After discovering the aberration of starlight in the late 1720’s, James Bradley proceeded to catalog the positions of more than three thousand stars between 1727 and 1747. His catalog led him to discover the nutation of the Earth’s axis as the Earth orbits the Sun.
While attending Balliol College, Oxford University, between 1711 and 1717, James Bradley spent a great deal of time with his uncle, the Reverend James Pound, who was a skilled astronomer. Under the instruction of Pound, Bradley gained expertise in using the astronomical instruments of the time and in making very precise astronomical observations. He learned how to repair, maintain, and build his own equipment. In 1721, Bradley gave up his position as vicar of Bridstow to become the Savilian professor of astronomy at Oxford. Using a telescope—more than 212 feet long so as to yield high magnifications—Bradley measured the diameter of Venus
In 1725, Bradley began a series of observations at Kew Green, near London, in the observatory of his friend Samuel Molyneux. They hoped to measure the parallax of a star
As Bradley and Molyneux made their observations, they found that the apparent displacement of Gamma Draconis was far too great to be a parallax and was, moreover, in the opposite direction of what they had expected. During the course of one year, the changing position of the star traced out a small ellipse. Bradley concluded that some other unexplained effect was masking the parallax of the star. Intrigued by the problem, he mounted a telescope at his aunt’s home in Wanstead and made continual observations.
After many months of careful observations and deep thinking, Bradley finally realized that the effect was produced by the finite velocity of light,
Between 1727 and 1732, Bradley recorded the annual change in position of a number of fixed stars. When Edmond Halley, the astronomer royal at Greenwich, died in 1742, Bradley was appointed as his successor. After repairing the instruments at Greenwich, Bradley continued his observations and cataloged
Early in the history of the Earth, it had been realized that the tides
From his numerous detailed observations, Bradley concluded that the precessional motion of the Earth’s axis was not uniform, as had previously been supposed. Instead, there was a sinusoidal motion superimposed on the regular conical motion. Bradley adduced this extra wobble, known as nutation, to the fact that the tidal forces produced by the Moon and the Sun vary over time. This variation is caused by the continual change in location of the Earth, Moon, and Sun relative to each other and to the fact that the Moon’s orbit is inclined at about five degrees to the plane of the Earth’s orbit around the Sun. As a result, the axis of the Moon’s orbit around the Earth precesses, just as the axis of the Earth precesses in its motion around the Sun.
Through his careful observations of star positions between 1727 and 1747, Bradley observed that a star returned to the same location at which it was originally observed in about 18.6 years. That duration corresponded to the time it took for one full precessional cycle of the Moon’s orbit. As was characteristic of all his scientific work, when all the facts were finally available, Bradley announced his discovery of the nutation of the Earth’s axis on February 14, 1748. His discovery of the aberration of starlight—as well as the discovery of the subtle, uneven nodding, or nutation, of the Earth’s axis to which it led—rank as two of the most important discoveries made by astronomers.
Bradley’s discovery of the nutation of the Earth’s axis provided an enhanced understanding of the interacting gravitational forces between the Earth, the Moon, and the Sun. French mathematician Jean le Rond d’Alembert provided the mathematical explanation for nutation in 1749. Contributions to the size of the Earth’s nutation were divided into the component along the ecliptic plane (nutation in longitude) and the component perpendicular to the ecliptic plane (nutation in obliquity). These values are significant, because they contribute to the accuracy with which astronomers can determine positions of astronomical objects that are observed from the Earth.
Observations of nutation values, including amplitude and phase, provide information about how the Earth responds to known forces. This information can then be used to place constraints on the properties of the interior of the Earth.
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Bradley, James. Miscellaneous Works: Correspondence of the Rev. James Bradley. New York: Oxford University Press, 1992. Publication of Bradley’s writings, including a biography of Bradley, how he analyzed his astronomical data, and a detailed description of the nutation of the Earth’s axis. -
Fix, John D. Astronomy: Journey to the Cosmic Frontier. New York: McGraw-Hill, 2001. Discusses one of Bradley’s main contributions to astronomy, the aberration of starlight, which led to his discovery of the nutation of the Earth’s axis. -
Mathews, P. M. Modeling of Nutation and Precession. Washington, D.C.: American Geophysical Union, 2002. Contains diagrams that clearly illustrate the precession and nutation of the Earth’s axis, how the motions are modeled, and implications that provide insights into the interior of the Earth. -
Seeds, Michael A. Foundations of Astronomy. Belmont, Calif.: Brooks/Cole, 2004. Seeds reviews the history of understandings of the Moon and explains the fundamental ideas of precession and nutation in conjunction with Bradley’s discoveries. -
Zelik, Michael. Astronomy: The Evolving Universe. 9th ed. New York: Cambridge University Press, 2002. Reviews and illustrates the concepts of stellar parallax, aberration of starlight, precession, nutation, and contributions of James Bradley to astronomy.
Astronomy Wars in England
Halley Predicts the Return of a Comet
Quest for Longitude
Flamsteed’s Star Catalog Marks the Transition to Modern Astronomy
Maclaurin’s Gravitational Theory
D’Alembert Develops His Axioms of Motion
Laplace Articulates His Nebular Hypothesis
Jean le Rond d’Alembert.