The English astronomer James Bradley (1693-1762), one of the most determined and meticulous astronomers, discovered the aberration of light and the nutation of the earth's axis.
James Bradley, who was the nephew of the astronomer James Pound, was born at Sherborne, Gloucestershire, in March 1693. He studied at Balliol College, Oxford, and took orders in 1719, when he was given his living at Bridstow. In the meantime he had become a skilled astronomer in the techniques of the day, under the instruction of his uncle. In 1718 he was elected a fellow of the Royal Society, and at the early age of 28 he became Savilian professor of astronomy at Oxford and so resigned from Bridstow.
Bradley lived at a time when an astronomer had to be his own technician—repairing, maintaining, and even making his own equipment. High magnifications were obtained by telescopes with lenses of great focal length, often so long that they were not fitted to tubes. In 1722 Bradley measured the diameter of Venus with a telescope over 212 feet in length.
Bradley was a friend of Samuel Molyneux, who had an observatory at Kew near London. There in 1725 Bradley systematically observed the star γ Draconis, hoping to discover the parallactic motion of the stars, that is, a seeming change in the positions of the stars, scattered through space, mirroring the change in the earth's position in its orbit around the sun. His observations were close to what he expected; the star described a tiny ellipse with an axis of only 40 seconds of arc. But the direction of the ellipse was wrong, and he concluded that the effect did not arise from parallactic motion. Greatly puzzled by the result, he at last realized that it was due to the finite velocity of light, owing to the velocity of the earth as it moved in an ellipse, which created an aberration of light. This was a very remarkable piece of work, all the more memorable for the fact that Bradley gave almost precisely the modern value for the constant of aberration, about 20.5 seconds (the modern value being 20.47 seconds).
Out of his work on aberration, Bradley discovered nutation, the oscillation of the earth's axis, caused by the changing direction of the gravitational pull of the moon on the equatorial bulge. He concluded that nutation must arise from the fact that the moon is sometimes above and sometimes below the ecliptic, and it should therefore have the periodicity of the lunar node, that is, approximately 18.6 years. His observations of this covered the period from 1727 to 1747, a full cycle of the motion of the moon's nodes.
At Greenwich, as astronomer royal, where he found the instruments in a poor state of repair, he obtained some fine new instruments, including an eight foot mural quadrant, with which he compiled a new catalog of star positions. It was published posthumously and involved some 60, 000 observations. F. W. Bessel's catalog in 1818, with 3, 000 star positions, was largely based on Bradley's observations. Bradley's health failed, and he retired to Chalford, Gloucestershire, where he died July 13, 1762.
There is a thorough biography of Bradley in the preface to Stephen Peter Rigaud's edition of Bradley's Miscellaneous Works and Correspondence (1832), which is the source of practically all the short notices of Bradley's life that have appeared since. Bradley's career is also discussed in Henry Smith Williams, The Great Astronomers (1930), and Sir Oliver Joseph Lodge, Pioneers of Science and the Development of Their Scientific Theories (1960).