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James Bradley was born in March 1693, in Sherborne, Gloucestershire, England. He was educated at Oxford and earned a master's degree in 1717. He had great aptitude in mathematics and was befriended by Isaac Newton and Edmond Halley. Since he felt he could not make a living in astronomy, he became a vicar in the Church of England, but gave that up in 1721 and returned to Oxford to teach.
Bradley's great goal in life was to measure the parallax of the stars--a measurable shift in the apparent position of an object resulting from the Earth's orbit around the Sun. When Nicholas Copernicus claimed the Earth was in motion around the Sun, it seemed obvious that parallax, due to the movement of the Earth, should be seen; it was not. While Copernicus contended that this was due to the great distance of the stars, his critics charged it was because the Earth simply did not move.
Using a telescope nearly 212 feet (65 m) long, Bradley was able to detect a tiny shift in stellar positions through the year. Unfortunately, the movement did not agree with the Earth's motion. It was not until 1728 that the explanation became evident to him; the shift was due to the speed of light and the velocity of the Earth.
The classic explanation of Bradley's discovery uses falling raindrops. If rain falls straight down, an umbrella is held straight up. If you're moving through the rain, you have to angle the umbrella forward. The faster you move, the greater the umbrella has to be angled. Consequently, to observe a star from a moving Earth, the telescope has to be angled very slightly.
The amount of angling (the amount of the "aberration of light" ) allowed Bradley to determine the ratio between the speed of light and the velocity of the Earth. Light moved ten thousand times faster than did the Earth, he claimed. The Earth travels 18.5 miles (29.7 km) in a single second, giving the speed of light as 185,000 miles (297,000 km) per second. Bradley's calculations came very close to the actual speed. (Danish astronomer, Olaus Roemer also calculated the speed of light during the 1670s, but his work had fallen into obscurity.)
Bradley made several other important discoveries during his lifetime. He found that the Earth's axis underwent a slight shift which he called "nutation." This was due to the gravitational tug of the moon as it orbited the Earth. He didn't publish his discovery until 1748, preferring to make observations for nineteen years to confirm it. In 1733 he obtained a measurement of Jupiter's diameter, making astronomers realize the true immensity of some of the other planets. He prepared an extremely accurate star chart which took into account the shift in stellar positions due to the aberration of light. He also earned the enmity of much of the public in 1752, when he came out in support of the adoption of the new Gregorian calendar, which would " rob" everyone of ten days.
James Bradley died on July 13, 1762. His hope of measuring parallax had not been realized; that had to wait a century for Friedrich Bessel.
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