Jordens Lutning

In general, the term precession is used for the slow, periodic conical motion of the rotation axis of a spinning body. It is the uniformly progressing motion of the pole rotation of a free rotating body undergoing torque from external gravitational forces. In the case of Earth, it is the slow gyration of the Earth's axis. The component of precession caused by the Sun and Moon acting on the Earth's equatorial bulge is called lunisolar precession, and the component caused by the action of the planets is called planetary precession. The sum of lunisolar and planetary precession is then called general precession.

Earth's axis of rotation is not perpendicular to the ecliptic but inclined about 23.5°, and thus affected by gravitational perturbations from other bodies in the solar system. The Sun and Moon pull harder on the nearest part of the Earth's equatorial bulge than the farthest part, and this causes a torque which precesses the Earth's rotational axis. This gravitational pull - mainly by the Sun and Moon on the Earth's equatorial bulge - causes the first point of Aries to move backward along the equator at about 50" per year, which produces the round 26,000 year canonical motion of the Earth's rotation axis.

While precessing, the Earth's axis wobbles due to the minutely greater gravitational effect of the Moon on the Earth's equatorial bulge, which is called nutation. It is a slight but recurrent oscillation of the axis of the Earth, due to lunar perturbations, and produces a small, irregular oscillation in the precessional motion of the Earth's rotational axis.

In general, obliquity is the angle between the equatorial and orbital planes of a body, which can be defined as the angular distance between the rotational and orbital poles. In the case of Earth, obliquity of the ecliptic is the angle between the planes of the equator and the ecliptic, and due to above summarized effects, the axial tilt of the Earth oscillates between round 22.0°-24.6°, with a period of round 41,000 years.