Description: In astronomy precession is the gradual change of the rotational axis or orbital parameters of an object.

The most significant precession in astronomy is the precession of the Earth's axis. The Earth's slightly non-spherical shape, combined with the gravitational pull of the Sun and Moon leads the Earth's rotation axis to trace out a cone over roughly 26,000 years. This leads to gradual changes in the positions of the celestial poles and the celestial coordinate system. The position of the Sun at the equinoxes also changes, with the Sun's positions at the spring and autumn equinox moving through the constellations of the Zodiac over a 26,000-year cycle.

The orientations of orbits can also precess. Orbits of planets and other objects around the Sun are elliptical. The orientations of these ellipses can change gradually over time. The best-known example is the precession of Mercury's orbit. The closest point of Mercury's orbit around the Sun changes gradually, moving around the Sun very slowly, over a period of over two million years. The rate Mercury's orbit changes in this way is not well-modelled by classical mechanics. The correct modelling of this effect was one of the key tests passed by Einstein's theory of general relativity. Another notable precession is the precession of the Moon's orbit around the Earth.

Related Terms:

• Equinox
• Celestial Coordinates
• Celestial Pole
• Earth's Rotation
• Orbit
• Theory of General Relativity
• Earth's Axis
• Rotation

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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