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Astro*Dictionary by Michael Erlewine





1 article for "Bradley, James"

Bradley, James [Astro*Index]

(1693-1762) English astronomer. Born at Sherborne, Gloucestershire; died at Chalford, Gloucestershire.

Friend of Newton and Halley, he was elected to the Royal Society in 1718. He undertook the measurement of the parallax of stars, hoping to supply further support for the Copernican theory. Using a 212-foot telescope, his observations of stars showed small displacements, in the form of an ellipse, during the course of a year. But, the displacements could not be explained on the basis of Parallax. Instead, he had discovered the effect known as Aberration of Light. From his observations, it was possible to determine the ratio of the velocity of the Earth (about the Sun) to the velocity of light, which led to a new method for estimating the velocity of light. (Roemer had first obtained an estimate from the observations of the satellities of Jupiter.)

Thus, his work supplied new evidence to support the Copernican view, as light would be subject to no aberration if the Earth were not moving. Parallax of the stars, however, had to await the work of Bessel.


See also:
♦ Copernican Theory ♦ Annual Parallax ♦ Bessel, Friedrich Wilhelm ♦ Aberration ♦ Light, Velocity of ♦ Roemer, Olaus


Astro*Index Copyright © 1997 Michael Erlewine