Bradley , James

(1693–1762) British astronomer

Bradley was born in Sherborne in Dorset and educated at Oxford University. He was taught astronomy by his uncle, the Rev. James Pound, who was also an astronomer. In 1721 Bradley became Savilian Professor of Astronomy at Oxford and in 1742 he succeeded Edmond Halley as Astronomer Royal. His astronomical career began with a determined effort to detect parallax – the angular displacement of a body when viewed from spatially separate positions (or, more significantly, one position on a moving Earth). He fixed a telescope in as vertical a position as possible to minimize the effects of atmospheric refraction and began to observe the star Gamma Draconis. He soon found that the star had apparently moved position but prolonged observation convinced him that it could not be parallax he had measured, for he found the greatest shift in position in September and March and not in December and June as it should have been if he was observing parallax. However, the change in position was so regular (every six months) that it could be due only to the observer being on a moving Earth. It took him until 1729 to find the precise cause of the change in position. He realized that as light has a finite speed it will therefore take some time, however small, to travel down the length of the telescope. While it is traveling from the top to the bottom of the telescope the bottom of the instrument will have been carried by the orbital motion of the Earth. The image of the star will therefore be slightly displaced. Bradley realized that he had at last produced hard observational evidence for the Earth's motion, for the finite speed of light, and for a new aberration that had to be taken into account if truly accurate stellar positions were to be calculated. He worked out the constant of aberration at between 20'' and 20''.5 – a very accurate figure. He also discovered another small displacement, which, because it had the same period as the regression of the nodes of the Moon, he identified as the result of the 5° inclination of the Moon's orbit to the ecliptic. This caused a slight wobble of the Earth's axis, which he called ‘nutation’. Friedrich Bessel later used Bradley's observations to construct a catalog of unprecedented accuracy.