Reference Terms
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Precession

Precession refers to a change in the direction of the axis of a rotating object.

In certain contexts, "precession" may refer to the precession that the Earth experiences, the effects of this type of precession on astronomical observation, or to the precession of orbital objects.

The revolution of a planet in its orbit around the Sun is also a form of rotary motion. (In this case, the combined system of Earth and Sun is rotating.) So the axis of a planet's orbital plane will also precess over time.

The major axis of each planet's elliptical orbit also precesses within its orbital plane, in response to perturbations in the form of the changing gravitational forces exerted by other planets.

Note:   The above text is excerpted from the Wikipedia article "Precession", which has been released under the GNU Free Documentation License.
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September 5, 2015

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