Precession — Precession is the phenomenon by which the axis of a spinning object “wobbles” when a torque is applied to it.

The phenomenon is commonly seen in a spinning toy top, but all rotating objects can undergo precession.

As a spinning object precesses, the tilt of its axis goes around in a circle in the opposite direction that the object is spinning. If the speed of the rotation and the magnitude of the torque are constant the axis will describe a cone, its movement at any instant being at right angles to the direction of the torque.

In the case of a toy top, if the axis is not perfectly vertical the torque is applied by the force of gravity trying to tip it over.

A rolling wheel will tend to remain upright due to precession; whenever it tends to tip over to either side, precession swerves its plane and changes its path so that it automatically steers itself. This is how bicycles operate. Precession is also the mechanism behind gyrocompasses.

Precession of the equinoxes

The Earth’s axis undergoes precession due to a combination of the Earth’s nonspherical shape (it is an oblate spheroid, bulging outward at the equator) and the gravitational tidal forces of the Moon and Sun applying torque as they attempt to pull the equatorial bulge into the plane of the ecliptic. It goes through one complete precession in a period of approximately 25,800 years during which the positions of stars within the celestial sphere will slowly change. Over this period, the axis’ north pole moves from where it is now, within 1 of Polaris, in a circle.

Polaris isn’t particularly well-suited for marking the north celestial pole, as its visual magnitude is only 1.97, fairly far down the list of brightest stars in the sky. On the other hand, in 3000 BC the faint star Thuban in the constellation Draco was the pole star; at magnitude 3.67 it is five times fainter than Polaris, and all but invisible from light-polluted urban skies. The brightest star to be North Star at any time in the forseeable past or future is the brilliant Vega, which will be the pole star in 14000 AD.

Polaris is not exactly at the pole; any long exposure unguided shot will show it having a short trail. It’s close enough, though. The south celestial pole precesses too, always remaining exactly opposite the north pole. The south pole is in a particularly bland portion of the sky, and the nominal south pole star is Sigma Octantis, which, while fairly close to the pole, is even weaker than Thuban — magnitude 5.5, which is barely visible even under a properly dark sky. The precession of the Earth is not entirely regular due to the fact that the Sun and Moon are not in the same plane and move relative to each other, causing the torque they apply to Earth to vary. This varying torque produces a slight irregular motion in the poles called nutation.

Precession of the Earth’s axis is a very slow effect, but at the level of accuracy at which astronomers work, it does need to be taken into account. Note that precession has no effect on Earth’s axial tilt. It is 23.5 degrees and precession does not change that. Other factors, however, do, including those factors that cause precession.

Hipparchus first estimated Earth’s precession around 130 BC using observations of a temple in Thebes, Egypt that was built in around 3200 BC with a known orientation relative to the star Spica.

Precession causes the cycle of seasons (tropical year) to be a few minutes less than the cycle of the sun as seen with respect to the stars (sidereal year). This results in a slow change in the position of the sun with respect to the stars at an equinox. It is significant for calendars and their leap year rules.

Precession of planetary orbits

The orbit of a planet around the Sun is also a form of rotation, and so the axis of a planet’s orbital plane will also precess over time. Discrepancies in the precession rate of the planet Mercury compared to those predicted by classical mechanics were one of the major pieces of evidence leading to the acceptance of Albert Einstein’s Theory of Relativity, which predicted the anomalies accurately.

Precession is also an important consideration in the dynamics of atoms and molecules.


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