James Bradley | |
|---|---|
 Bradley byThomas Hudson,c. 1744 | |
| Born | September 1692 Sherborne,Gloucestershire, England |
| Died | 13 July 1762(1762-07-13) (aged 69) Chalford,Gloucestershire, England |
| Alma mater | Balliol College, Oxford |
| Known for | |
| Children | 1 daughter |
| Awards | Copley Medal (1748) |
| Scientific career | |
| Fields | Astronomy |
| Institutions | |
| Ecclesiastical career | |
| Religion | Christianity (Anglican) |
| Church | Church of England |
| Ordained |
|
| 3rd Astronomer Royal | |
| In office 1742–1762 | |
| Preceded by | Edmond Halley |
| Succeeded by | Nathaniel Bliss |
James BradleyFRS (September 1692 – 13 July 1762) was an Englishastronomer andpriest who served as the thirdAstronomer Royal from 1742. He is best known for two fundamental discoveries in astronomy, theaberration of light (1725–1728), and thenutation of the Earth's axis (1728–1748).
These two discoveries were called "the most brilliant and useful of the century" byJean Baptiste Joseph Delambre, historian of astronomy, mathematical astronomer and director of theParis Observatory. In hisHistory of astronomy in the 18th century (1821), 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 (afterHipparchus andKepler) above the greatest astronomers of all ages and all countries."
Bradley was born atSherborne, nearCheltenham inGloucestershire, to William Bradley and Jane Pound in September 1692.[a] His nephew John was also an astronomer.[4] After attending Westwood's Grammar School[5] atNorthleach in Gloucestershire,[6] he enteredBalliol College, Oxford, on 15 March 1711, and took degrees ofBachelor of Arts andMaster of Arts in 1714 and 1717 respectively. His early observations were made at the rectory ofWanstead inEssex, under the tutelage ofJames Pound, his uncle and a skilled astronomer. Bradley was elected a fellow of theRoyal Society on 6 November 1718.[7]
He took orders on becoming vicar ofBridstow inHerefordshire in the following year, and a small sinecure living in Wales was also procured for him by his friendSamuel Molyneux. He resigned his ecclesiastical preferments in 1721, when appointed to theSavilian chair of astronomy atOxford. As reader on experimental philosophy from 1729 to 1760, he delivered 79 courses of lectures at theAshmolean Museum.[7]
In 1742, Bradley was appointed to succeedEdmond Halley as Astronomer Royal; his reputation enabled him to apply successfully for a set of instruments costing £1,000; and with an 8-footquadrant completed for him in 1750 byJohn Bird, he accumulated atGreenwich in ten years materials of inestimable value for the reform of astronomy. A crown pension of £250 a year was conferred upon him in 1752.[7]
Bradley retired in broken health, nine years later, to the Cotswold village ofChalford in Gloucestershire.[7] He had medical attention from a local doctor, andDaniel Lysons from Oxford. He died at Skiveralls House on 13 July 1762 and was buried in the churchyard of Holy Trinity church,Minchinhampton in Gloucestershire.[6][8] His wife died before him, in 1757 and he was survived by one daughter.
In 1722, Bradley measured the angular diameter ofVenus with a largeaerial telescope with anobjective focal length of 212 ft (65 m).[9]
Bradley's discovery of theaberration of light was made while attempting to detect stellarparallax.[10] Bradley worked withSamuel Molyneux until Molyneux's death in 1728, trying to measure the parallax ofGamma 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 theaberration 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.
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 theRiver 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[11] was, for all realistic purposes, conclusive evidence for the movement of the Earth, and hence for the correctness ofAristarchus',Copernicus', andKepler's theories; it was announced to the Royal Society in January 1729.[12]
The theory of the aberration also gave Bradley a means to improve on the accuracy of the previous estimate of thespeed of light, which had previously been estimated by the work ofOle 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 1.3% 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 atMolyneux's house on Kew Green, and were continued at the house of Bradley's uncle James Pound inWanstead, Essex. After publication of his work on the aberration, Bradley continued to observe, to develop and check his second major discovery, thenutation of the Earth's axis, but he did not announce this in print until 14 February 1748, when he had tested its reality by minute observations during an entire revolution (18.6 years) of the moon's nodes.[13]
The publication of Bradley's observations was delayed by disputes about their ownership; but they were finally issued by theClarendon Press, Oxford, in two folio volumes (1798, 1805). The insight and industry of the German astronomerFriedrich Wilhelm Bessel were, however, needed for the development of their fundamental importance.[7][14]
One or more of the preceding sentences incorporates text from a publication now in thepublic domain: Chisholm, Hugh, ed. (1911). "Bradley, James".Encyclopædia Britannica. Vol. 4 (11th ed.). Cambridge University Press. p. 373.
| |
Savilian Professors | |
|---|---|
Chairs established bySir Henry Savile | |
| Savilian Professors of Astronomy |
|
| Savilian Professors of Geometry |
|
Media from CommonsQuotations from WikiquoteTexts from Wikisource
Data from Wikidata
