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James Bradley (1692–1762) was an English
astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either ...
and
priest A priest is a religious leader authorized to perform the sacred rituals of a religion, especially as a mediatory agent between humans and one or more deities. They also have the authority or power to administer religious rites; in partic ...
who served as the third
Astronomer Royal Astronomer Royal is a senior post in the Royal Households of the United Kingdom. There are two officers, the senior being the Astronomer Royal dating from 22 June 1675; the junior is the Astronomer Royal for Scotland dating from 1834. The post ...
from 1742. He is best known for two fundamental discoveries in astronomy, the aberration of light (1725–1728), and the nutation of the Earth's axis (1728–1748). These two discoveries were called "the most brilliant and useful of the century" by
Jean Baptiste Joseph Delambre Jean Baptiste Joseph, chevalier Delambre (19 September 1749 – 19 August 1822) was a French mathematician, astronomer, historian of astronomy, and geodesist. He was also director of the Paris Observatory, and author of well-known books on t ...
, historian of astronomy, mathematical astronomer and director of the
Paris Observatory The Paris Observatory (french: Observatoire de Paris ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centers in the world. Its histo ...
. In his ''History of astronomy in the 18th century'' (1821), Delambre stated:
"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 Hipparchus (; el, Ἵππαρχος, ''Hipparkhos'';  BC) was a Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry, but is most famous for his incidental discovery of the precession of the e ...
and
Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws o ...
) above the greatest astronomers of all ages and all countries."


Biography

Bradley was born at
Sherborne Sherborne is a market town and civil parish in north west Dorset, in South West England. It is sited on the River Yeo, on the edge of the Blackmore Vale, east of Yeovil. The parish includes the hamlets of Nether Coombe and Lower Clatcombe. ...
, near
Cheltenham Cheltenham (), also known as Cheltenham Spa, is a spa town and borough on the edge of the Cotswolds in the county of Gloucestershire, England. Cheltenham became known as a health and holiday spa town resort, following the discovery of mineral s ...
in
Gloucestershire Gloucestershire ( abbreviated Glos) is a county in South West England. The county comprises part of the Cotswold Hills, part of the flat fertile valley of the River Severn and the entire Forest of Dean. The county town is the city of ...
, to William Bradley and Jane Pound in September 1692. His nephew John was also an astronomer. After attending Westwood's Grammar School at Northleach in Gloucestershire, he entered
Balliol College, Oxford Balliol College () is one of the constituent colleges of the University of Oxford in England. One of Oxford's oldest colleges, it was founded around 1263 by John I de Balliol, a landowner from Barnard Castle in County Durham, who provided the ...
, on 15 March 1711, and took degrees of
Bachelor of Arts Bachelor of arts (BA or AB; from the Latin ', ', or ') is a bachelor's degree awarded for an undergraduate program in the arts, or, in some cases, other disciplines. A Bachelor of Arts degree course is generally completed in three or four yea ...
and
Master of Arts A Master of Arts ( la, Magister Artium or ''Artium Magister''; abbreviated MA, M.A., AM, or A.M.) is the holder of a master's degree awarded by universities in many countries. The degree is usually contrasted with that of Master of Science. Tho ...
in 1714 and 1717 respectively. His early observations were made at the rectory of
Wanstead Wanstead () is a town in East London, England, in the London Borough of Redbridge. It borders South Woodford to the north, Redbridge to the east and Forest Gate to the south, with Leytonstone and Walthamstow to the west. It is located 8 mi ...
in
Essex Essex () is a Ceremonial counties of England, county in the East of England. One of the home counties, it borders Suffolk and Cambridgeshire to the north, the North Sea to the east, Hertfordshire to the west, Kent across the estuary of the Riv ...
, under the tutelage of James Pound, his uncle and a skilled astronomer. Bradley was elected a fellow of the
Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...
on 6 November 1718. He took orders on becoming vicar of Bridstow in
Herefordshire Herefordshire () is a county in the West Midlands of England, governed by Herefordshire Council. It is bordered by Shropshire to the north, Worcestershire to the east, Gloucestershire to the south-east, and the Welsh counties of Monmouths ...
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 1721, when appointed to the Savilian chair of astronomy at
Oxford Oxford () is a city in England. It is the county town and only city of Oxfordshire. In 2020, its population was estimated at 151,584. It is north-west of London, south-east of Birmingham and north-east of Bristol. The city is home to the ...
, while as reader on experimental philosophy from 1729 to 1760, he delivered 79 courses of lectures at the
Ashmolean Museum The Ashmolean Museum of Art and Archaeology () on Beaumont Street, Oxford, England, is Britain's first public museum. Its first building was erected in 1678–1683 to house the cabinet of curiosities that Elias Ashmole gave to the University o ...
. In 1742, Bradley was appointed to succeed
Edmond Halley Edmond (or Edmund) Halley (; – ) was an English astronomer, mathematician and physicist. He was the second Astronomer Royal in Britain, succeeding John Flamsteed in 1720. From an observatory he constructed on Saint Helena in 1676–77, H ...
as Astronomer Royal; his reputation enabled him to apply successfully for a set of instruments costing £1,000; and with an 8-foot quadrant completed for him in 1750 by John Bird, he accumulated at
Greenwich Greenwich ( , ,) is a town in south-east London, England, within the ceremonial county of Greater London. It is situated east-southeast of Charing Cross. Greenwich is notable for its maritime history and for giving its name to the Greenwich ...
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. Bradley retired in broken health, nine years later, to the Cotswold village of Chalford in Gloucestershire. He had medical attention from a local doctor, and Daniel Lysons from Oxford. He died at Skiveralls House on 13 July 1762. He was buried in the churchyard of Holy Trinity church, Minchinhampton in Gloucestershire. His wife died before him, in 1757. He was survived by one daughter.


Works

In 1722 Bradley measured the angular diameter of
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...
with a large
aerial telescope An aerial telescope is a type of very long focal length refracting telescope, built in the second half of the 17th century, that did not use a tube. Instead, the objective was mounted on a pole, tree, tower, building or other structure on a swive ...
with an objective focal length of . Bradley's discovery of the aberration of light was made while attempting to detect stellar
parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby object ...
. Bradley worked with Samuel Molyneux until Molyneux's death in 1728, trying to measure the parallax of
Gamma Draconis Gamma Draconis (γ Draconis, abbreviated Gamma Dra, γ Dra), formally named Eltanin , is a star in the northern constellation of Draco. Contrary to its gamma-designation (historically third-ranked), it is the brightest star in Draco at ...
. 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. 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 the
River Thames The River Thames ( ), known alternatively in parts as the The Isis, River Isis, is a river that flows through southern England including London. At , it is the longest river entirely in England and the Longest rivers of the United Kingdom, se ...
. 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 Nicolaus Copernicus (; pl, Mikołaj Kopernik; gml, Niklas Koppernigk, german: Nikolaus Kopernikus; 19 February 1473 – 24 May 1543) was a Renaissance polymath, active as a mathematician, astronomer, and Catholic canon, who formulat ...
', and Kepler's theories; it was announced to the Royal Society in January 1729. The theory of the aberration also gave Bradley a means to improve on the accuracy of the previous estimate of the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit fo ...
, which had previously been estimated by the work of
Ole Rømer Ole Christensen Rømer (; 25 September 1644 – 19 September 1710) was a Danish astronomer who, in 1676, made the first measurement of the speed of light. Rømer also invented the modern thermometer showing the temperature between two fi ...
and others. 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 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 1748, when he had tested its reality by minute observations during an entire revolution (18.6 years) of the moon's nodes. The publication of Bradley's observations was delayed by disputes about their ownership; but they were finally issued by the
Clarendon Press Oxford University Press (OUP) is the university press of the University of Oxford. It is the largest university press in the world, and its printing history dates back to the 1480s. Having been officially granted the legal right to print books ...
, Oxford, in two folio volumes (1798, 1805). The insight and industry of Friedrich Wilhelm Bessel were, however, needed for the development of their fundamental importance.F. W. Bessel (1818): ''Fundamenta Astronomiae pro anno MDCCLV deducta ex observationibus viri incomparabilis James Bradley in specula astronomica Grenovicensi, per annos 1750–1762 institutis'', Königsberg.


Editions

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Notes


References


External links


Online catalogue of Bradley's working papers (part of the Royal Greenwich Observatory Archives held at Cambridge University Library)
{{DEFAULTSORT:Bradley, James 1693 births 1762 deaths 18th-century British astronomers 18th-century English Anglican priests Alumni of Balliol College, Oxford Astronomers Royal Church of England priests Fellows of the Royal Society People from Chalford People from Cotswold District Recipients of the Copley Medal Savilian Professors of Astronomy