Quarter One Astronomer Biography

Quarter One Astronomer

            James Bradley was born in Sherborne, Gloucestershire in March of 1693.  Bradley was the third son of William Bradley and Jane Pound.  James’s parents had always intended for him to have a career in the church; but his uncle had other plans for him.  His uncle was the Reverend James Pound, who just so happened to be one of the most promising amateur astronomers of his time.  While James’s father’s income was limited, his education was paid for in large part by his uncle.  Not only did Bradley’s uncle facilitate his education, he also was one of the main forces driving Bradley’s love of astronomy.  Bradley received an education at Northleach Grammar School and at Balliol College, Oxford.  He entered Balliol College in 1711, received his B.A. in 1714, and his M.A. in 1717.  He was appointed as astronomer royal in 1742 and then Oxford awarded him an honorary D.D.  

            In 1719, Bradley took orders when he was given his living at Bridstow.  Meanwhile, he never ceased his studies in the field of astronomy under the instruction of his uncle.  In 1718 he received the honor of being elected as a fellow of the Royal Society.  At only 28 years old, he became Savilian professor of astronomy at Oxford and had to resign from his position at Bridstow. 

            Bradley lived in a time when astronomers were largely on their own when it came to repairing or modifying their equipment.  Amazingly, in 1722, Bradley measured the diameter of Venus with a telescope that was over 212 feet in length. 

            Bradley was very lucky in that he had many connections in the world of astronomy such as his uncle and Samuel Molyneux.  Molyneux had an observatory at Kew near London.  At this observatory in 1725, Bradley systematically observed the star y Draconis.  He was hoping to see the parallactic motion of the stars.  His observations did not stray much from what he had predicted: the star described a tiny ellipse with an axis of only 40 seconds of arc.  But to his surprise, the direction of the ellipse was wrong!  From this, he concluded that the effect did not arise from parallactic motion.  Bradley mulled over his findings, he couldn’t figure it out.  But then he realized that it was due to the finite velocity of light, owing to the velocity of the earth as it moved in an ellipse, which created an aberration of light.  This revelation was so remarkable especially because Bradley gave almost precisely the modern value for the constant of aberration, about 20.5 seconds.

            Bradley’s work on aberration lead to his further discoveries.   Namely he discovered nutation, the oscillation of the earth’s axis caused by the changing direction of the gravitational pull of the moon on the equatorial bulge.  Bradley asserted that nutation must result from the fact that the moon is sometimes above and sometimes below the ecliptic.  Therefore, it should have the periodicity of the lunar node.  The period from 1727 to 1747, a full cycle of the motion of the moon’s nodes, was covered by his observations. 

            At Greenwich, he assumed the position of royal astronomer.  He obtained an eight foot mural quadrant here, with which he compiled a new catalog of star positions.  This wasn’t published until after his death but it involved some 60,000 observations and lead to future research.  Bradley’s health began to deteriorate so he was forced to retire to Chalford, Gloucestershire, where he died on July 13, 1762.