{{short description|Relative directions of orbit or rotation}} {{about|retrograde motions of celestial bodies relative to a gravitationally central object|the apparent motion as seen from a particular vantage point|Apparent retrograde motion}} [[File:Retrogradeorbit.gif|thumb|Retrograde orbit: the satellite (red) orbits in the direction opposite to the rotation of its primary (blue/black)]]
'''Retrograde motion''' in astronomy is, in general, [[orbit]]al or [[rotation]]al motion of an object in the direction opposite the rotation of its [[Primary (astronomy)|primary]], that is, the central object (right figure). It may also describe other motions such as [[Axial precession|precession]] or [[Astronomical nutation|nutation]] of an object's [[Rotation around a fixed axis|rotational axis]]. '''Prograde''' or '''direct motion''' is more normal motion in the same direction as the primary rotates. However, "retrograde" and "prograde" can also refer to an object other than the primary if so described. The direction of rotation is determined by an [[inertial frame of reference]], such as distant [[fixed stars]].
In the [[Solar System]], the orbits around the [[Sun]] of all [[planet]]s and [[dwarf planets]] and most [[small Solar System body|small Solar System bodies]], except many [[comet]]s and few [[distant minor planet|distant object]]s, are prograde. They orbit around the Sun in the same direction as the sun rotates about its axis, which is [[counterclockwise]] when observed from above the Sun's north pole. Except for [[Venus]] and [[Uranus]], planetary rotations around their axis are also prograde. Most [[natural satellite]]s have prograde orbits around their planets. Prograde satellites of Uranus orbit in the direction Uranus rotates, which is retrograde to the Sun. Nearly all [[regular satellite]]s are [[tidally locked]] and thus have prograde rotation. Retrograde satellites are generally [[irregular satellite|small and distant]] from their planets, except [[Neptune]]'s satellite [[Triton (moon)|Triton]], which is large and close. All retrograde satellites are thought to have formed separately before being [[Asteroid capture|captured]] by their planets.
Most low-inclination [[artificial satellite]]s of Earth have been placed in a prograde orbit, because in this situation less propellant is required to reach the orbit.
== Formation of celestial systems == When a [[galaxy]] or a [[planetary system]] [[Nebular hypothesis|forms]], its material takes a shape similar to that of a disk. Most of the material orbits and rotates in one direction. This uniformity of motion is due to the collapse of a gas cloud.<ref name="NS_Aug">{{cite journal | last = Grossman | first = Lisa | title = Planet found orbiting its star backwards for first time | journal = New Scientist | date = 13 August 2008 | url = https://www.newscientist.com/article/dn17603-planet-found-orbiting-its-star-backwards-for-first-time.html | access-date = 10 October 2009}}</ref> The nature of the collapse is explained by [[conservation of angular momentum]]. In 2010 the discovery of several [[hot Jupiter]]s with backward orbits called into question the theories about the formation of planetary systems.<ref name="2010question">{{cite web|url=http://www.astro.gla.ac.uk/nam2010/pr10.php|title=NAM2010 at the University of Glasgow|access-date=2010-04-15|archive-date=2011-07-16|archive-url=https://web.archive.org/web/20110716051715/http://www.astro.gla.ac.uk/nam2010/pr10.php|url-status=dead}}</ref> This can be explained by noting that stars and their planets do not form in isolation but in [[star cluster]]s that contain [[molecular cloud]]s. When a [[protoplanetary disk]] collides with or steals material from a cloud this can result in retrograde motion of a disk and the resulting planets.<ref name=steal>{{cite web |url=https://www.newscientist.com/article/dn20818-stars-that-steal-give-birth-to-backwards-planets.html |title=Stars that steal give birth to backwards planets |website=New Scientist |date=23 August 2011 |author=Lisa Grossman}}</ref><ref name=natural-misalign>Ingo Thies, Pavel Kroupa, Simon P. Goodwin, Dimitris Stamatellos, Anthony P. Whitworth, [https://arxiv.org/abs/1107.2113 "A natural formation scenario for misaligned and short-period eccentric extrasolar planets"], 11 July 2011</ref>
== Orbital and rotational parameters ==
=== Orbital inclination === A celestial object's [[inclination]] indicates whether the object's ''orbit'' is prograde or retrograde. The inclination of a celestial object is the [[angle]] between its [[orbital plane (astronomy)|orbital plane]] and another reference frame such as the [[equatorial plane]] of the object's primary. In the [[Solar System]], inclination of the planets is measured from the [[ecliptic plane]], which is the [[Plane (geometry)|plane]] of [[Earth]]'s orbit around the [[Sun]].<ref>{{cite book | author=McBride, Neil | author2=Bland, Philip A. | author3=Gilmour, Iain | title=An Introduction to the Solar System | date=2004 | page=248 | publisher=Cambridge University Press | isbn=978-0-521-54620-1 }}</ref> The inclination of [[moons]] is measured from the equator of the planet they orbit. An object with an inclination between 0 and 90 degrees is orbiting or revolving in the same direction as the primary is rotating. An object with an inclination of exactly 90 degrees has a perpendicular orbit that is neither prograde nor retrograde. An object with an inclination between 90 degrees and 180 degrees is in a retrograde orbit.
=== Axial tilt === A celestial object's [[axial tilt]] indicates whether the object's ''rotation'' is prograde or retrograde. Axial tilt is the angle between an object's rotation axis and a line [[perpendicular]] to its [[Orbital plane (astronomy)|orbital plane]] passing through the object's centre. An object with an axial tilt up to 90 degrees is rotating in the same direction as its primary. An object with an axial tilt of exactly 90 degrees, has a perpendicular rotation that is neither prograde nor retrograde. An object with an axial tilt between 90 degrees and 180 degrees is rotating in the opposite direction to its orbital direction. Regardless of inclination or axial tilt, the [[Poles of astronomical bodies|north pole of any planet or moon]] in the Solar System is defined as the pole that is in the same celestial hemisphere as Earth's north pole.
==Solar System bodies== ===Planets=== All eight planets in the [[Solar System]] orbit the Sun in the direction of the Sun's rotation, which is [[counterclockwise]] when viewed from above the Sun's [[Poles of astronomical bodies#Geographic poles|north pole]]. Six of the planets also rotate about their axis in this same direction. The exceptions – the planets with retrograde rotation – are [[Venus]] and [[Uranus]]. Venus's [[axial tilt]] is 177°, which means it is rotating almost exactly in the opposite direction to its orbit. Uranus has an axial tilt of 97.77°, so its axis of rotation is approximately parallel with the plane of the Solar System.
The reason for Uranus's unusual axial tilt is not known with certainty, but the usual speculation is that it was caused by a collision with an Earth-sized [[protoplanet]] during the formation of the Solar System.<ref>{{cite book |last1=Bergstralh |first1=Jay T. |last2=Miner |first2=Ellis |last3=Matthews |first3=Mildred |title=Uranus |date=1991 |pages=485–86 |publisher=University of Arizona Press |isbn=978-0-8165-1208-9}}</ref>
It is unlikely that Venus was formed with its present slow retrograde rotation, which takes 243 days. Venus probably began with a fast prograde rotation with a period of several hours much like most of the planets in the Solar System. Venus is close enough to the Sun to experience significant gravitational [[tidal locking|tidal dissipation]], and also has a thick enough [[atmosphere of Venus|atmosphere]] to create thermally driven atmospheric [[tide]]s that create a retrograde [[torque]]. Venus's present slow retrograde rotation is an approximate [[mechanical equilibrium|equilibrium]] between gravitational tides trying to [[tidal locking|tidally lock]] Venus to the Sun and atmospheric tides trying to spin Venus in a retrograde direction. These effects are also sufficient to account for evolution of Venus's rotation from a primordial fast prograde direction to its present-day slow retrograde rotation,<ref name="TidalEvoExop">{{cite book |first1=Alexandre C. M. |last1=Correia |first2=Jacques |last2=Laskar |chapter=Tidal Evolution of Exoplanets |title=Exoplanets |editor=S. Seager |publisher=[[University of Arizona Press]] |year=2010 |arxiv=1009.1352 }}</ref> which is not completely stable. Venus's rotation period measured with ''Magellan'' spacecraft data over a 500-day period is smaller than the rotation period measured during the 16-year period between the Magellan spacecraft and ''Venus Express'' visits, with a difference of about 6.5{{spaces}}minutes.<ref name="slowing spin">{{cite web | title=Could Venus Be Shifting Gear? | date=10 February 2012 | series=Venus Express | publisher=European Space Agency | url=http://www.esa.int/Our_Activities/Space_Science/Venus_Express/Could_Venus_be_shifting_gear | access-date=7 January 2016 | archive-date=24 January 2016 | archive-url=https://web.archive.org/web/20160124032008/http://www.esa.int/Our_Activities/Space_Science/Venus_Express/Could_Venus_be_shifting_gear | url-status=live }}</ref> In the past, various alternative hypotheses have been proposed to explain Venus's retrograde rotation, such as collisions or it having originally formed that way.{{fact|date=June 2025}}
Despite being closer to the Sun than Venus, [[Mercury (planet)|Mercury]] is not tidally locked because it has entered a [[Mercury (planet)#Spin-orbit resonance|3:2 spin–orbit resonance]] due to the [[orbital eccentricity|eccentricity]] of its orbit. Mercury's prograde rotation is slow enough that due to its eccentricity, its angular orbital velocity exceeds its angular rotational velocity near [[perihelion]], causing the motion of the sun in Mercury's sky to temporarily reverse.<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-access=registration |url=https://archive.org/details/exploringmercury00stro | page=37 }}</ref> The rotations of Earth and Mars are also affected by [[tidal force]]s with the Sun, but they have not reached an equilibrium state like Mercury and Venus because they are further out from the Sun where tidal forces are weaker. The [[gas giant]]s of the Solar System are too massive and too far from the Sun for tidal forces to slow down their rotations.<ref name="TidalEvoExop" />
=== Dwarf planets === All known [[dwarf planet]]s and [[List of possible dwarf planets|dwarf planet candidates]] have prograde orbits around the Sun, but some have retrograde rotation. [[Pluto]] has retrograde rotation; its axial tilt is approximately 120 degrees.<ref>{{cite web |url=http://www.daviddarling.info/encyclopedia/P/Pluto.html|title=Pluto (minor planet 134340)}}</ref> Pluto and its moon [[Charon (moon)|Charon]] are tidally locked to each other. It is suspected that the Plutonian satellite system was created by a [[Collisional family|massive collision]].<ref name="Canup">{{cite journal| last = Canup | first = R. M.| author-link = Robin Canup | title = A Giant Impact Origin of Pluto-Charon| journal = [[Science (journal)|Science]] | volume = 307 | issue = 5709 | pages = 546–550| date = 2005-01-08| doi = 10.1126/science.1106818 |bibcode = 2005Sci...307..546C | pmid=15681378| s2cid = 19558835| url = https://authors.library.caltech.edu/51983/7/Canup.SOM.pdf}}</ref><ref name="Stern">{{cite journal |display-authors = 4 |last= Stern |first= S. A. |author-link= Alan Stern |author2= Weaver, H. A. |author3= Steff, A. J. |author4= Mutchler, M. J. |author5= Merline, W. J. |author6= Buie, M. W. |author7= Young, E. F. |author8= Young, L. A. |author9= Spencer, J. R. |title= A giant impact origin for Pluto's small moons and satellite multiplicity in the Kuiper belt |journal= [[Nature (journal)|Nature]] |volume= 439 |issue= 7079 |pages= 946–948 |date= 2006-02-23 |doi= 10.1038/nature04548 |bibcode= 2006Natur.439..946S |pmid= 16495992 |s2cid= 4400037 }}</ref>
=== Natural satellites and rings === [[File:RetrogradeBaan.gif|thumb|right|The orange moon is in a retrograde orbit.]] If formed in the gravity field of a planet as the planet is forming, a [[Natural satellite|moon]] will orbit the planet in the same direction as the planet is rotating and is a [[regular moon]]. If an object is formed elsewhere and later captured into orbit by a planet's gravity, it can be captured into either a retrograde or prograde orbit depending on whether it first approaches the side of the planet that is rotating towards or away from it. This is an [[irregular moon]].<ref>{{cite encyclopedia | title = Encyclopedia of the solar system | publisher = Academic Press | date = 2007}}</ref>
In the Solar System, many of the asteroid-sized moons have retrograde orbits, whereas all the large moons except [[Triton (moon)|Triton]] (the largest of Neptune's moons) have prograde orbits.<ref>{{cite journal | last = Mason | first = John | title = Science: Neptune's new moon baffles the astronomers | journal = New Scientist | date = 22 July 1989 | url = https://www.newscientist.com/article/mg12316742.600-science-neptunes-new-moon-baffles-the-astronomers.html | access-date = 10 October 2009}}</ref> The particles in Saturn's [[Rings of Saturn#Phoebe ring|Phoebe ring]] are thought to have a retrograde orbit because they originate from the irregular moon [[Phoebe (moon)|Phoebe]].
All retrograde satellites experience [[Tidal acceleration#Tidal deceleration|tidal deceleration]] to some degree. The only satellite in the Solar System for which this effect is non-negligible is Neptune's moon Triton. All the other retrograde satellites are on distant orbits and tidal forces between them and the planet are negligible.
Within the [[Hill sphere]], the region of stability for retrograde orbits at a large distance from the primary is larger than that for prograde orbits. This has been suggested as an explanation for the preponderance of retrograde moons around Jupiter. Because Saturn has a more even mix of retrograde/prograde moons, however, the underlying causes appear to be more complex.<ref name="Astakhov2003">{{cite journal|last1= Astakhov|first1=S. A.|last2=Burbanks|first2=A. D.|last3=Wiggins|first3= S.|last4= Farrelly|first4= D.|title= Chaos-assisted capture of irregular moons|journal= Nature|volume= 423|issue= 6937|year= 2003|pages=264–267|doi=10.1038/nature01622|pmid=12748635|bibcode=2003Natur.423..264A|s2cid=16382419}}</ref>
With the exception of [[Hyperion (moon)|Hyperion]], all the known [[regular moon|regular planetary natural satellites]] in the Solar System are [[Tidal locking|tidally locked]] to their host planet, so they have zero rotation relative to their host planet, but have the same type of rotation as their host planet relative to the Sun because they have prograde orbits around their host planet. That is, they all have prograde rotation relative to the Sun except those of Uranus.
If there is a collision, material could be ejected in any direction and coalesce into either prograde or retrograde moons, which may be the case for the moons of dwarf planet [[Haumea (dwarf planet)|Haumea]], although Haumea's rotation direction is not known.<ref>Matija Ćuk, Darin Ragozzine, David Nesvorný, [https://arxiv.org/abs/1308.1990 "On the Dynamics and Origin of Haumea's Moons"], 12 August 2013</ref>
=== Small Solar System bodies === ==== Asteroids ==== Many [[asteroids]] have a prograde orbit around the Sun. Only approximately a hundred [[List of notable asteroids#Retrograde and highly inclined|asteroids in retrograde orbits]] are known.
Some asteroids with retrograde orbits may be burnt-out comets,<ref name="NS_May">{{cite journal | last = Hecht | first = Jeff | title = Nearby asteroid found orbiting Sun backwards | journal = New Scientist | date = 1 May 2009 | url = https://www.newscientist.com/article/dn17073-nearby-asteroid-found-orbiting-sun-backwards.html | access-date = 10 October 2009}}</ref> but some may acquire their retrograde orbit due to gravitational interactions with [[Jupiter]].<ref name="greenstreet">S. Greenstreet, B. Gladman, H. Ngo, M. Granvik, and S. Larson, "Production of Near-earth Asteroids on Retrograde Orbits", ''The Astrophysical Journal Letters'', 749:L39 (5pp), 2012 April 20</ref>
Due to their small size and their large distance from Earth it is difficult to [[telescope|telescopically]] analyse the rotation of most asteroids. As of 2012, data is available for fewer than 200 asteroids and the different methods of determining the orientation of [[Orbital pole|poles]] often result in large discrepancies.<ref>{{cite journal|doi=10.1016/j.pss.2012.02.017|title=Spin vectors of asteroids: Updated statistical properties and open problems|journal=Planetary and Space Science|volume= 73|issue=1|pages= 70–74|year= 2012|last1= Paolicchi|first1= P.|last2= Kryszczyńska|first2= A.|bibcode= 2012P&SS...73...70P}}</ref> The asteroid spin vector catalog at Poznan Observatory<ref>{{cite web |url=http://vesta.astro.amu.edu.pl/Science/Asteroids/|title=Physical studies of asteroids at Poznan Observatory}}</ref> avoids use of the phrases "retrograde rotation" or "prograde rotation" as it depends which reference plane is meant and asteroid coordinates are usually given with respect to the [[ecliptic plane]] rather than the asteroid's orbital plane.<ref>[http://vesta.astro.amu.edu.pl/Science/Asteroids/Spindata/spin.txt Documentation for Asteroid Spin Vector Determinations]</ref>
Asteroids with satellites, also known as binary asteroids, make up about 15% of all asteroids less than 10 km in diameter in the [[Asteroid belt|main belt]] and [[Near-Earth asteroids|near-Earth]] population and most are thought to be formed by the [[YORP effect]] causing an asteroid to spin so fast that it breaks up.<ref>Kevin J. Walsh, Derek C. Richardson & Patrick Michel, [https://crimson.oca.eu/IMG/pdf/Walshetal2008Nature454.pdf "Rotational breakup as the origin of small binary asteroids"] {{Webarchive|url=https://web.archive.org/web/20160304042006/https://crimson.oca.eu/IMG/pdf/Walshetal2008Nature454.pdf |date=2016-03-04 }}, ''Nature'', Vol. 454, 10 July 2008</ref> As of 2012, and where the rotation is known, all [[Minor-planet moon|satellites of asteroids]] orbit the asteroid in the same direction as the asteroid is rotating.<ref>N. M. Gaftonyuk, N. N. Gorkavyi, [https://link.springer.com/article/10.1134%2FS0038094613020032 "Asteroids with satellites: Analysis of observational data"], ''Solar System Research'', May 2013, Volume 47, Issue 3, pp. 196–202</ref>
Most known objects that are in [[orbital resonance]] are orbiting in the same direction as the objects they are in resonance with, however a few retrograde asteroids have been found in resonance with [[Jupiter]] and [[Saturn]].<ref name="Morais2013">{{cite journal | last1 = Morais | first1 = M. H. M. | last2= Namouni | first2 = F. | title = Asteroids in retrograde resonance with Jupiter and Saturn | journal = [[Monthly Notices of the Royal Astronomical Society Letters]] | date = 2013-09-21 | bibcode = 2013MNRAS.436L..30M | volume = 436 | issue = 1 | pages = L30–L34 | doi = 10.1093/mnrasl/slt106 | doi-access = free | arxiv = 1308.0216| s2cid = 119263066 }}</ref>
==== Comets ==== [[Comets]] from the [[Oort cloud]] are much more likely than asteroids to be retrograde.<ref name="NS_May" /> [[Halley's Comet]] has a retrograde orbit around the Sun.<ref>{{cite web|url=http://csep10.phys.utk.edu/astr161/lect/comets/halley.html|title=Comet Halley}}</ref>
==== Centaurs ==== Most [[centaur (minor planet)|centaur]]s have a prograde orbit around the Sun. The first centaur with a retrograde orbit to be discovered was [[20461 Dioretsa]].<ref>{{cite web|url=https://www.aanda.org/articles/aa/full_html/2024/07/aa48985-23/aa48985-23.html|title=Physical parameters and orbital evolution of asteroids in retrograde orbits}}</ref> Other known centaurs with retrograde orbits include {{mp|2004 NN|8}}, {{mp|2012 TL|139}}, {{mpl|(434620) 2005 VD}}, {{mpl|2006 BZ|8}}, and {{mpl|2006 RJ|2}}. All of these orbits are highly inclined, with inclinations in the range of 160 to 180°.<ref>{{cite web|url=https://minorplanetcenter.net/db_search/show_by_properties?utf8=%E2%9C%93&semimajor_axis_min=&semimajor_axis_max=&eccentricity_min=&eccentricity_max=&inclination_min=160&inclination_max=180&argument_of_perihelion_min=&argument_of_perihelion_max=&ascending_node_min=&ascending_node_max=&mean_anomaly_min=&mean_anomaly_max=&mean_daily_motion_min=&mean_daily_motion_max=&perihelion_distance_min=&perihelion_distance_max=&aphelion_distance_min=&aphelion_distance_max=&period_min=&period_max=&absolute_magnitude_min=&absolute_magnitude_max=&orbit_uncertainty_min=&orbit_uncertainty_max=|title=i > 160 < 180 |publisher = Minor Planet Center}}</ref>
==== Kuiper belt objects ==== Most [[Kuiper belt]] objects have prograde orbits around the Sun. The first Kuiper belt object discovered to have a retrograde orbit was {{mpl|2008 KV|42}}.<ref name="NS_Sep">{{cite journal | last = Hecht | first = Jeff | title = Distant object found orbiting Sun backwards | journal = New Scientist | date = 5 September 2008 | url = https://www.newscientist.com/article/dn14669-distant-object-found-orbiting-sun-backwards.html | access-date = 10 October 2009}}</ref> Other Kuiper belt objects with retrograde orbits are [[471325 Taowu]],<ref name="arxiv5aug"> {{cite journal |first1=Ying-Tung |last1=Chen |first2=Hsing Wen |last2=Lin |first3=Matthew J |last3=Holman |first4=Matthew J |last4=Payne |first5=Wesley C |last5=Fraser |first6=Pedro |last6=Lacerda |first7=Wing-Huen |last7=Ip |first8=Wen-Ping |last8=Chen |first9=Rolf-Peter |last9=Kudritzki|first10=Robert|last10=Jedicke |first11=Richard J |last11=Wainscoat |first12=John L |last12=Tonry |first13=Eugene A |last13=Magnier |first14=Christopher |last14=Waters |first15=Nick |last15=Kaiser |first16=Shiang-Yu |last16=Wang |first17=Matthew |last17=Lehner |arxiv=1608.01808 |title=Discovery of A New Retrograde Trans-Neptunian Object: Hint of A Common Orbital Plane for Low Semi-Major Axis, High Inclination TNOs and Centaurs |date=5 August 2016 |display-authors=4 |doi=10.3847/2041-8205/827/2/L24 |volume=827 |issue=2 |journal=The Astrophysical Journal |page=L24|bibcode = 2016ApJ...827L..24C |s2cid=4975180 |doi-access=free }}</ref> {{mpl|(342842) 2008 YB|3}}, {{mpl|(468861) 2013 LU|28}} and [[2011 MM4|2011 MM<sub>4</sub>]].<ref name="Marcos"> {{Cite journal | author = C. de la Fuente Marcos | author2 = R. de la Fuente Marcos | title = Large retrograde Centaurs: visitors from the Oort cloud? | journal = Astrophysics and Space Science | date = 2014 | doi = 10.1007/s10509-014-1993-9 | volume = 352 | issue = 2 | pages = 409–419 | arxiv=1406.1450 | bibcode = 2014Ap&SS.352..409D | s2cid = 119255885 }} </ref> All of these orbits are highly tilted, with [[#Inclination|inclinations]] in the 100°–125° range.
=== Meteoroids === [[Meteoroids]] in a retrograde orbit around the Sun hit the Earth with a faster relative speed than prograde meteoroids and tend to burn up in the atmosphere and are more likely to hit the side of the Earth facing away from the Sun (i.e. at night) whereas the prograde meteoroids have slower closing speeds and more often land as [[meteorites]] and tend to hit the Sun-facing side of the Earth. Most meteoroids are prograde.<ref>A{{cite book|author1=Alex Bevan|author2=John De Laeter|title=Meteorites: A Journey Through Space and Time|url=https://books.google.com/books?id=zzccaEpC2loC&pg=PA31|year=2002|publisher=UNSW Press|isbn=978-0-86840-490-5|page=31}}</ref>
=== Sun === The Sun's motion about the [[Center of mass#Barycenter in astrophysics and astronomy|centre of mass]] of the Solar System is complicated by perturbations from the planets. Every few hundred years this motion switches between prograde and retrograde.<ref>{{cite journal | last = Javaraiah | first = J. | title = Sun's retrograde motion and violation of even-odd cycle rule in sunspot activity | journal = Monthly Notices of the Royal Astronomical Society | volume = 362 | issue = 2005 | pages = 1311–1318 | date = 12 July 2005 | arxiv = astro-ph/0507269 | doi = 10.1111/j.1365-2966.2005.09403.x| doi-access = free |bibcode = 2005MNRAS.362.1311J | s2cid = 14022993 }}</ref>
== Planetary atmospheres == Retrograde motion, or retrogression, within the Earth's atmosphere is seen in weather systems whose motion is opposite the general regional direction of airflow, i.e. from east to west against the [[westerlies]] or [[Westerly wind burst|from west to east]] through the [[trade wind]] easterlies. Prograde motion with respect to planetary rotation is seen in the [[atmospheric super-rotation]] of the [[thermosphere]] of Earth and in the upper [[troposphere]] of [[Atmosphere of Venus#Circulation|Venus]]. Simulations indicate that the atmosphere of [[Pluto]] should be dominated by winds retrograde to its rotation.<ref name="Bertrand2020">{{cite journal|last1= Bertrand|first1= T.|last2= Forget|first2= F.|last3= White|first3= O.|last4= Schmitt|first4= B.|last5= Stern|first5= S.A.|last6= Weaver|first6= H.A.|last7= Young|first7= L.A.|last8= Ennico|first8= K.|last9= Olkin|first9= C.B.|title= Pluto's beating heart regulates the atmospheric circulation: results from high resolution and multi-year numerical climate simulations |journal= Journal of Geophysical Research: Planets|year= 2020|volume= 125|issue= 2|doi= 10.1029/2019JE006120|bibcode= 2020JGRE..12506120B|s2cid= 214085883|url= https://hal.archives-ouvertes.fr/hal-03097621/file/Bertrand2020-JGRE-125-e2019JE006120_revised.pdf}}</ref>
== Artificial satellites == {{further|Artificial satellites in retrograde orbit}} [[Satellite|Artificial satellites]] destined for low inclination orbits are usually launched in the prograde direction, since this minimizes the amount of propellant required to reach orbit by taking advantage of the Earth's rotation (an equatorial launch site is optimal for this effect). However, Israeli [[Ofeq]] satellites are launched in a westward, retrograde direction over the Mediterranean to ensure that launch debris does not fall onto populated land areas.
== Exoplanets == Stars and planetary systems tend to be born in [[star cluster]]s rather than forming in isolation. [[Protoplanetary disk]]s can collide with or steal material from [[molecular cloud]]s within the cluster and this can lead to disks and their resulting planets having inclined or retrograde orbits around their stars.<ref name=steal /><ref name=natural-misalign /> Retrograde motion may also result from gravitational interactions with other celestial bodies in the same system (See [[Kozai mechanism]]) or a near-collision with another planet,<ref name="NS_Aug" /> or it may be that the star itself flipped over early in their system's formation due to interactions between the star's magnetic field and the planet-forming disk.<ref>[https://www.newscientist.com/article/mg20727765.200-tilting-stars-may-explain-backwards-planets.html "Tilting stars may explain backwards planets"], ''New Scientist'', 1 September 2010, Issue 2776.</ref><ref>Dong Lai, Francois Foucart, Douglas N. C. Lin, [https://arxiv.org/abs/1008.3148 "Evolution of Spin Direction of Accreting Magnetic Protostars and Spin-Orbit Misalignment in Exoplanetary Systems"]</ref>
The [[accretion disk]] of the protostar [[IRAS 16293-2422]] has parts rotating in opposite directions. This is the first known example of a counterrotating accretion disk. If this system forms planets, the inner planets will likely orbit in the opposite direction to the outer planets.<ref>[http://www.nrao.edu/pr/2006/counterdisk/ "Still-Forming Solar System May Have Planets Orbiting Star in Opposite Directions, Astronomers Say"], National Radio Astronomy Observatory, February 13, 2006</ref>
[[WASP-17b]] was the first [[exoplanet]] that was discovered to be orbiting its star opposite to the direction the star is rotating.<ref name="Anderson2010">{{cite journal |display-authors=4 |last1=Anderson|first1=D. R. |last2=Hellier|first2=C. |last3=Gillon|first3=M. |last4=Triaud|first4=A. H. M. J. |last5=Smalley|first5=B. |last6=Hebb|first6=L. |last7=Cameron|first7=A. Collier |last8=Maxted|first8=P. F. L. |last9=Queloz|first9=D. |last10=West|first10=R. G. |last11=Bentley|first11=S. J. |last12=Enoch|first12=B. |last13=Horne|first13=K. |last14=Lister|first14=T. A. |last15=Mayor|first15=M. |last16=Parley|first16=N. R. |last17=Pepe|first17=F. |last18=Pollacco|first18=D. |last19=Ségransan|first19=D. |last20=Udry|first20=S. |last21=Wilson|first21=D. M. |title=WASP-17b: An ultra-low density planet in a probable retrograde orbit |journal=The Astrophysical Journal |volume=709 |issue=1 |date=2010-01-20 |pages=159–167 |doi=10.1088/0004-637X/709/1/159 |arxiv = 0908.1553 |bibcode = 2010ApJ...709..159A |s2cid=53628741}}</ref> A second such planet was announced just a day later: [[HAT-P-7b]].<ref>[https://www.newscientist.com/article/dn17613-second-backwards-planet-found-a-day-after-the-first.html "Second backwards planet found, a day after the first"], ''New Scientist'', 13 August 2009</ref>
In one study more than half of all the known [[hot Jupiter]]s had orbits that were misaligned with the rotation axis of their parent stars, with six having backwards orbits.<ref name="2010question" /> One proposed explanation is that hot Jupiters tend to form in dense clusters, where [[Perturbation (astronomy)|perturbations]] are more common and [[gravitational capture]] of planets by neighboring stars is possible.<ref>{{cite web |url=https://phys.org/news/2022-12-spaces-swapping-stars-hot-jupiters.html |title=Trading spaces: How swapping stars create hot Jupiters |author=Paul M. Sutter |agency=Universe Today |date=December 9, 2022 }}</ref>
The last few [[Impact event|giant impacts]] during [[Nebular hypothesis#Formation of planets|planetary formation]] tend to be the main determiner of a [[terrestrial planet]]'s rotation rate. During the giant impact stage, the thickness of a [[protoplanetary disk]] is far larger than the size of planetary embryos so collisions are equally likely to come from any direction in three dimensions. This results in the [[axial tilt]] of accreted planets ranging from 0 to 180 degrees with any direction as likely as any other with both prograde and retrograde spins equally probable. Therefore, prograde spin with small axial tilt, common for the solar system's terrestrial planets except for Venus, is not common for terrestrial planets in general.<ref>Sean N. Raymond, Eiichiro Kokubo, Alessandro Morbidelli, Ryuji Morishima, Kevin J. Walsh, [https://arxiv.org/abs/1312.1689 "Terrestrial Planet Formation at Home and Abroad"], Submitted on 5 Dec 2013 (v1), last revised 28 Jan 2014 (this version, v3)</ref>
== Stars' galactic orbits == The pattern of stars appears fixed in the sky, insofar as human vision is concerned; this is because their massive distances relative to the Earth result in motion imperceptible to the naked eye. In reality, stars orbit the center of their galaxy.
Stars with an orbit retrograde relative to a [[disk galaxy]]'s [[Galaxy rotation curve| general rotation]] are more likely to be found in the [[galactic halo]] than in the [[galactic disk]]. The [[Milky Way]]'s outer halo has many [[globular clusters]] with a retrograde orbit<ref> {{cite journal | last = Kravtsov | first = V. V. | title = Globular clusters and dwarf spheroidal galaxies of the outer galactic halo: On the putative scenario of their formation | journal = Astronomical and Astrophysical Transactions | volume = 20 | issue = 1 | pages = 89–92 | date = 2001 | url = http://images.astronet.ru/pubd/2008/09/28/0001230622/89-92.pdf | doi = 10.1080/10556790108208191 | access-date = 13 October 2009 | bibcode=2001A&AT...20...89K}}</ref> and with a retrograde or zero rotation.<ref>{{cite journal | last = Kravtsov | first = Valery V. | title = Second parameter globulars and dwarf spheroidals around the Local Group massive galaxies: What can they evidence? | journal = Astronomy & Astrophysics | volume = 396 | date = 2002 | pages = 117–123 | doi = 10.1051/0004-6361:20021404| bibcode=2002A&A...396..117K|arxiv = astro-ph/0209553 | s2cid = 16607125 }}</ref> The structure of the halo is the topic of an ongoing debate. Several studies have claimed to find a halo consisting of two distinct components.<ref> {{cite journal | display-authors = 4 | author = Daniela Carollo | author2 = Timothy C. Beers | author3 = Young Sun Lee| author4 = Masashi Chiba | author5 = John E. Norris| author6 = Ronald Wilhelm | author7 = Thirupathi Sivarani| author8 = Brian Marsteller | author9 = Jeffrey A. Munn| author10 = Coryn A. L. Bailer-Jones | author11 = Paola Re Fiorentin| author12 = Donald G. York | title = Two stellar components in the halo of the Milky Way | journal = Nature | volume = 450 | issue=7172 | pages = 1020–5 | date = 13 December 2007 | url = http://stromlo.anu.edu.au/news/media_releases/nature06460.pdf | doi = 10.1038/nature06460 | access-date = 13 October 2009 | pmid=18075581 |bibcode = 2007Natur.450.1020C |arxiv = 0706.3005 | s2cid = 4387133 }}</ref><ref>{{cite journal | author = Daniela Carollo | title = Structure and Kinematics of the Stellar Halos and Thick Disks of the Milky Way Based on Calibration Stars from Sloan Digital Sky Survey DR7 | journal = The Astrophysical Journal | volume = 712 | issue = 1 | pages = 692–727 | date = 2010 | doi = 10.1088/0004-637X/712/1/692 |bibcode = 2010ApJ...712..692C |arxiv = 0909.3019 | s2cid = 15633375 |display-authors=etal}}</ref><ref>{{cite journal | author = Timothy C. Beers | title = The Case for the Dual Halo of the Milky Way | journal = The Astrophysical Journal | volume = 746 | issue = 1 | page = 34 | date = 2012 | doi = 10.1088/0004-637X/746/1/34 |bibcode = 2012ApJ...746...34B |arxiv = 1104.2513 | s2cid = 51354794 |display-authors=etal}}</ref> These studies find a "dual" halo, with an inner, more metal-rich, prograde component (i.e. stars orbit the galaxy on average with the disk rotation), and a metal-poor, outer, retrograde (rotating against the disc) component.<!-- PLEASE NOTE that the reference for this sentence uses the word rotation to refer to the rotation of the halo itself about the centre of the galaxy, not the rotation of individual stars about their individual axes. --> However, these findings have been challenged by other studies,<ref>{{cite journal | author = R. Schoenrich | author2 = M. Asplund | author3 = L. Casagrande | title = On the alleged duality of the Galactic halo | journal = MNRAS | volume = 415 | issue = 4 | pages = 3807–3823 | date = 2011 | doi = 10.1111/j.1365-2966.2011.19003.x | doi-access = free |bibcode = 2011MNRAS.415.3807S |arxiv = 1012.0842 | s2cid = 55962646 }}</ref><ref>{{cite journal | author = R. Schoenrich | author2 = M. Asplund | author3 = L. Casagrande | title = Does SEGUE/SDSS indicate a dual Galactic halo? | journal = The Astrophysical Journal | volume = 786 | issue = 1 | page = 7 | date = 2014 | doi = 10.1088/0004-637X/786/1/7 | bibcode = 2014ApJ...786....7S | arxiv = 1403.0937 | s2cid = 118357068 }}</ref> arguing against such a duality. These studies demonstrate that the observational data can be explained without a duality, when employing an improved statistical analysis and accounting for measurement uncertainties.
The nearby [[Kapteyn's Star]] is thought to have ended up with its high-velocity retrograde orbit around the galaxy as a result of being ripped from a [[dwarf galaxy]] that [[galaxy merger|merged]] with the Milky Way.<ref>{{cite journal |url= https://www.newscientist.com/article/mg20427334.100-backward-star-aint-from-round-here.html |title= Backward star ain't from round here|journal= New Scientist}}</ref>
== Galaxies ==
=== Satellite galaxies === Close-flybys and mergers of galaxies within [[galaxy cluster]]s can pull material out of galaxies and create small [[satellite galaxies]] in either prograde or retrograde orbits around larger galaxies.<ref>M. S. Pawlowski, P. Kroupa, and K. S. de Boer, [https://arxiv.org/abs/1106.2804 "Making Counter-Orbiting Tidal Debris – The Origin of the Milky Way Disc of Satellites"]</ref>
A galaxy called Complex H, which was orbiting the Milky Way in a retrograde direction relative to the Milky Way's rotation, is colliding with the Milky Way.<ref>{{cite web|last=Cain |first=Fraser |title=Galaxy Orbiting Milky Way in the Wrong Direction |publisher=Universe Today |date=22 May 2003 |url=http://www.universetoday.com/2003/05/22/galaxy-orbiting-milky-way-in-the-wrong-direction/ |access-date=13 October 2009 |url-status=dead |archive-url=https://web.archive.org/web/20080819191304/http://www.universetoday.com/2003/05/22/galaxy-orbiting-milky-way-in-the-wrong-direction/ |archive-date=August 19, 2008 }}</ref><ref>{{cite journal | doi = 10.1086/376961 | last = Lockman | first = Felix J. | date = 2003 | title = High-velocity cloud Complex H: a satellite of the Milky Way in a retrograde orbit? | journal = The Astrophysical Journal Letters | volume = 591 | issue = 1 | pages = L33–L36| bibcode=2003ApJ...591L..33L|arxiv = astro-ph/0305408 | s2cid = 16129802 }}</ref>
=== Counter-rotating bulges === [[NGC 7331]] is an example of a galaxy that has a bulge that is rotating in the opposite direction to the rest of the disk, probably as a result of infalling material.<ref>{{cite journal | first = F. | last = Prada | author2 = C. Gutierrez | author3 = R. F. Peletier | author4 = C. D. McKeith | title = A Counter-rotating Bulge in the Sb Galaxy NGC 7331 | date = 14 March 1996 | arxiv = astro-ph/9602142 | doi=10.1086/310044 | volume=463 | journal=The Astrophysical Journal | pages=L9–L12 | bibcode=1996ApJ...463L...9P| s2cid = 17386894 }}</ref>
=== Central black holes === The center of a spiral galaxy contains at least one [[supermassive black hole]].<ref name="Merritt2005">{{cite journal|last1= Merritt|first1= D.|last2= Milosavljević|first2= M.|title= Massive Black Hole Binary Evolution|journal= Living Reviews in Relativity|volume= 8|pages= 8|year= 2005|doi= 10.12942/lrr-2005-8|doi-access= free|arxiv= astro-ph/0410364v2|bibcode= 2005LRR.....8....8M|s2cid= 119367453}}</ref> A retrograde black hole – one whose spin is opposite to that of its disk – spews jets much more powerful than those of a prograde black hole, which may have no jet at all. Scientists have produced a theoretical framework for the formation and evolution of retrograde black holes based on the gap between the inner edge of an accretion disk and the black hole.<ref>{{cite news | title = Some black holes make stronger jets of gas | publisher = UPI | date = 1 June 2010 | url = http://www.upi.com/Science_News/2010/06/01/Some-black-holes-make-stronger-jets-of-gas/UPI-80711275420355/ | access-date =1 June 2010}}</ref><ref>{{cite web | last = Atkinson | first = Nancy | title = What's more powerful than a supermassive black hole? A supermassive black hole that spins backwards | newspaper = The Christian Science Monitor | date = 1 June 2010 | url = https://www.csmonitor.com/Science/Cool-Astronomy/2010/0601/What-s-more-powerful-than-a-supermassive-black-hole-A-supermassive-black-hole-that-spins-backwards | access-date =1 June 2010}}</ref><ref name= "Garofalo2010">{{cite journal|last1= Garofalo|first1= D.|last2= Evans|first2= D.A.|last3= Sambruna|first3= R.M.|title=The evolution of radio-loud active galactic nuclei as a function of black hole spin|journal= Monthly Notices of the Royal Astronomical Society|date= August 2010|volume= 406|issue= 2|pages= 975–986|doi= 10.1111/j.1365-2966.2010.16797.x|arxiv= 1004.1166|bibcode= 2010MNRAS.406..975G|doi-access= free}}</ref>
== See also == * [[Artificial satellites in retrograde orbit]] * [[Gravitomagnetic clock effect]] * [[Yarkovsky effect]] * [[Apparent retrograde motion]] * [[Eskimo yo-yo|Alaska yo-yo]], a toy involving simultaneous circular motion of two balls in opposite directions
== Footnotes == {{notelist}}
== References == {{Reflist|30em}}
== Further reading == {{Commons category|Retrograde motion}} * [https://arxiv.org/abs/2003.13864 Retrograde-rotating exoplanets experience obliquity excitations in an eccentricity-enabled resonance], Steven M. Kreyche, Jason W. Barnes, Billy L. Quarles, Jack J. Lissauer, John E. Chambers, Matthew M. Hedman, 30 Mar 2020 * {{cite journal | first = Julie | last = Gayon |author2=Eric Bois | title = Are retrograde resonances possible in multi-planet systems? | date = 21 April 2008 | arxiv = 0801.1089 | doi = 10.1051/0004-6361:20078460 | bibcode=2008A&A...482..665G | journal = Astronomy and Astrophysics | volume = 482 | issue = 2 | pages = 665–672 | s2cid = 15436738 }} * {{cite journal | last = Kalvouridis | first = T. J. | title = Retrograde Orbits in Ring Configurations of N Bodies | journal = Astrophysics and Space Science | volume = 284 | issue = 3 | pages = 1013–1033 | date = May 2003 | doi = 10.1023/A:1023332226388 |bibcode = 2003Ap&SS.284.1013K | s2cid = 117212083 }} * {{cite journal |doi = 10.1006/icar.1999.6170 | bibcode=1999Icar..141...13L |title = Orbital Evolution of Retrograde Interplanetary Dust Particles and Their Distribution in the Solar System |date = 1999 |last1 = Liou |first1 = J |journal = Icarus |volume = 141 | issue=1 |pages = 13–28}} * [http://www.agu.org/pubs/crossref/1991/91GL01882.shtml How large is the retrograde annual wobble?] {{Webarchive|url=https://web.archive.org/web/20120920205136/http://www.agu.org/pubs/crossref/1991/91GL01882.shtml |date=2012-09-20 }}, N. E. King, Duncan Carr Agnew, 1991. * {{cite journal | last = Fernandez | first = Julio A. | title = On the observed excess of retrograde orbits among long-period comets | journal = Monthly Notices of the Royal Astronomical Society | volume = 197 | issue = 2 | pages = 265–273 | bibcode = 1981MNRAS.197..265F | date = 1981|doi = 10.1093/mnras/197.2.265 | doi-access = free }} * [https://arxiv.org/abs/1304.4377 Dynamical Effects on the Habitable Zone for Earth-like Exomoons], Duncan Forgan, David Kipping, 16 April 2013 * [https://arxiv.org/abs/1205.2297 What collisional debris can tell us about galaxies], Pierre-Alain Duc, 10 May 2012 * [https://arxiv.org/abs/astro-ph/9910418 The Formation and Role of Vortices in Protoplanetary Disks], Patrick Godon, Mario Livio, 22 October 1999
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