Editing Polar motion (section)

==Observations==
Polar motion is observed routinely by [[space geodesy]] methods such as [[very-long-baseline interferometry]],<ref>{{cite book|first=H |last=Schuh |chapter=Earth's rotation measured by VLBI|pages=1&ndash;12 |doi=10.1007/978-3-642-75587-3_1 |editor1=Peter Brosche |editor2=Jürgen Sündermann|title=Earth's Rotation from Eons to Days: Proceedings of a Workshop Held at the Centre for Interdisciplinary Research (ZiF) of the University of Bielefeld, FRG. September 26-30, 1988|date=1990|publisher=Springer Berlin Heidelberg|isbn=978-3-642-75587-3}}</ref> [[Lunar Laser Ranging experiment|lunar laser ranging]] and [[satellite laser ranging]].<ref name=Eubanks>{{cite book|last1=Eubanks |first1=T.M. |chapter=Variations in the orientation of the earth  |editor1=David E. Smith |editor2=Donald L. Turcotte|title=Contributions of space geodesy to geodynamics: Earth dynamics|date=1993|publisher=American Geophysical Union|location=Washington, D.C.|isbn=9781118669723}}</ref> The annual component is rather constant in amplitude, and its frequency varies by not more than 1 to 2%. The amplitude of the Chandler wobble, however, varies by a factor of three, and its frequency by up to 7%. Its maximum amplitude during the last 100 years never exceeded 230 mas.

The [[Chandler wobble]] is usually considered a resonance phenomenon, a free [[nutation]] that is excited by a source and then dies away with a time constant τ<sub>D</sub> of the order of 100&nbsp;years. It is a measure of the elastic reaction of the Earth.<ref>{{cite journal|last1=Dickey|first1=Jean|last2=Eubanks|first2=T.|title=Earth Rotation and Polar Motion: Measurements and Implications|journal=IEEE Transactions on Geoscience and Remote Sensing|date=July 1985|volume=GE-23|issue=4|pages=373–384|doi=10.1109/TGRS.1985.289427|bibcode=1985ITGRS..23..373D|s2cid=46607194}}</ref> It is also the explanation for the deviation of the Chandler period from the Euler period. However, rather than dying away, the Chandler wobble, continuously observed for more than 100 years, varies in amplitude and shows a sometimes rapid frequency shift within a few years.<ref>Guinot, B., The Chandlerian wobble from 1900 to 1970, Astron. Astrophys., '''19''', 07, 1992</ref> This reciprocal behavior between amplitude and frequency has been described by the empirical formula:<ref>Vondrak, J., Long-periodic behaviour of polar motion between 1900 and 1980, A. Geophys., '''3''', 351, 1985</ref>

(2) {{pad|4em}} m = 3.7/(ν − 0.816) {{pad|2em}} (for 0.83 < ν < 0.9)

with m the observed amplitude (in units of mas), and ν the frequency (in units of reciprocal sidereal years) of the Chandler wobble. In order to generate the Chandler wobble, recurring excitation is necessary. Seismic activity, groundwater movement, snow load, or atmospheric interannual dynamics have been suggested as such recurring forces, e.g.<ref name=Eubanks/><ref>Runcorn, S.K., et al., The excitation of the Chandler wobble, Surv. Geophys., '''9''', 419, 1988</ref> Atmospheric excitation seems to be the most likely candidate.<ref>Hide, 1984 Rotation of the atmosphere of the earth and planets, Phil. Trans. R. Soc., '''A313''', 107</ref><ref name=VollandAR>{{cite journal | last1 = Volland | first1 = H | year = 1996 | title = Atmosphere and Earth' Rotation | journal = Surv. Geophys. | volume = 17 | issue = 1| page = 101 | doi=10.1007/bf01904476| bibcode = 1996SGeo...17..101V | s2cid = 129884741 }}</ref> Others propose a combination of atmospheric and oceanic processes, with the dominant excitation mechanism being ocean‐bottom pressure fluctuations.<ref>{{cite journal | last1 = Gross | first1 = R | year = 2001 | title = The excitation of the Chandler Wobble | journal = Geophys. Res. Lett. | volume = 27 | issue = 15| page = 2329 | doi=10.1029/2000gl011450 | bibcode=2000GeoRL..27.2329G| doi-access =  }}</ref>

Current and historic polar motion data is available from the [[International Earth Rotation and Reference Systems Service]]'s [[Earth orientation parameters]].<ref>{{cite web|title=Earth orientation data|url=http://www.iers.org/IERS/EN/DataProducts/EarthOrientationData/eop.html|website=International Earth Rotation and Reference Systems Service|publisher=Federal Agency for Cartography and Geodesy|access-date=7 September 2015}}</ref> Note in using this data that the convention is to define {{math|''p''<sub>''x''</sub>}} to be positive along 0° longitude and {{math|''p''<sub>''y''</sub>}} to be positive along 90°E longitude.<ref>
{{cite web|title=IERS Conventions 2010: Chapter 8|url=http://tai.bipm.org/iers/conv2010/conv2010_c8.html|page=§8.3}}</ref>