Editing Precession (section)

{{short description|Periodic change in the direction of a rotation axis}}
{{Other uses}}
[[File:Gyroscope precession.gif|thumb|Precession of a [[gyroscope]]{{clarify|reason=This image, while it moves, does not clarify which aspect of the gyroscope is demonstrating precession! This could be improved with a caption but would be better improved by a label in the image.|date=November 2022}}]]
[[File:Praezession.svg|thumb|{{legend-line|green solid 2px|[[Rotation]]}} {{legend-line|blue solid 2px|Precession}}{{legend-line|red solid 2px|[[Nutation]]}} in [[obliquity]] of a planet]] 

'''Precession''' is a change in the [[orientation (geometry)|orientation]] of the rotational axis of a [[rotation|rotating]] body. In an appropriate [[frame of reference|reference frame]] it can be defined as a change in the first [[Euler angles|Euler angle]], whereas the third Euler angle defines the [[rotation around a fixed axis|rotation itself]]. In other words, if the axis of rotation of a body is itself rotating about a second axis, that body is said to be precessing about the second axis. A motion in which the second Euler angle changes is called ''[[nutation]]''. In [[physics]], there are two types of precession: [[torque]]-free and torque-induced.

In astronomy, ''precession'' refers to any of several slow changes in an astronomical body's rotational or orbital parameters. An important example is the steady change in the orientation of the axis of rotation of the [[Earth]], known as the [[axial precession|precession of the equinoxes]].