James Bradley facts for kids
Quick facts for kids
James Bradley
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Portrait of Bradley by Thomas Hudson, c. 1744
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| Born | September 1692 Sherborne, Gloucestershire, England
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| Died | 13 July 1762 (aged 69) Chalford, Gloucestershire, England
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| Alma mater | Balliol College, Oxford | ||||||
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| Awards | Copley Medal (1748) | ||||||
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| Fields | Astronomy | ||||||
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James Bradley (1692–1762) was an English astronomer and priest. He became the third Astronomer Royal in 1742. He is famous for two big discoveries in astronomy. These are the aberration of light (which he studied from 1725–1728) and the nutation of the Earth's axis (from 1728–1748).
A famous historian of astronomy, Jean Baptiste Joseph Delambre, called these discoveries "the most brilliant and useful of the century." Bradley's work helped us understand how the Earth moves and how light travels.
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About James Bradley
James Bradley was born in September 1692 in Sherborne, England. This town is near Cheltenham in Gloucestershire. His parents were William Bradley and Jane Pound. His nephew, John, also became an astronomer.
James went to Westwood's Grammar School. Then, on March 15, 1711, he started at Balliol College, Oxford. He earned his Bachelor of Arts degree in 1714 and his Master of Arts degree in 1717.
His first observations of the stars were made at his uncle's home. His uncle, James Pound, was a skilled astronomer who taught him a lot. On November 6, 1718, Bradley was chosen to be a member of the Royal Society. This is a very important group for scientists.
In 1719, he became a vicar (a type of priest) in Bridstow, Herefordshire. He also got a small church job in Wales. But in 1721, he left these church jobs. He was appointed to teach astronomy at Oxford University. From 1729 to 1760, he also gave many lectures on experimental philosophy.
In 1742, Bradley took over from Edmond Halley as the Astronomer Royal. This was a very important position. He was able to get £1,000 for new instruments. With a special 8-foot quadrant built in 1750, he made many valuable observations at Greenwich. These observations helped improve astronomy greatly. In 1752, he started receiving a yearly payment of £250 from the King.
Nine years later, in 1761, Bradley's health got worse. He retired to the village of Chalford in Gloucestershire. He passed away on July 13, 1762, at Skiveralls House. He was buried in the churchyard of Holy Trinity church in Minchinhampton. His wife had died before him in 1757, and he had one daughter who survived him.
Bradley's Amazing Discoveries
In 1722, Bradley used a huge aerial telescope to measure Venus. This telescope had a very long lens, about 212 feet!
Aberration of Light
One of Bradley's biggest discoveries was the aberration of light. He found this while trying to measure something called "stellar parallax." Stellar parallax is a small shift in a star's apparent position. It happens because the Earth moves around the Sun. Bradley worked with Samuel Molyneux to try and measure the parallax of a star called Gamma Draconis.
They expected to see a small yearly circle in the star's position. But they didn't find the expected parallax motion. Instead, they found a different, unexplained yearly movement. After Molyneux died, Bradley realized what was causing it. It was the aberration of light. He noticed that the timing of this new motion was different from what parallax would cause.
If it were parallax, the star should have appeared furthest south in December and furthest north in June. But Bradley found the star appeared furthest south in March and furthest north in September. This pattern couldn't be explained by parallax.
There's a story, possibly not true, about how Bradley figured it out. He was on a sailing boat on the River Thames. He saw that a small flag at the top of the mast changed direction when the boat turned. This happened even though the wind hadn't changed. Only the boat's direction and speed had changed. Bradley thought about how the Earth's movement around the Sun, combined with the constant speed of light from a star, could make the star seem to move. He found that this idea matched his observations perfectly. It also gave him a way to estimate the speed of light. His work showed that any parallax was too small to measure with his tools. This also meant stars were much, much farther away than people thought!
This discovery of the aberration of light was strong proof that the Earth moves. It supported the ideas of Aristarchus, Copernicus, and Kepler. Bradley announced his findings to the Royal Society in January 1729. His theory also helped improve the measurement of the speed of light. Before him, Ole Rømer had estimated it. Bradley's value for the speed of light was very close to the actual value. He said it took 8 minutes and 12 seconds for light to travel from the Sun to Earth. This was only about 1.3% too high! It was the first accurate measurement of this important number.
Bradley's first observations for this discovery were made at Molyneux's house in Kew Green. He continued them at his uncle James Pound's house in Wanstead, Essex.
Nutation of Earth's Axis
After publishing his work on aberration, Bradley kept observing the stars. He wanted to check his second big discovery: the nutation of the Earth's axis. This is a small wobble in the Earth's tilt as it spins. He didn't announce this discovery until February 14, 1748. He waited until he had observed it for a full cycle of the Moon's nodes (about 18.6 years). This way, he was sure his findings were correct.
Bradley's detailed observations were finally published much later. They came out in two large books in 1798 and 1805. But it took the hard work of another astronomer, Friedrich Wilhelm Bessel, to fully understand how important they were.
Images for kids
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Miscellaneous Works and Correspondence, 1832
See also
In Spanish: James Bradley para niños
