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James Bradley

Bradley, James (1693-1762), an English astronomer, discovered the aberration of starlight that provided the first direct proof that the earth revolves around the sun, and the nutation, or nodding motion, of the earth's axis.

Bradley received his bachelor's and master's degrees from Balliol College, Oxford. He was trained in observational astronomy by a clergyman uncle, a skilled amateur astronomer, who introduced him to famous astronomer Edmond Halley. In 1718, on Halley's recommendation, Bradley was elected to the Royal Society. After a short stint in the clergy, Bradley was appointed professor of astronomy at Oxford in 1721.

In 1729, Bradley announced to the Royal Society his discovery of the aberration of starlight, an apparent slight change in the positions of stars that results from the earth's annual motion. One story tells that he had drawn his conclusion following a sail on the Thames River, when he observed that the wind vane of the mast shifted position with the boat's varying motion, even though the wind's direction was constant. Thus, he reasoned that the apparent shift in the position of the star Gamma Draconis resulted from the motion of the earth in its orbit. This provided the first direct observational proof of Nicolaus Copernicus 's assertion that the earth revolves around the sun.

Additional star measurements over 20 years led Bradley to conclude that the changing direction of the moon's gravitational pull caused nutation, an uneven nodding motion of the earth's axis. He announced this finding in 1748.

In 1742, Bradley succeeded Halley as astronomer royal, the chief astronomer of England. In additional to an annual stipend, he received a grant of 1,000 pounds for instruments, including two quadrants, which allowed more precise measurements. One quadrant was first used in a series of observations in the early 1750's to determine the latitude of the Royal Greenwich Observatory.

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