It can be defined as a change in direction of the rotation axis in which the second Euler angle  (
nutation ) is constant.  In physics ,
Kaynak: Precessionand hence the celestial equator, known as 
nutation  This adds a periodic component to the position of the equinoxes; the positions of the
Kaynak: EclipticThe vector of the figure axis  F of the system wobbles around M. This motion is called Euler 's free 
nutation .  For a rigid Earth which is
Kaynak: Polar motionHe is best known for two fundamental discoveries in astronomy, the aberration of light  (1725–1728), and the 
nutation of the Earth's axis
Kaynak: James Bradley"tip" or "
nutation" angle) for a particular spin that gives the maximal signal in the least amount of time when signal averaging over many
Kaynak: Ernst angleNutation These expressions are for the so-called mean obliquity, that is, the obliquity free from short-term variations.  known as 
nutationKaynak: Axial tilt date May 2011 This wobble, which is a 
nutation , combines with another wobble with a period of one year, so that the total polar motion
Kaynak: Chandler wobbleThis provides the absolute reference  for the determination of universal time , precession , and 
nutation  Changes in rotation
Kaynak: Earth's rotationThe tilt does undergo 
nutation ; a slight, irregular motion with a main period of 18.6 years.  The orientation (rather than the angle) of
Kaynak: EarthThe term "precession" typically refers only to this largest secular motion ; other changes in the alignment of Earth's axis — 
nutationKaynak: Axial precessiondate March 2013 Precession  and 
nutation  shift the equinox slightly from one day to the next, so sidereal time is not an exact measure of
Kaynak: Sidereal timeabout some other point, or the rotation  of a single body about its axis , or other phenomena such as precession  or 
nutation  of the axis.
Kaynak: Retrograde motionThese rotations are called precession , 
nutation , and intrinsic rotation  (spin).  As an example, consider a top .  The top spins around
Kaynak: Euler anglesuses the International Celestial Reference System, Barycentric Coordinate Time and up-to-date precession, 
nutation and polar motion models.
Kaynak: Tempo (astronomy)Aberration vs 
nutation: This motion was evidently not due to parallax, for the reasons given in the discussion of Figure 2, and neither was
Kaynak: Aberration of lightLondon, 82, 1905  primarily to explain 
nutation s, but are date June 2011 also used in Earth rotation predictions.  The semi-diurnal tide
Kaynak: Earth tideSuperimposed on this is a smaller motion of the ecliptic , and a small oscillation of the Earth's axis, 
nutation  In order to fix the
Kaynak: Equatorial coordinate system