The precession of Earth's axis of rotation with respect to inertial space is also called the precession of the equinoxes. Like a wobbling top, the direction of the Earth's axis is changing; while today, the North Pole points roughly to Polaris, over time it will change. Because of this wobble, the position of the earth in its orbit around the sun at the moment of the equinoxes and solstices will also change.

The term precession typically refers only to the largest periodic motion. Other changes of Earth's axis are nutation and polar motion; their magnitude is very much smaller.

Currently, this annual motion is about 50.3 seconds of arc per year or 1 degree every 71.6 years. The process is slow, but cumulative. A complete precession cycle covers a period of approximately 25,765 years, the so called Platonic year, during which time the equinox regresses a full 360° through all twelve constellations of the zodiac. Precessional movement is also the determining factor in the length of an astrological age.

In ancient times the precession of the equinox referred to the motion of the equinox relative to the background stars in the zodiac; this is equivalent to the modern understanding. It acted as a method of keeping time in the Great Year.

Polar shift and equinoxes shift

The figures to the right attempt to explain the relation between the precession of the Earth's axis and the shift in the equinoxes. These figures show the position of the Earth's axis on the celestial sphere, a fictitious sphere which places the stars according to their position as seen from Earth, regardless of their actual distance. The first image shows the celestial sphere from the outside, with the constellations in mirror image. The second figure shows the perspective of a near-Earth position as seen through a very wide angle lens (from which the apparent distortion).

The rotation axis of the Earth describes, over a period of 25,700 years, a small circle (blue) among the stars, centered around the ecliptic north pole (the blue E) and with an angular radius of about 23.4°, an angle known as the obliquity of the ecliptic. The direction of precession is opposite to the daily rotation of the Earth on its axis. The orange axis was the Earth's rotation axis 5,000 years ago, when it pointed to the star Thuban. The yellow axis, pointing to Polaris, marks the axis now.

The equinoxes occur where the celestial equator intersects the ecliptic (red line), that is, where the Earth's axis is perpendicular to the line connecting the centers of the Sun and Earth. When the axis precesses from one orientation to another, the equatorial plane of the Earth (indicated by the circular grid around the equator) moves. The celestial equator is just the Earth's equator projected onto the celestial sphere, so it moves as the Earth's equatorial plane moves, and the intersection with the ecliptic moves with it. The positions of the poles and equator on Earth do not change, only the orientation of the Earth against the fixed stars.

As seen from the orange grid, 5,000 years ago, the vernal equinox was close to the star Aldebaran of Taurus. Now, as seen from the yellow grid, it has shifted (indicated by the red arrow) to somewhere in the constellation of Pisces.

Still pictures like these are only first approximations as they do not take into account the variable speed of the precession, the variable obliquity of the ecliptic, the planetary precession (whose center lies on a circle about 6° away from the poles) and the proper motions of the stars.

Explanation

The precession of the equinoxes is caused by the differential gravitational forces of the Sun and the Moon on the Earth.

In popular science books, precession is often explained with the example of a spinning top. While the physical effect is the same, some crucial details differ. For a spinning top, gravity causes the top to wobble, which in turn causes precession. The applied force in this case is parallel to the rotation axis. For the Earth, however, the applied forces of the Sun and the Moon are perpendicular to the axis of rotation.

The Sun and the Moon pull on the equatorial bulge; due to its own rotation, the Earth is not a perfect sphere but an oblate spheroid, with an equatorial diameter about 43 kilometers larger than its polar diameter. If the Earth were a perfect sphere, there would be no precession.

The figure below explains how this process works. (Viewing the diagram at its maximum resolution is recommended.) The Earth is given as a perfect sphere with the mass of the bulge approximated by a blue torus around its equator. The green arrows indicate the gravitational forces from the Sun on some extreme points. These forces are not parallel, as they all point toward the center of the Sun. Therefore, the forces working on the northernmost and southernmost parts of the equatorial bulge have a component perpendicular to the ecliptical plane and a component directed parallel to it. The parallel component is centripetal force for the Earth in its orbit around the Sun. The perpendicular components are shown as cyan arrows tangential to the Earth's surface. These tangential forces create a torque (orange), and this torque, added to the rotation (magenta), shifts the rotational axis to a slightly new position (yellow). Over time, the axis precesses along the white circle, which is centered around the ecliptic pole.

This torque is always in the same direction, perpendicular to the direction in which the rotation axis is tilted away from the ecliptic pole, so that it does not change the axial tilt itself. The magnitude of the torque from the sun (or the moon) varies with the gravitational object's alignment with the earth's spin axis and approaches zero when it is orthogonal.

Although the above explanation involved the Sun, the same explanation holds true for any object moving around the Earth, along or close to the ecliptic, notably, the Moon. The combined action of the Sun and the Moon is called the lunisolar precession. In addition to the steady progressive motion (resulting in a full circle in 25,700 years) the Sun and Moon also cause small periodic variations, due to their changing positions. These oscillations, in both precessional speed and axial tilt, are known as the nutation. The most important term has a period of 18.6 years and an amplitude of less than 20 seconds of arc.

In addition to lunisolar precession, the actions of the other planets of the solar system cause the whole ecliptic to rotate slowly around an axis which has an ecliptic longitude of about 174° measured on the instantaneous ecliptic. This planetary precession shift is only 0.47 seconds of arc per year (more than a hundred times smaller than lunisolar precession), and takes place along the instantaneous equator.

The sum of the two precessions is known as the general precession.

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