James Bradley
Encyclopedia
James Bradley FRS was an English astronomer
and served as Astronomer Royal
from 1742, succeeding Edmund Halley. 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 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 (1821), because "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."
, near Cheltenham
in Gloucestershire
, in March 1693. He entered Balliol College, Oxford
, on 15 March 1711, and took degrees of B.A. and M.A. in 1714 and 1717 respectively. His early observations were made at the rectory of Wanstead
in Essex, under the tutelage of the Rev. James Pound
, his uncle and a skilled astronomer. Bradley was elected a fellow of the Royal Society
on 6 November 1718.
He took orders on becoming vicar of Bridstow
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
, while as reader on experimental philosophy
from 1729 to 1760, he delivered 79 courses of lectures at the Ashmolean Museum
.
In 1722 Bradley measured the diameter of Venus
with a large aerial telescope
with an objective
focal length of 212 ft (64.6 m).
Bradley's discovery of the aberration of light
was made while attempting to detect stellar parallax
. Bradley worked with Samuel Molyneux
until Molyneux's death in 1728 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 realized 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, 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 little flag at the top of the mast 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
' and Kepler's theories; it was announced to the Royal Society in January 1729 (Phil. Trans.
xxxv. 637). 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 shown to be finite by the work of Ole Rømer and others.
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 (Phil. Trans. xlv. I), when he had tested its reality by minute observations during an entire revolution (18.6 years) of the moon's nodes.
In 1742, Bradley had been appointed to succeed Edmund Halley as Astronomer Royal; his enhanced reputation enabled him to apply successfully for a set of instruments costing GB£1,000; and with an 8-foot quadrant completed for him in 1750 by John Bird
, he accumulated at Greenwich
in ten years materials of inestimable value for the reform of astronomy. A crown pension of GB£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
, where he died at Skiveralls House on 13 July 1762. The publication of his observations was delayed by disputes about their ownership; but they were finally issued by the Clarendon Press, 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.
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
and served as Astronomer Royal
Astronomer Royal
Astronomer Royal is a senior post in the Royal Household of the Sovereign of the United Kingdom. There are two officers, the senior being the Astronomer Royal dating from 22 June 1675; the second is the Astronomer Royal for Scotland dating from 1834....
from 1742, succeeding Edmund Halley. He is best known for two fundamental discoveries in astronomy, the aberration of light
Aberration of light
The aberration of light is an astronomical phenomenon which produces an apparent motion of celestial objects about their real locations...
(1725–1728), and the nutation of the Earth's axis
Nutation
Nutation is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behavior of a mechanism...
(1728–1748). These 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 (1821), because "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
Hipparchus, the common Latinization of the Greek Hipparkhos, can mean:* Hipparchus, the ancient Greek astronomer** Hipparchic cycle, an astronomical cycle he created** Hipparchus , a lunar crater named in his honour...
and Kepler) above the greatest astronomers of all ages and all countries."
Biography
Bradley was born at SherborneSherborne, Gloucestershire
Sherborne is a village and civil parish almost east of Northleach in Gloucestershire. Sherborne is a linear village, extending more than a mile along the valley of Sherborne Brook, a tributary of the River Windrush....
, near Cheltenham
Cheltenham
Cheltenham , also known as Cheltenham Spa, is a large spa town and borough in Gloucestershire, on the edge of the Cotswolds in the South-West region of England. It is the home of the flagship race of British steeplechase horse racing, the Gold Cup, the main event of the Cheltenham Festival held...
in Gloucestershire
Gloucestershire
Gloucestershire 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....
, in March 1693. He entered Balliol College, Oxford
Balliol College, Oxford
Balliol College , founded in 1263, is one of the constituent colleges of the University of Oxford in England but founded by a family with strong Scottish connections....
, on 15 March 1711, and took degrees of B.A. and M.A. in 1714 and 1717 respectively. His early observations were made at the rectory of Wanstead
Wanstead
Wanstead is a suburban area in the London Borough of Redbridge, North-East London. The main road going through Wanstead is the A12. The name is from the Anglo-Saxon words wænn and stede, meaning "settlement on a small hill"....
in Essex, under the tutelage of the Rev. James Pound
James Pound
-Life:He was the son of John Pound, of Bishop's Canning, Wiltshire, where he was born . He matriculated at St. Mary Hall, Oxford, on 16 March 1687; graduated B.A. from Hart Hall on 27 February 1694, and M.A...
, his uncle and a skilled astronomer. Bradley was elected a fellow of the Royal Society
Royal Society
The Royal Society of London for Improving Natural Knowledge, known simply as the Royal Society, is a learned society for science, and is possibly the oldest such society in existence. Founded in November 1660, it was granted a Royal Charter by King Charles II as the "Royal Society of London"...
on 6 November 1718.
He took orders on becoming vicar of Bridstow
Bridstow
Bridstow is a village and civil parish in Herefordshire, England. The village lies 2 km west of Ross-on-Wye and 17 km southeast of Hereford. The parish is bounded to the east and south by the River Wye. The A40 road linking the M50 motorway to South Wales runs through the parish,...
in the following year, and a small sinecure living in Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
was also procured for him by his friend Samuel Molyneux
Samuel Molyneux
Samuel Molyneux FRS , son of William Molyneux, was an 18th-century member of the British parliament from Kew and an amateur astronomer whose work with James Bradley attempting to measure stellar parallax led to the discovery of the aberration of light...
. He resigned his ecclesiastical preferments in 1721, when appointed to the Savilian chair of astronomy
Savilian Chair of Astronomy
The Savilian Chair of Astronomy at the University of Oxford in England was founded in 1619 and is named after Sir Henry Savile. The Professor is a Fellow of New College....
at Oxford
University of Oxford
The University of Oxford is a university located in Oxford, United Kingdom. It is the second-oldest surviving university in the world and the oldest in the English-speaking world. Although its exact date of foundation is unclear, there is evidence of teaching as far back as 1096...
, while as reader on experimental philosophy
Philosophy
Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...
from 1729 to 1760, he delivered 79 courses of lectures at the Ashmolean Museum
Ashmolean Museum
The Ashmolean Museum on Beaumont Street, Oxford, England, is the world's first university museum...
.
In 1722 Bradley measured the diameter of Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
with a large aerial telescope
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 swivel ball-joint. The observer stood on the ground and held...
with an objective
Objective (optics)
In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...
focal length of 212 ft (64.6 m).
Bradley's discovery of the aberration of light
Aberration of light
The aberration of light is an astronomical phenomenon which produces an apparent motion of celestial objects about their real locations...
was made while attempting to detect stellar parallax
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. The term is derived from the Greek παράλλαξις , meaning "alteration"...
. Bradley worked with Samuel Molyneux
Samuel Molyneux
Samuel Molyneux FRS , son of William Molyneux, was an 18th-century member of the British parliament from Kew and an amateur astronomer whose work with James Bradley attempting to measure stellar parallax led to the discovery of the aberration of light...
until Molyneux's death in 1728 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 realized that this was caused by what is now known as the aberration of light
Aberration of light
The aberration of light is an astronomical phenomenon which produces an apparent motion of celestial objects about their real locations...
. 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, 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 little flag at the top of the mast 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
Aristarchus
-People:* Aristarchus of Samos , Greek astronomer and mathematician* Aristarchus of Samothrace , Greek grammarian* Aristarchus of Tegea , Greek writer* Aristarchus of Thessalonica , Eastern saint...
' and Kepler's theories; it was announced to the Royal Society in January 1729 (Phil. Trans.
Philosophical Transactions of the Royal Society
The Philosophical Transactions of the Royal Society is a scientific journal published by the Royal Society of London. It was established in 1665, making it the first journal in the world exclusively devoted to science, and it has remained in continuous publication ever since, making it the world's...
xxxv. 637). The theory of the aberration also gave Bradley a means to improve on the accuracy of the previous estimate of the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
, which had previously been shown to be finite by the work of Ole Rømer and others.
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
Nutation
Nutation is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behavior of a mechanism...
of the Earth's axis, but he did not announce this in print until 14 February 1748 (Phil. Trans. xlv. I), when he had tested its reality by minute observations during an entire revolution (18.6 years) of the moon's nodes.
In 1742, Bradley had been appointed to succeed Edmund Halley as Astronomer Royal; his enhanced reputation enabled him to apply successfully for a set of instruments costing GB£1,000; and with an 8-foot quadrant completed for him in 1750 by John Bird
John Bird (astronomer)
John Bird , the great mathematical instrument maker, was born at Bishop Auckland. He worked in London for Jeremiah Sisson, and by 1745 he had his own business in the Strand. Bird was commissioned to make a brass quadrant 8 feet across for the Royal Observatory at Greenwich, where it is still...
, he accumulated at Greenwich
Greenwich
Greenwich is a district of south London, England, located in the London Borough of Greenwich.Greenwich is best known for its maritime history and for giving its name to the Greenwich Meridian and Greenwich Mean Time...
in ten years materials of inestimable value for the reform of astronomy. A crown pension of GB£250 a year was conferred upon him in 1752.
Bradley retired in broken health, nine years later, to the Cotswold village of Chalford
Chalford
Chalford is a village in the Frome Valley of the Cotswolds in Gloucestershire, England. It is about 8 km upstream of Stroud. It gives its name to Chalford parish, which covers the villages of Chalford, Chalford Hill, France Lynch, Bussage and Brownshill, spread over 2 mi² of the...
in Gloucestershire
Gloucestershire
Gloucestershire 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....
, where he died at Skiveralls House on 13 July 1762. The publication of his observations was delayed by disputes about their ownership; but they were finally issued by the Clarendon Press, 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.
Further reading
- Rigaud's Memoir, prefixed to Miscellaneous Works and Correspondence of James Bradley, D.D. (Oxford, 1832),
- New and General Biographical Dictionary, xii. 54 (1767)
- Biog. Brit. (KippisAndrew KippisAndrew Kippis was an English nonconformist clergyman and biographer.The son of Robert Kippis, a silk-hosier, he was born at Nottingham. Having gone to school at Sleaford in Lincolnshire he passed at the age of sixteen to the Dissenting academy at Northampton, of which Dr Philip Doddridge was then...
) - Fouchy's Eloge, Paris Memoirs (1762), p. 231 (Histoire)
- Delambre's Hist. de l’astronomie au 18e siècle, p. 413.