James Bradley discovered in 1728 the annual shifting of the stars due to the aberration of light, and in 1748, the complicating effects upon precession of the "nutation" of the earth's axis.
By De la Rue's advice, Pritchard began his career there with a determination of the physical libration of the moon, or the nutation of its axis.
Amongst the most important of his works not already mentioned may be named the following: - Mathematical Tracts (1826) on the Lunar Theory, Figure of the Earth, Precession and Nutation, and Calculus of Variations, to which, in the second edition of 1828, were added tracts on the Planetary Theory and the Undulatory Theory of Light; Experiments on Iron-built Ships, instituted for the purpose of discovering a correction for the deviation of the Compass produced by the Iron of the Ships (1839); On the Theoretical Explanation of an apparent new Polarity in Light (1840); Tides and Waves (1842).
In 1749 he furnished a method of applying his principles to the motion of any body of a given figure; and in 1754 he solved the problem of the precession of the equinoxes, determined its quantity and explained the phenomenon of the nutation of the earth's axis.
The oscillation of the earth's axis may arise in two distinct ways; distinguished as " nutation of the axis " and " variation of latitude.
Nutation of the axis would determine a similar apparent motion for all stars: thus, all stars having the same polar distance as y Draconis should exhibit the same apparent motion after or before this star by a constant interval.
This star was seen to possess an apparent motion similar to that which would be a consequence of the nutation of the earth's axis; but since its declination varied only one half as much as in the case of y Draconis, it was obvious that nutation did not supply the requisite solution.
He also determined the mass of the moon, and from a discussion of the Greenwich transit circle observations between 1851 and 1865 he found for the constant of nutation the value 9.134".
It is affected by the motions of Precession and Nutation, of which the former has been known since the time of Hipparchus.
He refrained from announcing the supplementary detection of nutation until the 14th of February 1748 (Phil.
Bradley, was the first to represent the effects of nutation in the solar tables, and introduced, in 1741, the use of the transitinstrument at the Paris observatory.
" Nutation, the only form of oscillation imagined by Bradley, postulates that while the earth's C A FIG.