peter Rigaud's edition of Bradley's Miscellaneous Works and Correspondence , which is the source of practically all the short notices of Bradley's life that have appeared since. University of Oxford Ashmolean Museum.

      )". Oxford Dictionary of National Biography. Ed. H.&#;C.&#;G. Matthew and Brian Harrison. Oxford: OUP, Online ed.

      Jon elist His influence however, stretched far wider than just over the tracking of the fixed stars. Matthew and Brian Harrison. He made laudable attempts at the very difficult feat of measuring the diameters of Venus, Mars, Jupiter, and of Saturn and its ring system, a task that taxed the resources of astronomers with much larger and better telescopes a century and a half later. References [ edit ].

      Ed. Lawrence Goldman. Oct. 18 Nov. <>.

    10. ^ abcde&#;One or more of the preceding sentences&#;incorporates text from a publication now in the public domain:&#;Chisholm, Hugh, ed. (). "Bradley, James".

      Encyclopædia Britannica. Vol.&#;4 (11th&#;ed.). Cambridge University Press. p.&#;

    11. ^Bradley was buried at the parish church in Minchinhampton, Gloucestershire. See Stratford, Joseph ().

      James bradley for president: He died at Skiveralls House on 13 July Within a few days Molyneux and Bradley detected a small but increasing deviation of the star, a displacement too large and in the wrong direction to be due to its parallax. A hard worker, he was able to endure long hours of observing and intensive calculating with no apparent ill effects. Astronomers Royal.

      Gloucestershire Biographical Notes. Gloucester: "Journal" Office. p.&#;

    12. ^This paragraph is adapted from the edition of the Encyclopædia Britannica.
    13. ^ abcHirshfeld, Alan (). Parallax: The Race to Measure the Cosmos.

      New York: Henry Holt. ISBN&#;.

    14. ^Bradley, James (). "A Letter from the Reverend Mr. James Bradley Savilian Professor of Astronomy at Oxford, and F.R.S. to Dr. Edmond Halley Astronom. Reg. &c. Giving an Account of a New Discovered Motion of the Fix'd Stars".

      James bradley ca senate James Bay. James Brindley. University of Oxford Ashmolean Museum. Writings on Bradley or his work are G.

      Philosophical Transactions of the Royal Society of London. 35: – On pp. – Bradley explains how the finite speed of light causes distant stars to appear to move along an elliptical orbit. On p. he presents his estimate of the speed of light.

    15. ^Bradley, James (). "A letter to the Right honourable George Earl of Macclesfield concerning an apparent motion observed in some of the fixed stars".

      Philosophical Transactions of the Royal Society of London. 45 (): 1–

    16. ^F. W. Bessel (): Fundamenta Astronomiae pro anno MDCCLV deducta ex observationibus viri incomparabilis James Bradley in specula astronomica Grenovicensi, per annos – institutis, Königsberg.

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