By Tim Carr
Being a contemporary of Isaac Newton and Edmund Halley is not a recipe for fame but James Bradley was not the sort to worry about such things. His contribution to
modernising astronomy speaks for itself.
Born in 1693, he studied theology at Oxford but developed a facination for astronomy from his uncle, James Pound, who was a friend of Halley. Despite a steady career
in the church beckoning, he resigned his position as chaplain to the Bishop of Hertford and became professor of astronomy in Oxford in 1721 and never looked back.
What Bradley wanted to do was to prove that stars showed parallax in their positions as the Earth moved around the Sun. One of the objections to the Copernican system was that there was no observable shift in the observed positions of the stars as the Earth moved from one side of it's orbit to the other. A shift by the observer of
many millions of miles should result in the stars' position 'appearing' to move very slightly. Now this had occured to Copernicus (and to Aristarchus, who, thousands of
years ago also said the Earth orbited the Sun ) but both realised that the parallax effect could not be seen as the stars were just too far away for any equipment to
measure it. 'Well you would say that wouldn't you', said the church. Very convenient.
Bradley realised that the accurate measurement of parallax would not only silence some of the critics but, more importantly, give astronomers an idea of just how far
away the stars were. Using a telescope with a focal length of over two hundred feet, he soon discovered the long elusive parallax. The problem was that not only was the
displacement too big it was in the wrong direction!
In 1728, after a couple of years of this, he realised that the cause was the movement of his observation platform - the Earth itself. By tilting the telescope to compensate, he got more accurate results. Sadly, however, a value for stellar parallax was still beyond him. But in discovering this 'abberation' of starlight and measuring it
(between 20 and 20.5 seconds of arc), Bradley had done two important things. The ratio between the speed of light and the speed of the Earth s orbit confirmed
Romer's value for the speed of light. Also, the very fact that light was showing any abberation at all, proved that the Earth must be moving in space.
Although this was by far the most important work of his life, Bradley was not finished there. His meticulous measuring of stars was upset by tiny shifts of the Earth's axis,
caused by the gravitational pull of the Moon. This effect is called nutation and requires accuracy in measurement of two arc-seconds. As even this level of accuracy was
insufficient for detecting stellar parallax, it showed that the stars were truly a very, very long way away. These results were only published in 1748 after nearly 20 years
of measurements. That same year, he received the Copely medal for his work.
In 1733 he made the first accurate measurement of Jupiter's diameter, disabusing scientists of the notion that their world was anything special. In 1742 he was appointed Astronomer Royal. James Bradley died in 1762.
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