Bradley Discovers the Nutation of Earth’s Axis Summary

  • Last updated on November 10, 2022

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.

Summary of Event

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 Venus in 1722 and the transit of Mercury Mercury over the disc of the Sun in 1723. He also tracked the movement of Comet Halley Halley’s comet[Halleys comet] during the latter part of 1723. He soon gained a reputation as a meticulous astronomer who made very careful observations. The precision of his approach may be confirmed through examination of an extant book of his data and calculations. [kw]Bradley Discovers the Nutation of Earth’s Axis (1748) [kw]Axis, Bradley Discovers the Nutation of Earth’s (1748) [kw]Earth’s Axis, Bradley Discovers the Nutation of (1748) [kw]Nutation of Earth’s Axis, Bradley Discovers the (1748) [kw]Discovers the Nutation of Earth’s Axis, Bradley (1748) Earth’s nutation[Earths nutation] Nutation of Earth’s axis Nutation of Earth’s axis [g]England;1748: Bradley Discovers the Nutation of Earth’s Axis[1210] [c]Astronomy;1748: Bradley Discovers the Nutation of Earth’s Axis[1210] [c]Science and technology;1748: Bradley Discovers the Nutation of Earth’s Axis[1210] Bradley, James Pound, James Halley, Edmond Molyneux, Samuel Hooke, Robert Flamsteed, John Alembert, Jean le Rond d’

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 Parallax of a star —that is, the apparent shift in the position of a star that occurs as a result of the Earth’s movement from a given point in its orbit to the point at the opposite end of that orbit. The astronomers used a vertical telescope more than 24 feet long that was carefully mounted to observe the position of a star that passed directly overhead, thus avoiding complications because of the refraction of light. The star Gamma Draconis Gamma Draconis (star) fit the necessary criteria for their project: It passed through the zenith of London and was bright enough to be observed with instruments. A number of years earlier, Robert Hooke and John Flamsteed had independently observed the same star and recorded its apparent displacement relative to the Earth. Bradley and Molyneux hoped to improve the accuracy of those measurements.

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, Light;speed of which had been reported by Danish astronomer Ole Rømer Rømer, Ole while observing eclipses of the moons of Jupiter in 1675. As a result of the finite speed of light, light coming from a particular star would reach an observer on the orbiting Earth from a direction that was slightly different from the actual position of the star from which it came. Bradley’s deduction, based on the resulting apparent displacement of the star, constituted the celebrated discovery of the aberration of starlight. Starlight, aberration of It was the first observational proof to confirm the theory of Nicolaus Copernicus Copernicus, Nicolaus that the Earth revolves around the Sun. Heliocentrism

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 Star catalogs the positions of more than three thousand stars between July, 1742, and September, 1747. He discovered that when a given star had completed its shift due to aberration over the course of twelve months it did not return to the same position it had previously occupied. Initially, he thought this observation was due to instrument error. After correcting for instrument error and checking the positions of numerous other stars that he had observed between 1727 and 1747, Bradley found the same outcome and sought an explanation. He surmised that there must be a change in the location of the points on Earth from which the positions of a given star were measured.

Early in the history of the Earth, it had been realized that the tides Tides and astronomy were related to the motion of the Moon. Moon (of Earth) Around 134 b.c.e., the Greek astronomer Hipparchus Hipparchus discovered the precession, or conical motion, Conical motion of the Earth’s axis because of the Moon’s motion. A satisfactory explanation of the tides and precession was finally supplied by Sir Isaac Newton Newton, Sir Isaac Newton, Sir Isaac;gravitational theory in the late 1680’s. The changing gravitational [p]Gravity pull of the Moon and the Sun on the Earth’s equatorial bulge produced the tides and the conical motion, or precession, of the rotational axis of the Earth. This rotation is analogous to the precessional wobble of the axis of a top that is spinning in the Earth’s gravitational field.

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.

Significance

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. Earth’s interior[Earths interior] The core and mantle of the Earth respond differently to the gravitational forces of the Moon and the Sun, causing a differential rotation between the core and the mantle. Consequently, nutation observations have proven invaluable in determining the limits both of the possible geometrical shape of the core-mantle boundary of the Earth and of the possible dissipative effects produced by core-mantle boundary interactions. These limits help to determine more accurate values for the density contrast across the core-mantle boundary. Researchers in astrophysics also use nutation amplitudes to provide evidence for the differential rotation between the inner and outer cores of the Earth. This information then refines values of the density contrast that exists across the boundary between inner core and outer core.

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">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.
  • citation-type="booksimple"

    xlink:type="simple">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.
  • citation-type="booksimple"

    xlink:type="simple">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.
  • citation-type="booksimple"

    xlink:type="simple">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.
  • citation-type="booksimple"

    xlink:type="simple">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.

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