Editing Mercury (planet) (section)

== Orbit, rotation, and longitude ==
{{multiple image |direction=horizontal |align=right |total_width=400
|image1=ThePlanets Orbits Mercury PolarView.svg |caption1=Orbit of Mercury (2006)
|image2=Mercuryorbitsolarsystem.gif |caption2=Animation of Mercury's and Earth's revolution around the Sun
}}

Mercury has the most [[Orbital eccentricity|eccentric]] orbit of all the planets in the Solar System; its eccentricity is 0.21 with its distance from the Sun ranging from {{convert|46000000|to|70000000|km|mi|abbr=on}}. It takes 87.969 Earth days to complete an orbit. The diagram illustrates the effects of the eccentricity, showing Mercury's orbit overlaid with a circular orbit having the same [[semi-major axis]]. Mercury's higher velocity when it is near perihelion is clear from the greater distance it covers in each 5-day interval. In the diagram, the varying distance of Mercury to the Sun is represented by the size of the planet, which is inversely proportional to Mercury's distance from the Sun.

This varying distance to the Sun leads to Mercury's surface being flexed by [[tidal bulge]]s raised by the [[Sun]] that are about 17 times stronger than the Moon's on Earth.<ref>{{cite journal |last1=Van Hoolst |first1=Tim |last2=Jacobs |first2=Carla |year=2003 |title=Mercury's tides and interior structure |journal=Journal of Geophysical Research |volume=108 |issue=E11 |page=7 |doi=10.1029/2003JE002126 |bibcode=2003JGRE..108.5121V|doi-access=free }}</ref> Combined with a 3:2 [[#Spin-orbit resonance|spin–orbit resonance]] of the planet's rotation around its axis, it also results in complex variations of the surface temperature.<ref name="strom">{{cite book |first1=Robert G. |last1=Strom |last2=Sprague |first2=Ann L. |date=2003 |title=Exploring Mercury: the iron planet |publisher=Springer |isbn=978-1-85233-731-5 |url=https://archive.org/details/exploringmercury00stro }}</ref> The resonance makes a single [[solar day]] (the length between two [[meridian (astronomy)|meridian]] transits of the Sun) on Mercury last exactly two Mercury years, or about 176 Earth days.<ref name="compare">{{cite web |title=Space Topics: Compare the Planets: Mercury, Venus, Earth, The Moon, and Mars |publisher=Planetary Society |url=http://www.planetary.org/explore/topics/compare_the_planets/terrestrial.html |access-date=April 12, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20110728044444/http://www.planetary.org/explore/topics/compare_the_planets/terrestrial.html |archive-date=July 28, 2011}}</ref>

Mercury's orbit is inclined by 7 degrees to the plane of Earth's orbit (the [[ecliptic]]), the largest of all eight known solar planets.<ref name=Williams2019/> As a result, [[transits of Mercury]] across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between Earth and the Sun, which is in May or November. This occurs about every seven years on average.<ref>{{cite web |last=Espenak |first=Fred |date=April 21, 2005 |url=http://eclipse.gsfc.nasa.gov/transit/catalog/MercuryCatalog.html |title=Transits of Mercury |publisher=NASA/Goddard Space Flight Center |access-date=May 20, 2008 |archive-date=August 29, 2015 |archive-url=https://archive.today/20150829155710/http://eclipse.gsfc.nasa.gov/transit/catalog/MercuryCatalog.html |url-status=live }}</ref>

Mercury's [[axial tilt]] is almost zero,<ref name="Cosmic1" /> with the best measured value as low as 0.027 degrees.<ref name="Margot2007">{{cite journal |last1=Margot |first1=J. L. |display-authors=4 |last2=Peale |first2=S. J. |last3=Jurgens |first3=R. F. |last4=Slade |first4=M. A. |last5=Holin |first5=I. V. |title=Large Longitude Libration of Mercury Reveals a Molten Core |journal=Science |year=2007 |volume=316 |pages=710–714 |doi=10.1126/science.1140514 |bibcode=2007Sci...316..710M |pmid=17478713 |issue=5825|s2cid=8863681 }}</ref> This is significantly smaller than that of [[Jupiter]], which has the second smallest axial tilt of all planets at 3.1 degrees. This means that to an observer at Mercury's poles, the center of the Sun never rises more than 2.1 [[arcminutes]] above the horizon.<ref name="Margot2007" /> By comparison, the [[angular size]] of the Sun as seen from Mercury ranges from {{fraction|1|1|4}} to 2 degrees across.<ref>{{cite book | title=From The Sun To The Stars | first=James B. | last=Kaler | author-link=James B. Kaler | year=2016 | page=56 | isbn=9789813143265 | publisher=World Scientific Publishing Company | url=https://books.google.com/books?id=ZYv4DAAAQBAJ&pg=PA56 | access-date=October 25, 2023 | archive-date=October 31, 2023 | archive-url=https://web.archive.org/web/20231031140355/https://books.google.com/books?id=ZYv4DAAAQBAJ&pg=PA56 | url-status=live }}</ref>

At certain points on Mercury's surface, an observer would be able to see the Sun peek up a little more than two-thirds of the way over the horizon, then reverse and set before rising again, all within the same [[Extraterrestrial skies#Mercury|Mercurian day]].{{efn|name=angular}} This is because approximately four Earth days before perihelion, Mercury's angular [[orbital speed|orbital velocity]] equals its angular [[rotational velocity]] so that the Sun's [[improper motion|apparent motion]] ceases; closer to perihelion, Mercury's angular orbital velocity then exceeds the angular rotational velocity. Thus, to a hypothetical observer on Mercury, the Sun appears to move in a [[apparent retrograde motion|retrograde]] direction. Four Earth days after perihelion, the Sun's normal apparent motion resumes.<ref name="strom" /> A similar effect would have occurred if Mercury had been in synchronous rotation: the alternating gain and loss of rotation over a revolution would have caused a libration of 23.65° in longitude.<ref>{{cite book |title=Popular Astronomy: A Review of Astronomy and Allied Sciences |url=https://books.google.com/books?id=ePc-AQAAIAAJ |year=1896 |publisher=Goodsell Observatory of Carleton College |quote=although in the case of [[Venus]] the libration in longitude due to the eccentricity of the orbit amounts to only 47' on either side of the mean position, in the case of Mercury it amounts to 23° 39' |access-date=December 24, 2016 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301162153/https://books.google.com/books?id=ePc-AQAAIAAJ |url-status=live }}</ref>

For the same reason, there are two points on Mercury's equator, 180 degrees apart in [[longitude]], at either of which, around perihelion in alternate Mercurian years (once a Mercurian day), the Sun passes overhead, then reverses its apparent motion and passes overhead again, then reverses a second time and passes overhead a third time, taking a total of about 16 Earth-days for this entire process. In the other alternate Mercurian years, the same thing happens at the other of these two points. The amplitude of the retrograde motion is small, so the overall effect is that, for two or three weeks, the Sun is almost stationary overhead, and is at its most brilliant because Mercury is at perihelion, its closest to the Sun. This prolonged exposure to the Sun at its brightest makes these two points the hottest places on Mercury. Maximum temperature occurs when the Sun is at an angle of about 25 degrees past noon due to [[Diurnal temperature variation#Temperature lag|diurnal temperature lag]], at 0.4 Mercury days and 0.8 Mercury years past sunrise.<ref>{{cite web |last=Seligman |first=C. |title=The Rotation of Mercury |url=https://cseligman.com/text/planets/mercuryrot.htm |publisher=cseligman.com |at=NASA Flash animation |access-date=July 31, 2019 |archive-date=August 6, 2019 |archive-url=https://web.archive.org/web/20190806213722/http://cseligman.com/text/planets/mercuryrot.htm |url-status=live }}</ref> Conversely, there are two other points on the equator, 90 degrees of longitude apart from the first ones, where the Sun passes overhead only when the planet is at aphelion in alternate years, when the apparent motion of the Sun in Mercury's sky is relatively rapid. These points, which are the ones on the equator where the apparent retrograde motion of the Sun happens when it is crossing the horizon as described in the preceding paragraph, receive much less solar heat than the first ones described above.<ref>{{cite journal | title=On the Variations in the Insolation at Mercury Resulting from Oscillations of the Orbital Eccentricity | last=van Hemerlrijck | first=E. | journal=The Moon and the Planets | volume=29 | issue=1 | pages=83–93 | date=August 1983 | doi=10.1007/BF00928377 | bibcode=1983M&P....29...83V | s2cid=122761699 }}</ref>

Mercury attains an inferior conjunction (nearest approach to Earth) every 116 Earth days on average,<ref name="fact" /> but this interval can range from 105 days to 129 days due to the planet's eccentric orbit. Mercury can come as near as {{Convert|82200000|km|AU e6mi|abbr=in}} to Earth, and that is slowly declining: The next approach to within {{Convert|82100000|km|e6mi|abbr=unit|sigfig=2}} is in 2679, and to within {{Convert|82000000|km|e6mi|abbr=unit}} in 4487, but it will not be closer to Earth than {{Convert|80000000|km|e6mi|abbr=unit}} until 28,622.<ref>Mercury Closest Approaches to Earth generated with: <br /> 1. [http://chemistry.unina.it/~alvitagl/solex/ Solex 10]&nbsp; {{Webarchive|url=https://web.archive.org/web/20081220235836/http://chemistry.unina.it/~alvitagl/solex/ |date=December 20, 2008 }} ([http://home.surewest.net/kheider/astro/SolexMerc.txt Text Output file] {{webarchive|url=https://web.archive.org/web/20120309120624/http://home.surewest.net/kheider/astro/SolexMerc.txt |date=March 9, 2012 }}) <br /> 2. [http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1235936812 Gravity Simulator charts] {{Webarchive|url=https://web.archive.org/web/20140912091426/http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1235936812 |date=September 12, 2014 }} <br /> 3. [http://home.surewest.net/kheider/astro/Mercury.txt JPL Horizons 1950–2200]&nbsp; {{webarchive|url=https://web.archive.org/web/20151106172707/http://home.surewest.net/kheider/astro/Mercury.txt |date=November 6, 2015 }} {{noprint|(3 sources are provided to address [[WP:OR|original research]] concerns and to support general long-term trends) }}</ref> Its period of retrograde motion as seen from Earth can vary from 8 to 15 days on either side of an inferior conjunction. This large range arises from the planet's high orbital eccentricity.<ref name="strom" /> Essentially, because Mercury is closest to the Sun, when taking an average over time, Mercury is most often the closest planet to the Earth,<ref name="AIP Publishing 2019 p.">{{cite journal | title=Venus is not Earth's closest neighbor | journal=Physics Today | publisher=AIP Publishing | date=March 12, 2019 | issue=3 | issn=1945-0699 | doi=10.1063/pt.6.3.20190312a | page=30593| bibcode=2019PhT..2019c0593. | s2cid=241077611 }}</ref><ref name="MoreOrLess">{{cite web |last1=Harford |first1=Tim |title=BBC Radio 4 – More or Less, Sugar, Outdoors Play and Planets |url=https://www.bbc.co.uk/programmes/m0001y9p |publisher=BBC |date=January 11, 2019 |quote=Oliver Hawkins, more or less alumnus and statistical legend, wrote some code for us, which calculated which planet was closest to the Earth on each day for the past 50 years, and then sent the results to [[David A. Rothery]], professor of planetary geosciences at the Open University. |access-date=January 12, 2019 |archive-date=January 12, 2019 |archive-url=https://web.archive.org/web/20190112044935/https://www.bbc.co.uk/programmes/m0001y9p |url-status=live }}</ref> and—in that measure—it is the closest planet to each of the other planets in the Solar System.<ref>{{cite journal |last1=Stockman |first1=Tom |last2=Monroe |first2=Gabriel |last3=Cordner |first3=Samuel |title=Venus is not Earth's closest neighbor |journal=Physics Today |date=March 12, 2019 |issue=3 |page=30593 |doi=10.1063/PT.6.3.20190312a|bibcode=2019PhT..2019c0593. |s2cid=241077611 }}</ref><ref>{{cite AV media |last1=Stockman |first1=Tom |date=March 7, 2019 |title=Mercury is the closest planet to all seven other planets |medium=video |url=https://www.youtube.com/watch?v=GDgbVIqGADQ | archive-url=https://ghostarchive.org/varchive/youtube/20211028/GDgbVIqGADQ| archive-date=October 28, 2021|access-date=May 29, 2019 |via=YouTube }}{{cbignore}}</ref><ref>{{Citation|title=🌍 Which Planet is the Closest?| date=October 30, 2019 |url=https://www.youtube.com/watch?v=SumDHcnCRuU| archive-url=https://ghostarchive.org/varchive/youtube/20211028/SumDHcnCRuU| archive-date=October 28, 2021|language=en|access-date=July 22, 2021}}{{cbignore}}</ref>{{efn|In astronomical literature, the term "closest planets" often means "the two planets that approach each other most closely". In other words, the orbits of the two planets approach each other most closely. However, this does ''not'' mean that the two planets are closest over a long period of time. For example, essentially because Mercury is closer to the Sun than Venus, Mercury spends more time in proximity to Earth; it could, therefore, be said that Mercury is the planet that is "closest to Earth when averaged over time". However, it turns out that using this time-average definition of 'closeness', Mercury can be the "closest planet" to ''all'' other planets in the solar system.}}

=== Longitude convention ===
The longitude convention for Mercury puts the zero of longitude at one of the two hottest points on the surface, as described above. However, when this area was first visited, by {{nowrap|''Mariner 10''}}, this zero meridian was in darkness, so it was impossible to select a feature on the surface to define the exact position of the meridian. Therefore, a small crater further west was chosen, called [[Hun Kal]], which provides the exact reference point for measuring longitude.<ref>{{cite journal | last=Davies | first=M. E. | title=Surface Coordinates and Cartography of Mercury | journal=Journal of Geophysical Research | volume=80 | issue=B17 | pages=2417–2430 | date=June 10, 1975 | doi=10.1029/JB080i017p02417 | bibcode=1975JGR....80.2417D }}</ref><ref>{{cite book | last1=Davies | first1=M. E. | first2=S. E. | last2=Dwornik | first3=D. E. | last3=Gault | first4=R. G. | last4=Strom | title=NASA Atlas of Mercury | publisher=NASA Scientific and Technical Information Office | date=1978 }}</ref> The center of Hun Kal defines the 20°&nbsp;west meridian. A 1970 International Astronomical Union resolution suggests that longitudes be measured positively in the westerly direction on Mercury.<ref name="usgs">{{cite web |url=https://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table1.html |access-date=October 22, 2009 |title=USGS Astrogeology: Rotation and pole position for the Sun and planets (IAU WGCCRE) |archive-url=https://web.archive.org/web/20111024101856/http://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table1.html |archive-date=October 24, 2011 |url-status=dead}}</ref> The two hottest places on the equator are therefore at longitudes 0°&nbsp;W and 180°&nbsp;W, and the coolest points on the equator are at longitudes 90°&nbsp;W and 270°&nbsp;W. However, the ''MESSENGER'' project uses an east-positive convention.<ref name="ArchinalA'Hearn2010">{{cite journal |last1=Archinal |first1=Brent A. |display-authors=4 |last2=A'Hearn |first2=Michael F. |last3=Bowell |first3=Edward L. |last4=Conrad |first4=Albert R. |last5=Consolmagno |first5=Guy J. |last6=Courtin |first6=Régis |last7=Fukushima |first7=Toshio |last8=Hestroffer |first8=Daniel |last9=Hilton |first9=James L. |last10=Krasinsky |first10=George A. |last11=Neumann |first11=Gregory A. |last12=Oberst |first12=Jürgen |last13=Seidelmann |first13=P. Kenneth |last14=Stooke |first14=Philip J. |last15=Tholen |first15=David J. |last16=Thomas |first16=Peter C. |last17=Williams |first17=Iwan P. |title=Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009 |journal=Celestial Mechanics and Dynamical Astronomy |volume=109 |issue=2 |year=2010 |pages=101–135 |issn=0923-2958 |doi=10.1007/s10569-010-9320-4 |bibcode=2011CeMDA.109..101A|s2cid=189842666 }}</ref>

=== Spin-orbit resonance ===
[[File:Mercury's orbital resonance.svg|thumb|After one orbit, Mercury has rotated 1.5 times, so after two complete orbits the same hemisphere is again illuminated.]]

For many years it was thought that Mercury was synchronously [[tidally locked]] with the Sun, [[rotating]] once for each orbit and always keeping the same face directed towards the Sun, in the same way that the same side of the Moon always faces Earth. Radar observations in 1965 proved that the planet has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun. The eccentricity of Mercury's orbit makes this resonance stable—at perihelion, when the solar tide is strongest, the Sun is nearly stationary in Mercury's sky.<ref>{{cite journal |last1=Liu |first1=Han-Shou |last2=O'Keefe |first2=John A. |title=Theory of Rotation for the Planet Mercury |journal=Science |year=1965 |volume=150 |issue=3704 |page=1717 |doi=10.1126/science.150.3704.1717 |pmid=17768871 |bibcode=1965Sci...150.1717L|s2cid=45608770 }}</ref>

The 3:2 resonant tidal locking is stabilized by the variance of the tidal force along Mercury's eccentric orbit, acting on a permanent dipole component of Mercury's mass distribution.<ref name="Colombo" /> In a circular orbit there is no such variance, so the only resonance stabilized in such an orbit is at 1:1 (e.g., Earth–Moon), when the tidal force, stretching a body along the "center-body" line, exerts a torque that aligns the body's axis of least inertia (the "longest" axis, and the axis of the aforementioned dipole) to always point at the center. However, with noticeable eccentricity, like that of Mercury's orbit, the tidal force has a maximum at perihelion and therefore stabilizes resonances, like 3:2, ensuring that the planet points its axis of least inertia roughly at the Sun when passing through perihelion.<ref name="Colombo" />

The original reason astronomers thought it was synchronously locked was that, whenever Mercury was best placed for observation, it was always nearly at the same point in its 3:2 resonance, hence showing the same face. This is because, coincidentally, Mercury's rotation period is almost exactly half of its synodic period with respect to Earth. Due to Mercury's 3:2 spin-orbit resonance, a solar day lasts about 176 Earth days.<ref name="strom" /> A [[sidereal day]] (the period of rotation) lasts about 58.7 Earth days.<ref name="strom" />

Simulations indicate that the orbital eccentricity of Mercury varies [[chaos theory|chaotically]] from nearly zero (circular) to more than 0.45 over millions of years due to [[Perturbation (astronomy)|perturbations]] from the other planets.<ref name="strom" /><ref name="Correia2009">{{cite journal |last1=Correia |first1=Alexandre C. M. |last2=Laskar |first2=Jacques |title=Mercury's capture into the 3/2 spin-orbit resonance including the effect of core–mantle friction |journal=Icarus |year=2009 |doi=10.1016/j.icarus.2008.12.034 |arxiv=0901.1843 |volume=201 |issue=1 |pages=1–11 |bibcode=2009Icar..201....1C|s2cid=14778204 }}</ref> This was thought to explain Mercury's 3:2 spin-orbit resonance (rather than the more usual 1:1), because this state is more likely to arise during a period of high eccentricity.<ref name="Correia">{{cite journal |last1=Correia |first1=Alexandre C. M. |last2=Laskar |first2=Jacques |year=2004 |title=Mercury's capture into the 3/2 spin-orbit resonance as a result of its chaotic dynamics |journal=[[Nature (journal)|Nature]] |volume=429 |pages=848–850 |doi=10.1038/nature02609 |pmid=15215857 |issue=6994 |bibcode=2004Natur.429..848C|s2cid=9289925 }}</ref> However, accurate modeling based on a realistic model of tidal response has demonstrated that Mercury was captured into the 3:2 spin-orbit state at a very early stage of its history, within 20 (more likely, 10) million years after its formation.<ref>{{Cite journal |bibcode=2014Icar..241...26N |last1=Noyelles |first1=B. |last2=Frouard |first2=J. |last3=Makarov |first3=V. V. |last4=Efroimsky |first4=M. |name-list-style=amp |title=Spin-orbit evolution of Mercury revisited |journal=Icarus |pages=26–44 |year=2014 |volume=241 |issue=2014 |doi=10.1016/j.icarus.2014.05.045 |arxiv=1307.0136|s2cid=53690707 }}</ref>

Numerical simulations show that a future [[Secular resonance|secular]] [[Orbital resonance|orbital resonant]] interaction with the perihelion of Jupiter may cause the eccentricity of Mercury's orbit to increase to the point where there is a 1% chance that the orbit will be destabilized in the next five billion years. If this happens, Mercury may fall into the Sun, collide with Venus, be ejected from the Solar System, or even disrupt the rest of the inner Solar System.<ref name="Laskar2008">{{cite journal |last=Laskar |first=Jacques |date=March 18, 2008 |title=Chaotic diffusion in the Solar System |journal=[[Icarus (journal)|Icarus]] |volume=196 |issue=1 |pages=1–15 |bibcode=2008Icar..196....1L |doi=10.1016/j.icarus.2008.02.017 |arxiv=0802.3371|s2cid=11586168 }}</ref><ref name="Laskar2009">{{cite journal |last1=Laskar |first1=Jacques |last2=Gastineau |first2=Mickaël |date=June 11, 2009 |title=Existence of collisional trajectories of Mercury, Mars and Venus with the Earth |journal=[[Nature (journal)|Nature]] |volume=459 |issue=7248 |pages=817–819 |doi=10.1038/nature08096 |bibcode=2009Natur.459..817L |pmid=19516336|s2cid=4416436 }}</ref>

=== 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>