Bradley, James

(1693–1762) English astronomer: discovered stellar aberration; obtained first accurate measurement of the speed of light and direct proof of Earth’s motion.

Bradley was successor as Astronomer Royal. While attempting to observe parallax in the position of ? Draconis (caused by the Earth’s movement across the diameter of its orbit), Bradley found that the star did indeed appear to move, but that the greatest contrast was between September and March, not between December and June, as would be expected from parallax. He deduced that the movement (aberration) he saw was related to the ratio of the velocity of light to the velocity of the Earth about the Sun (the latter is about 30 km s –1 , and the ratio about 10 000:1). This discovery allowed him to estimate the speed of light to be 3.083 × 10 8 m –1 , which is more accurate than value. It also gave the first direct evidence for the Earth’s motion about the Sun. Bradley also discovered nutation, the wobble of the Earth’s axis caused by the changing gravitational attraction of the Moon due to its slightly inclined orbit. It was not until work, a century later, that stellar parallax was observed.