who in 1728 discovered the aberration of starlight. From the amount
of aberration in star positions, he was able to calculate the speed
of light. In 1748, he announced the discovery of nutation (variation
in the Earth's axial tilt).
Bradley was born in Sherborne, Dorset, and studied theology at Oxford.
His keen interest in astronomy caused him to resign a clerical post
1721 to become professor of astronomy at Oxford.
As Astronomer Royal from 1742 he sought to modernize the observatory
in Greenwich, and embarked on an extensive programme of stellar observation.
The determination of stellar parallax was the goal of many astronomers
of Bradley's day because it would confirm Copernicus' hypothesis that
the Earth moved around the Sun. When Bradley did find a displacement,
it was not only too large, but was in an unexpected direction. Eventually
Bradley realized that the displacement was simply a consequence of
observing a stationary object from a moving one, namely the Earth.
Bradley called this effect the 'aberration of light'.
This discovery allowed Bradley to produce more accurate tables of
stellar positions, but he found that his observations on the distances
of stars were still variable. He studied the distribution of these
variations and deduced that they were caused by the oscillation of
the Earth's axis, which in turn was caused by the gravitational interaction
between the Moon and the Earth's equatorial bulge, so that the orbit
of the Moon was sometimes above the ecliptic and sometimes below it.
Bradley named this 'nutation'.