Precession of equinoxes

(astronomy) A slow conical motion of the earth's axis about the vertical to the plane of the ecliptic, having a period of 26,000 years, caused by the attractive force of the sun, moon, and other planets on the equatorial protuberance of the earth; it results in a gradual westward motion of the equinoxes.

A slow change in the direction of orientation of Earth's axis of rotation which results in a gradual westward motion of the equinoxes. There are two types, known as lunisolar precession and planetary precession.

The lunisolar precession of the equinoxes is caused by the gravitational attraction of the Sun and the Moon which, as a result of the polar flattening of Earth and the inclination of Earth's axis, gives rise to a small turning moment, or torque, on the Earth in its orbit. As a result of this torque, Earth's axis describes a cone about the normal to the plane of its orbit (see illustration); the period of this precession is approximately 26,000 years in a direction opposite to that of Earth's rotation.

Conical motion of Earth's axis. One complete cycle of precession requires 26,000 years.

The planetary precession is a comparatively small eastward motion of the equinoxes caused by the action of the other planets in altering the plane of Earth's orbit.

As a result of precession, Polaris will not always be the pole star. Vega will be nearer to the north celestial pole in about 12,000 years; α Draconis was the pole star about 4600 years ago. Another effect of precession is that the signs of the zodiac no longer correspond to their respective constellations. See also Precession.

The "precession of the equinoxes" is the 26,000-year circular movement of the Earth's axis. It is caused by the bulging at the equator, which makes the Earth's axis twist in such a way that the North and South Poles complete a circle every 26,000 years. Every year when the sun crosses the equator at the time of the equinox, it is in a slightly different position than the previous year. This movement proceeds eastward until a circle is completed.