1911 Encyclopædia Britannica/Nutation

NUTATION (from 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, depending 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.