March 5: James Bradley

James Bradley

March 1693 – July 13, 1762

(exact date of birth is unknown)

James Bradley was an English astronomer and Astronomer Royal from 1742. He is best known for discovering the aberration of light while attempting to detect stellar parallax.

Bradley entered Balliol College, Oxford, on March 15, 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, himself a skilled astronomer, and was elected a fellow of the Royal Society on November 6, 1718. In 1721 he was appointed to the Savilian chair of astronomy at Oxford.

Bradley worked with Samuel Molyneux until Molyneux's death in 1728 trying to measure the parallax of Gamma Draconis. However, while not finding the expected parallax, they instead found an unexplained motion which shortly after Molyneux's death Bradley realized was caused by the aberration of light. The discovery of the aberration of light, which was conclusive evidence for the movement of earth and hence the correctness of Aristarchus and Kepler's theories, was announced to the Royal Society in January 1729. 

The measurement of parallax had been the Holy Grail of astronomers since the time of Aristarchus and Archimedes. If the Earth orbited round the Sun, as postulated by Aristarchus and formulated much later by Copernicus, then nearby stars should show some apparent change of position over an annual cycle because of the changed viewpoint from the Earth at each extreme of its orbit. This is the same effect as when nearby objects seen from the window of a moving train, appear to change position much faster than distant objects — ‘parallax’.

Unfortunately for Helio-centrists (notably Galileo), the best endeavours of astronomers for centuries had produced no reliable evidence whatsoever of stellar parallax — just as expected by the Geo-centrists: if the Earth was stationary at the centre of the universe then of course the stars would not show any change of position.

The best the Helio-centrists could do was to argue that the stars were so hugely distant that the parallax effect was tiny and beyond the ability of present instruments to detect. Succeeding generations worked to build better instruments and devise new strategies for measurement. When Bradley had directly detected the effect of the Earth’s motion around the Sun, Aristarchus, Copernicus and Galileo were vindicated:

“eppur si muove” — and yet it does move.

This aberration also allowed Bradley to accurately estimate the speed of light which had previously been shown to be finite by the work Ole Rømer and others. The observations upon which it was founded were made at Molyneux’s house on Kew Green. Bradley did not announce the supplementary detection of nutation until February 14, 1748, when he had tested its reality by minute observations during an entire revolution (18.6 years) of the moon’s nodes. 

In 1742, he 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.

He died on July 13, 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.

The Lunar promontory Mons Bradley is named in his honor.