Editing Precession (section)

===Apsidal precession===
[[File:Precessing Kepler orbit 280frames e0.6 smaller.gif|thumb|upright=1.25|[[Apsidal precession]]—the orbit rotates gradually over time.]]
{{main|Apsidal precession}}{{See also|Anomalous perihelion precession}}
The [[orbit]]s of planets around the [[Sun]] do not really follow an identical ellipse each time, but actually trace out a flower-petal shape because the major axis of each planet's elliptical orbit also precesses within its orbital plane, partly in response to perturbations in the form of the changing gravitational forces exerted by other planets. This is called perihelion precession or [[apsidal precession]].

In the adjunct image, Earth's apsidal precession is illustrated. As the Earth travels around the Sun, its elliptical orbit rotates gradually over time. The eccentricity of its ellipse and the precession rate of its orbit are exaggerated for visualization. Most orbits in the Solar System have a much smaller eccentricity and precess at a much slower rate, making them nearly circular and nearly stationary.

Discrepancies between the observed perihelion precession rate of the planet [[Mercury (planet)|Mercury]] and that predicted by [[classical mechanics]] were prominent among the forms of experimental evidence leading to the acceptance of [[Albert Einstein|Einstein]]'s [[Theory of Relativity]] (in particular, his [[General relativity|General Theory of Relativity]]), which accurately predicted the anomalies.<ref>[[Max Born]] (1924), ''Einstein's Theory of Relativity'' (The 1962 Dover edition, page 348 lists a table documenting the observed and calculated values for the precession of the perihelion of Mercury, Venus, and Earth.)</ref><ref>{{cite web| url = http://www.dailygalaxy.com/my_weblog/2008/03/18-billion-suns.html| title = An even larger value for a precession has been found, for a black hole in orbit around a much more massive black hole, amounting to 39 degrees each orbit.| date = 18 March 2008| access-date = 2023-11-15| archive-date = 2018-08-07| archive-url = https://web.archive.org/web/20180807131603/http://www.dailygalaxy.com/my_weblog/2008/03/18-billion-suns.html| url-status = bot: unknown}}</ref> Deviating from Newton's law, Einstein's theory of gravitation predicts an extra term of {{math|{{sfrac|''A''|''r''<sup>4</sup>}}}}, which accurately gives the observed excess turning rate of 43 [[arcsecond]]s every 100 years.