James Bradley FRS (March 1693 – 13 July 1762) was an English
astronomerAn 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...
the
Astronomer RoyalAstronomer 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. Bradley is best known for two fundamental discoveries in astronomy, the
aberration of lightThe aberration of light is an astronomical phenomenon which produces an apparent motion of celestial objects about their real locations...
(1725-28), and the
nutation of the Earth's axisNutation is a slight irregular motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight...
(1728-48). These discoveries were called "the most brilliant and useful of the century" by
Jean Baptiste Joseph DelambreJean Baptiste Joseph, chevalier Delambre was a French mathematician and astronomer. He was also director of the Paris Observatory, and author of well-known books on the history of astronomy from ancient times to the 18th century.After a childhood fever, he suffered from very sensitive eyes, and...
, 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
HipparchusHipparchus, 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 Sherborne, near
CheltenhamCheltenham , or Cheltenham Spa, is a spa town and borough in Gloucestershire, England with a population of 110,013 at the 2001 census. The inhabitants are known as "Cheltonians". Its motto is: Salubritas et Eruditio ....
in
GloucestershireGloucestershire 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, OxfordBalliol College , founded in 1263, is one of the constituent colleges of the University of Oxford in England.Traditionally, the undergraduates are amongst the most politically active in the university, and the college's alumni include three former prime ministers. H. H...
, 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 his uncle, the Rev.
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...
, himself a skilled astronomer, and was elected a fellow of the
Royal SocietyThe Royal Society of London for the Improvement of Natural Knowledge, known simply as the Royal Society, or even the Royal, is a learned society for science that was founded in 1660 and is considered by most to be the oldest such society still in existence...
on 6 November 1718.
He took orders on becoming vicar of Bridstow in the following year, and a small sinecure living in
WalesWales is a country that is part of the United Kingdom, bordered by England to its east, and the Atlantic Ocean and Irish Sea to its west. It is also an elective region of the European Union...
was also procured for him by his friend
Samuel MolyneuxSamuel 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 astronomyThe 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
OxfordThe University of Oxford , located in the UK city of Oxford, is the oldest surviving university in the English-speaking world and is regarded as one of the world's leading academic institutions. Although the exact date of foundation remains unclear, there is evidence of teaching there as far back...
, while as reader on experimental
philosophyPhilosophy is the study of general and fundamental problems concerning matters such as existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing these questions by its critical, generally systematic approach and its reliance on reasoned...
from 1729 to 1760, he delivered 79 courses of lectures at the
Ashmolean MuseumThe Ashmolean Museum on Beaumont Street, Oxford, England, is the world's first university museum...
.
In 1722 Bradley measured the diameter of
VenusVenus is the second-closest planet to 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...
with a large
aerial telescopeAn 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...
(
objectiveIn optics, an objective is the lens or mirror in a microscope, telescope, camera or other optical instrument that gathers the light coming from the object being observed, and focuses the rays to produce a real image...
focal length of 212 ft (65 m)).
Bradley's discovery of the
aberration of lightThe 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
parallaxParallax is an apparent displacement or difference of orientation 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...
. Bradley worked with
Samuel MolyneuxSamuel 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 DraconisGamma Draconis is a star in the constellation Draco. It has the traditional name Eltanin ....
.
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 lightThe 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 , on the moon* Aristarchus of Samos , Greek astronomer and mathematician* Aristarchus of Samothrace , Greek grammarian* Aristarchus of Tegea , Greek writer...
' and Kepler's theories; it was announced to the Royal Society in January 1729 (
Phil. Trans.The Philosophical Transactions of the Royal Society, or Phil. Trans., is a scientific journal published by the Royal Society.Begun in 1665, it is the oldest scientific journal printed in the English-speaking world and the second oldest in the world, after the French Journal des sçavans...
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 lightIn physics, the speed of light is a physical constant, the speed at which electromagnetic radiation, such as light, travels in free space . Its value is 299,792,458 metres per second...
, which had previously been shown to be finite by the work
Ole RømerOle Christensen Rømer was a Danish astronomer who in 1676 made the first quantitative measurements of the speed of light...
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
nutationNutation is a slight irregular motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight...
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 BirdJohn 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 preserved...
, he accumulated at
GreenwichGreenwich is a district in south-east London, England, on the south bank of the River Thames in the London Borough of Greenwich. It is best known for its maritime history and as giving its name to the Greenwich Meridian and Greenwich Mean Time.The town became the site of a Royal palace, the...
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
ChalfordChalford 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 w of the Cotswold...
in
GloucestershireGloucestershire 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), is practically exhaustive. Other sources of information are:
New and General Biographical Dictionary, xii. 54 (1767);
Biog. Brit. (
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 nonconformist academy at Northampton, of which Dr Philip Doddridge was...
); Fouchy’s
Eloge,
Paris Memoirs (1762), p. 231 (Histoire); Delambre’s
Hist. de l’astronomie au 18e siècle, p. 413.
External links