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See also: celestial See also: pole around its mean position, due to inequalities in the See also: action of the See also: sun and See also: moon, on an See also: earth of ellipsoidal See also: form
.
When either of these attracting bodies is in the See also: 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 See also: Mechanics)
.
The result is a motion which can be divided into two components
.
One of these is the progressive and nearly See also: 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 See also: nutation
.
Owing to the revolution of the moon's See also: node and the inclination of its orbit, this See also: body moves through a wider range of declination in some positions of the node than in others
.
The See also: period of the revolution of the node is 18.6 years
.
At one See also: time of this period the limits of its declination are more than 28° See also: north and See also: 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 See also: great number of minute inequalities arising from the See also: ellipticity of the orbits of the sun and moon and their varying declinations
.
The amount and formulae of nutation from See also: year to year are given in the Nautical See also: Almanac
.
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