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physical phenomena > process > physical process > motion > axis motion

axis motion
subject			fact

axis motion	has definition the motion of the axis of a system
	is a kind of motion
physical process	has domain physics

Kinds of axis motion :

• nutation (3 facts) - Slight but recurrent oscillation of the axis of the Earth, caused by the Moon's minutely greater gravitational effect on the Earth's equatorial "bulge"., A small, irregular oscillation in the precessional motion of Earth's rotational axis, caused primarily by lunar perturbations. It has a principal period of 18.6 years, and moves the equinox as much as 17" ahead of or behind its mean position.
• polar motion (2 facts) - The irregularly varying motion of the Earth's pole of rotation with respect to the Earth's crust. (See celestial ephemeris pole.)
• precession (6 kinds, 21 facts) - The slow (once per twenty-six thousand years) gyration of the Earth's axis., A slow, periodic conical motion of the rotation axis of a spinning body. In the case of Earth's precession it is due to the fact that Earth's axis of rotation is not perpendicular to the ecliptic but is inclined about 23°.5 and is thus affected by gravitational perturbations from other bodies in the solar system. The Moon and Sun pull harder on that part of the Earth's equatorial bulge nearest them than on that farthest away; this causes a torque which precesses the Earth's rotational axis., The uniformly progressing motion of the pole of rotation of a freely rotating body undergoing torque from external gravitational forces. In the case of the Earth, the component of precession caused by the Sun and Moon acting on the Earth's equatorial bulge is called lunisolar precession; the component caused by the action of the planets is called planetary precession. The sum of lunisolar and planetary precession is called general precession. (See nutation.)