Editing Mercury (planet) (section)

=== Advance of perihelion ===
{{Main|Perihelion precession of Mercury}}
[[File:Drehung der Apsidenlinie light.svg|right|thumb|[[Apsidal precession]] of Mercury's orbit]]
In 1859, the French mathematician and astronomer [[Urbain Le Verrier]] reported that the slow [[precession]] of Mercury's orbit around the Sun could not be completely explained by [[Newtonian mechanics]] and perturbations by the known planets. He suggested, among possible explanations, that another planet (or perhaps instead a series of smaller "corpuscules") might exist in an orbit even closer to the Sun than that of Mercury, to account for this perturbation.<ref>{{cite journal | last=Le Verrier | first=Urbain | year=1859 | language=French | url=https://archive.org/stream/comptesrendusheb49acad#page/378/mode/2up | title=Lettre de M. Le Verrier à M. Faye sur la théorie de Mercure et sur le mouvement du périhélie de cette planète | journal=Comptes rendus hebdomadaires des séances de l'Académie des sciences | publication-place=Paris | volume=49 | pages=379–383 }} (At p. 383 in the same volume Le Verrier's report is followed by another, from Faye, enthusiastically recommending to astronomers to search for a previously undetected intra-mercurial object.)</ref> Other explanations considered included a slight oblateness of the Sun. The success of the search for [[Neptune]] based on its perturbations of the orbit of [[Uranus]] led astronomers to place faith in this possible explanation, and the hypothetical planet was named [[Vulcan (hypothetical planet)|Vulcan]], but no such planet was ever found.<ref>{{cite book |first1=Richard |last1=Baum |last2=Sheehan |first2=William |title=In Search of Planet Vulcan, The Ghost in Newton's Clockwork Machine |date=1997 |isbn=978-0-306-45567-4 |publisher=Plenum Press |location=New York |url-access=registration |url=https://archive.org/details/insearchofplanet0000baum }}</ref>

The observed [[perihelion precession]] of Mercury is 5,600 [[arcseconds]] (1.5556°) per century relative to Earth, or {{val|574.10|0.65|u=arcseconds}} per century<ref name="Clemence">{{cite journal |first=Gerald M. |last=Clemence |title=The Relativity Effect in Planetary Motions |journal=Reviews of Modern Physics |volume=19 |issue=4 |pages=361–364 |year=1947 |doi=10.1103/RevModPhys.19.361 |bibcode=1947RvMP...19..361C}}</ref> relative to the inertial [[International Celestial Reference Frame|ICRF]]. Newtonian mechanics, taking into account all the effects from the other planets and including 0.0254 arcseconds per century due to the oblateness of the Sun, predicts a precession of 5,557 arcseconds (1.5436°) per century relative to Earth, or {{val|531.63|0.69|u=arcseconds}} per century relative to ICRF.<ref name="Clemence" /> In the early 20th century, [[Albert Einstein]]'s [[general theory of relativity]] provided the explanation for the observed precession, by formalizing gravitation as being mediated by the curvature of spacetime. The effect is small: just {{val|42.980|0.001|u=arcseconds}} per century (or 0.43 arcsecond per year, or 0.1035 arcsecond per orbital period) for Mercury; it therefore requires a little over 12.5 million orbits, or 3 million years, for a full excess turn. Similar, but much smaller, effects exist for other Solar System bodies: 8.6247 arcseconds per century for Venus, 3.8387 for Earth, 1.351 for Mars, and 10.05 for [[1566 Icarus]].<ref>{{cite journal |last=Gilvarry |first=John J. |title=Relativity Precession of the Asteroid Icarus |journal=Physical Review |year=1953 |volume=89 |issue=5 |page=1046 |doi=10.1103/PhysRev.89.1046 |bibcode=1953PhRv...89.1046G}}</ref><ref>{{cite web |first=Kevin |last=Brown |url=http://www.mathpages.com/rr/s6-02/6-02.htm |title=6.2 Anomalous Precession |website=Reflections on Relativity |publisher=MathPages |access-date=May 22, 2008 |archive-date=August 3, 2019 |archive-url=https://web.archive.org/web/20190803235349/https://www.mathpages.com/rr/s6-02/6-02.htm |url-status=live }}</ref>