James Bradley bigraphy, stories - British astronomer

James Bradley : biography

1693 - 13 July 1762

James Bradley FRS (March 1693 – 13 July 1762) 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." (edited by Claude-Louis Mathieu, and published by Bachelier, Paris, 1827). See also pp. xvii and 420.

Biography

Bradley was born at Sherborne, near Cheltenham in Gloucestershire, to William Bradley and Jane Pound 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 .This paragraph is adapted from the 1888 edition of the Encyclopædia Britannica.

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.Bradley's new discovery was communicated in a correspondence with Dr. Edmond Halley 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.)

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