NUTATION (from

See also:

• LAT

Lat. nutare, to nod)  , a revolution of the celestial pole around its mean position, due to inequalities in the action of the sun and moon, on an earth of ellipsoidal form . When either of these attracting bodies is in the plane of the equator, it produces no change in the direction of the celestial pole . The greater their distance from this plane, the greater the change, for reasons shown in the article ASTRONOMY (Celestial Mechanics) . The result is a motion which can be divided into two components . One of these is the progressive and nearly uniform motion of a fictitious mean pole, called precession (q.v.), and the other a revolution of the true around the mean pole, de-pending on the varying declinations of the sun and moon, and called nutation . Owing to the revolution of the moon's node and the inclination of its orbit, this body moves through a wider range of declination in some positions of the node than in others . The period of the revolution of the node is 18.6 years . At one time of this period the limits of its declination are more than 28° north and south, while, at the opposite point, they are little more than 18° . The result of these periodic changes is that the nutation takes place nearly in an ellipse, differing little from a circle, at a distance of about 9", in a period of about 18.6 years . The motion is not exactly an ellipse, having a great number of minute inequalities arising from the ellipticity of the orbits of the sun and moon and their varying declinations . The amount and formulae of nutation from year to year are given in the Nautical Almanac .