Editing Nutation (section)

==Astronomy==
{{main|Astronomical nutation|Perturbation (astronomy)}}

The nutation of a planet occurs because the gravitational effects of other bodies cause the speed of its [[axial precession]] to vary over time, so that the speed is not constant. English astronomer [[James Bradley]] discovered the nutation of [[Earth's rotation|Earth's axis]] in 1728.

===Earth===
{{split section|Earth's nutation|date=October 2020|discuss=Talk:Astronomical nutation#Split Earth's nutation}}
{{further|Geodynamics}}
[[File:Trópico de Cáncer en México - Carretera 83 (Vía Corta) Zaragoza-Victoria, Km 27+800.jpg|thumb|240px|Yearly changes in the location of the [[Tropic of Cancer]] near a highway in Mexico]]

Nutation subtly changes the [[axial tilt]] of Earth with respect to the [[ecliptic]] plane, shifting the [[Circle of latitude#Major circles of latitude|major circles of latitude]] that are defined by the Earth's tilt (the [[tropical circle]]s and the [[polar circle]]s).

In the case of Earth, the principal sources of tidal force are the [[Sun]] and [[Moon]], which continuously change location relative to each other and thus cause nutation in Earth's axis. The largest component of Earth's nutation has a period of 18.6 years, the same as that of the precession of the [[Lunar node|Moon's orbital nodes]].<ref name=Lowrie>{{cite book |last=Lowrie |first=William |title=Fundamentals of Geophysics |url=https://archive.org/details/fundamentalsgeop00lowr |url-access=limited |date=2007 |publisher=[[Cambridge University Press]] |location=Cambridge [u.a.] |isbn=9780521675963 |pages=[https://archive.org/details/fundamentalsgeop00lowr/page/n69 58]–59 |edition=2nd}}</ref> However, there are other significant periodic terms that must be accounted for depending upon the desired accuracy of the result. A mathematical description (set of equations) that represents nutation is called{{by whom|date=February 2022}} a "theory of nutation".{{citation needed|date=February 2022}} In the theory, parameters are adjusted in a more or less ''ad hoc'' method to obtain the best fit to data. Simple [[rigid body dynamics]] do not give the best theory; one has to account for deformations of the Earth, including [[asthenosphere|mantle inelasticity]] and changes in the [[core–mantle boundary]].<ref>{{cite web |url=http://www.iers.org/nn_10382/IERS/EN/Science/Recommendations/resolutionB3.html |title=Resolution 83 on non-rigid Earth nutation theory |work=[[International Earth Rotation and Reference Systems Service]] |publisher=Federal Agency for Cartography and Geodesy |date=2 April 2009 |access-date=2012-08-06}}</ref>

The principal term of nutation is due to the regression of the Moon's [[nodal line]] and has the same period of 6798 days (18.61 years). It reaches plus or minus 17&Prime; in [[longitude]] and 9.2&Prime; in [[axial tilt|obliquity]].<ref>{{cite web |url=http://www2.jpl.nasa.gov/basics/bsf2-1.php#nutation |title=Basics of Space Flight, Chapter 2 |date=28 August 2013 |access-date=2015-03-26 |publisher=[[Jet Propulsion Laboratory]]/NASA}}</ref> All other terms are much smaller; the next-largest, with a period of 183 days (0.5 year), has amplitudes 1.3&Prime; and 0.6&Prime; respectively. The periods of all terms larger than 0.0001&Prime; (about as accurately as available technology can measure) lie between 5.5 and 6798 days; for some reason (as with ocean tidal periods) they seem to avoid the range from 34.8 to 91 days, so it is customary{{according to whom|date=February 2022}} to split the nutation into long-period and short-period terms. The long-period terms are calculated and mentioned in the almanacs, while the additional correction due to the short-period terms is usually taken from a table. They can also be calculated from the [[Julian day]] according to IAU 2000B methodology.<ref>{{Cite web | url=http://www.neoprogrammics.com/nutations/ |title = NeoProgrammics - Science Computations}}</ref>