Portrait of the British astronomer James Bradley, 1693-1762. Bradley was Halley's successor as Astronomer Royal, and his major concern was to measure the parallax of the stars and thus obtain some idea of their distance. His studies of tiny changes in position of certain stars led him to attribute them not to parallax, but to the ratio of the velocity of light to the velocity of the Earth about the Sun. This discovery allowed him to estimate the speed of light very accurately. It also gave the first direct evidence for the Earth's motion about the Sun.