James Bradley has worked out the consequences…

January 1729 CE

James Bradley has worked out the consequences of supposing that the direction and speed of the earth in its orbit, combined with a consistent speed of light from the star, might cause the apparent changes of stellar position that he observed.

He found that this fitted the observations well, and also gave an estimate for the speed of light, and showed that the stellar parallax, if any, with extremes in June and December, was far too small to measure at the precision available to Bradley. (The smallness of any parallax, compared with expectations, also showed that the stars must be many times more distant from the Earth than anybody had previously believed.)

This discovery of what becomes known as the aberration of light is, for all realistic purposes, conclusive evidence for the movement of the Earth, and hence for the correctness of Aristarchus' and Kepler's theories; it is announced to the Royal Society in January 1729 (Phil. Trans. xxxv. 637).

The theory of the aberration also gives Bradley a means to improve on the accuracy of the previous estimate of the speed of light, which had previously been shown to be finite by the work of Ole Rømer and others.

Bradley was born at Sherborne, near Cheltenham in Gloucestershire, in March 1693.

Entering Balliol College, Oxford, on March 15, 1711, he took degrees of B.A. and M.A. in 1714 and 1717 respectively.

He had made his early astronomical observations at the rectory of Wanstead in Essex, under the tutelage of the Reverend James Pound, his uncle and a skilled astronomer.

Elected a fellow of the Royal Society on November 6, 1718, he had taken orders on becoming vicar of Bridstow in the following year, and a small sinecure living in Wales had also been procured for him by his friend Samuel Molyneux.

He resigned his ecclesiastical preferments in 1721, when appointed to the Savilian chair of astronomy at Oxford, while as reader on experimental philosophy from 1729 to 1760, he delivered seventy-nine courses of lectures at the Ashmolean Museum.

Bradley had measured the diameter of Venus in 1722 with a large aerial telescope with an objective focal length of two hundred and twelve feet (sixty-five meters).

Bradley makes his discovery of the aberration of light while attempting to detect stellar parallax.

He works with Samuel Molyneux until Molyneux's death in 1728 trying to measure the parallax of Gamma Draconis.

This stellar parallax ought to have shown up, if it existed at all, as a small annual cyclical motion of the apparent position of the star.

However, while Bradley and Molyneux did not find the expected apparent motion due to parallax, they found instead a different and unexplained annual cyclical motion.

Shortly after Molyneux's death, Bradley realizes that this is caused by what is now known as the aberration of light.

The basis on which Bradley distinguishes the annual motion actually observed from the expected motion due to parallax, is that its annual timetable is different.

Calculation show that if there had been any appreciable motion due to parallax, then the star should have reached its most southerly apparent position in December, and its most northerly apparent position in June.

What Bradley has found instead is an apparent motion that reaches its most southerly point in March, and its most northerly point in September; and that cannot be accounted for by parallax: the cause of a motion with the pattern actually seen is at first obscure.

A story has often been told, that the solution to the problem eventually occurred to Bradley while he was in a sailing-boat on the River Thames.

He noticed that when the boat turned about, a little flag at the top of the mast changed its direction, even though the wind had not changed; the only thing that had changed was the direction and speed of the boat.