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'''Retrograde motion''' is in the [[direction]] [[opposite]] to the [[Motion|movement]] of something else, and is the contrary of direct or ''prograde motion''. This [[motion]] can refer to the [[orbit]] of one body about another body or about some other point, or to the [[rotation]] of a single body about its [[axis]], or other orbital [[parameters]] such as [[precession]] or nutation of the axis. In [[reference]] to [[celestial]] [[systems]], retrograde motion usually means [[motion]] which is contrary to the rotation of the primary, that is, the object which forms the [[system]]'s [[Center|hub]].
'''Retrograde motion''' is in the [[direction]] [[opposite]] to the [[Motion|movement]] of something else, and is the contrary of direct or ''prograde motion''. This [[motion]] can refer to the [[orbit]] of one body about another body or about some other point, or to the [[rotation]] of a single body about its [[axis]], or other orbital [[parameters]] such as [[precession]] or nutation of the axis. In [[reference]] to [[celestial]] [[systems]], retrograde motion usually means [[motion]] which is contrary to the rotation of the primary, that is, the object which forms the [[system]]'s [[Center|hub]].
==Formation of celestial systems==
==Formation of celestial systems==
When a [[galaxy]] or a [[planetary system]] [[forms]], its [[material]] takes the shape 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. The [[nature]] of the collapse is explained by the principle called [http://en.wikipedia.org/wiki/Conservation_of_angular_momentum conservation of angular momentum]. In 2010 the [[discovery]] of several [http://en.wikipedia.org/wiki/Hot_jupiter hot jupiters] with backward [[orbits]] called into question the [[theories]] about the formation of [[planetary system]]s.
When a [[galaxy]] or a [[planetary system]] [[forms]], its [[material]] takes the shape 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. The [[nature]] of the collapse is explained by the principle called [https://en.wikipedia.org/wiki/Conservation_of_angular_momentum conservation of angular momentum]. In 2010 the [[discovery]] of several [https://en.wikipedia.org/wiki/Hot_jupiter hot jupiters] with backward [[orbits]] called into question the [[theories]] about the formation of [[planetary system]]s.
==Inclination==
==Inclination==
A [[celestial]] object's [http://en.wikipedia.org/wiki/Inclination inclination] indicates whether the object's [[orbit]] is direct or ''retrograde''. The inclination of a [[celestial]] object is the [[angle]] between its [[orbital]] [[plane]] and another [[reference]] frame such as the [[equatorial]] [[plane]] of the object's primary. In our [[solar system]], inclination of the [[planets]] is often [[measured]] from the [http://en.wikipedia.org/wiki/Ecliptic_plane ecliptic plane], which is the plane of [[Earth]]'s [[orbit]] around the [[sun]]. The inclination of [[moons]] is measured from the [[equator]] of the [[planet]] they [[orbit]]. An object with an inclination between -90 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 which is neither direct nor retrograde. An object with an inclination beyond 90 degrees up to 270 degrees is in a ''retrograde'' [[orbit]].
A [[celestial]] object's [https://en.wikipedia.org/wiki/Inclination inclination] indicates whether the object's [[orbit]] is direct or ''retrograde''. The inclination of a [[celestial]] object is the [[angle]] between its [[orbital]] [[plane]] and another [[reference]] frame such as the [[equatorial]] [[plane]] of the object's primary. In our [[solar system]], inclination of the [[planets]] is often [[measured]] from the [https://en.wikipedia.org/wiki/Ecliptic_plane ecliptic plane], which is the plane of [[Earth]]'s [[orbit]] around the [[sun]]. The inclination of [[moons]] is measured from the [[equator]] of the [[planet]] they [[orbit]]. An object with an inclination between -90 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 which is neither direct nor retrograde. An object with an inclination beyond 90 degrees up to 270 degrees is in a ''retrograde'' [[orbit]].
==Axial tilt==
==Axial tilt==
A [[celestial]] object's [http://en.wikipedia.org/wiki/Axial_tilt axial tilt] indicates whether the object's [[rotation]] is direct or retrograde. [http://en.wikipedia.org/wiki/Axial_tilt Axial tilt] is the [[angle]] between an object's [[rotation]] [[axis]] and a line perpendicular to its [[orbital]] [[plane]] passing through the object's [[center]]. 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 which is neither direct nor retrograde. An object with an axial tilt beyond 90 degrees up to 270 degrees has a retrograde rotation relative to its orbital direction.
A [[celestial]] object's [https://en.wikipedia.org/wiki/Axial_tilt axial tilt] indicates whether the object's [[rotation]] is direct or retrograde. [https://en.wikipedia.org/wiki/Axial_tilt Axial tilt] is the [[angle]] between an object's [[rotation]] [[axis]] and a line perpendicular to its [[orbital]] [[plane]] passing through the object's [[center]]. 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 which is neither direct nor retrograde. An object with an axial tilt beyond 90 degrees up to 270 degrees has a retrograde rotation relative to its orbital direction.
==Earth and the planets==
==Earth and the planets==
All eight [[planets]] in our [[solar system]] [[orbit]] the [[sun]] in the direction that the sun is rotating, which is counterclockwise when viewed from above the [[Earth]]'s north pole. Six of the planets also [[rotate]] in this same direction. The exceptions—the planets with ''retrograde'' [[rotation]]—are [http://en.wikipedia.org/wiki/Venus Venus] and [http://en.wikipedia.org/wiki/Uranus Uranus]. Venus's axial tilt is 177 degrees, which means it is spinning almost exactly in the [[opposite]] direction to its [[orbit]].
All eight [[planets]] in our [[solar system]] [[orbit]] the [[sun]] in the direction that the sun is rotating, which is counterclockwise when viewed from above the [[Earth]]'s north pole. Six of the planets also [[rotate]] in this same direction. The exceptions—the planets with ''retrograde'' [[rotation]]—are [https://en.wikipedia.org/wiki/Venus Venus] and [https://en.wikipedia.org/wiki/Uranus Uranus]. Venus's axial tilt is 177 degrees, which means it is spinning almost exactly in the [[opposite]] direction to its [[orbit]].
==Moons and rings==
==Moons and rings==
If [[formed]] in the [[gravity]]-field of a planet as the [[planet]] is forming, a [[moon]] will orbit the planet in the same direction as the planet is [[rotating]]. If an object is [[formed]] elsewhere and later captured into [[orbit]] by a [[planet]]'s [[gravity]], it will be captured into a ''retrograde'' or prograde [[orbit]] depending on whether it first approaches the side of the planet that is rotating towards or away from it. The retrograde [[orbits]] of a [[planet]]'s [[satellites]] are said to be irregular. ''Prograde orbits'' are said to be [[regular]].
If [[formed]] in the [[gravity]]-field of a planet as the [[planet]] is forming, a [[moon]] will orbit the planet in the same direction as the planet is [[rotating]]. If an object is [[formed]] elsewhere and later captured into [[orbit]] by a [[planet]]'s [[gravity]], it will be captured into a ''retrograde'' or prograde [[orbit]] depending on whether it first approaches the side of the planet that is rotating towards or away from it. The retrograde [[orbits]] of a [[planet]]'s [[satellites]] are said to be irregular. ''Prograde orbits'' are said to be [[regular]].
In our [[solar system]], many of the [[asteroid]]-sized [[moons]] have retrograde [[orbits]] whereas all the large moons except [http://en.wikipedia.org/wiki/Triton Triton] (the largest of [http://en.wikipedia.org/wiki/Neptune Neptune]'s moons), have prograde orbits. The [[particles]] in [http://en.wikipedia.org/wiki/Saturn Saturn]'s Phoebe ring are thought to have a retrograde orbit because they [[originate]] from the irregular moon Phoebe.
In our [[solar system]], many of the [[asteroid]]-sized [[moons]] have retrograde [[orbits]] whereas all the large moons except [https://en.wikipedia.org/wiki/Triton Triton] (the largest of [https://en.wikipedia.org/wiki/Neptune Neptune]'s moons), have prograde orbits. The [[particles]] in [https://en.wikipedia.org/wiki/Saturn Saturn]'s Phoebe ring are thought to have a retrograde orbit because they [[originate]] from the irregular moon Phoebe.
Within the [http://en.wikipedia.org/wiki/Hill_sphere Hill sphere], the region of [[stability]] for retrograde [[orbits]] at a large distance from the primary is larger than the region for prograde orbits at a large distance from the primary. This was [[thought]] to explain the preponderance of retrograde moons around [http://en.wikipedia.org/wiki/Jupiter Jupiter], however [http://en.wikipedia.org/wiki/Saturn Saturn] has a more even mix of retrograde/prograde moons so the reasons are more [[Complex|complicated]].
Within the [https://en.wikipedia.org/wiki/Hill_sphere Hill sphere], the region of [[stability]] for retrograde [[orbits]] at a large distance from the primary is larger than the region for prograde orbits at a large distance from the primary. This was [[thought]] to explain the preponderance of retrograde moons around [https://en.wikipedia.org/wiki/Jupiter Jupiter], however [https://en.wikipedia.org/wiki/Saturn Saturn] has a more even mix of retrograde/prograde moons so the reasons are more [[Complex|complicated]].
==Asteroids, comets, and Kuiper belt objects==
==Asteroids, comets, and Kuiper belt objects==
[[Asteroids]] usually have a direct orbit. By 1 May 2009, astronomers had identified a mere 20 [http://en.wikipedia.org/wiki/List_of_notable_asteroids#Retrograde_and_highly-inclined asteroids in retrograde orbits]. The retrograde asteroids may be burnt-out [[comets]].
[[Asteroids]] usually have a direct orbit. By 1 May 2009, astronomers had identified a mere 20 [https://en.wikipedia.org/wiki/List_of_notable_asteroids#Retrograde_and_highly-inclined asteroids in retrograde orbits]. The retrograde asteroids may be burnt-out [[comets]].
[[Comets]] from the [http://en.wikipedia.org/wiki/Oort_cloud Oort cloud] are much more likely than asteroids to be retrograde. [http://en.wikipedia.org/wiki/Halley's_Comet Halley's Comet] has a retrograde orbit around the [[sun]].
[[Comets]] from the [https://en.wikipedia.org/wiki/Oort_cloud Oort cloud] are much more likely than asteroids to be retrograde. [https://en.wikipedia.org/wiki/Halley's_Comet Halley's Comet] has a retrograde orbit around the [[sun]].
The first [http://en.wikipedia.org/wiki/Kuiper_belt Kuiper belt] object discovered to have a retrograde orbit is 2008 KV42. The [http://en.wikipedia.org/wiki/Dwarf_planet dwarf planet] [http://en.wikipedia.org/wiki/Pluto Pluto] has retrograde rotation. Pluto's axial tilt is approximately 120 degrees.
The first [https://en.wikipedia.org/wiki/Kuiper_belt Kuiper belt] object discovered to have a retrograde orbit is 2008 KV42. The [https://en.wikipedia.org/wiki/Dwarf_planet dwarf planet] [https://en.wikipedia.org/wiki/Pluto Pluto] has retrograde rotation. Pluto's axial tilt is approximately 120 degrees.
==The Sun==
==The Sun==
The [[sun]]'s [[motion]] about the [[center]] of [[mass]] of the [[solar system]] is complicated by perturbations from the [[planets]]. Every few hundred years this [[motion]] switches between prograde and retrograde.
The [[sun]]'s [[motion]] about the [[center]] of [[mass]] of the [[solar system]] is complicated by perturbations from the [[planets]]. Every few hundred years this [[motion]] switches between prograde and retrograde.
==Exoplanets==
==Exoplanets==
Astronomers have discovered some [http://en.wikipedia.org/wiki/Exoplanets exoplanets] with retrograde orbits. [http://en.wikipedia.org/wiki/WASP-17b WASP-17b] is the first exoplanet that was discovered to be orbiting its [[star]] opposite to the direction the star is [[rotating]]. HAT-P-7b also has a ''retrograde'' [[orbit]]. The retrograde [[motion]] may be the result of [[gravitational]] interactions with other [[celestial]] bodies (See [http://en.wikipedia.org/wiki/Kozai_mechanism Kozai mechanism].) or a collision with another [[planet]]. It has been found that several [http://en.wikipedia.org/wiki/Hot_jupiter hot jupiters] have backward [[orbits]] and this calls into question the [[theories]] about the formation of [[planetary systems]]. By combining new [[observations]] with the old [[data]] it was found that more than half of all the [http://en.wikipedia.org/wiki/Hot_jupiter hot Jupiters] [[studied]] have orbits that are misaligned with the [[rotation]] [[axis]] of their [[parent]] [[stars]], and six exoplanets in this study have retrograde motion.
Astronomers have discovered some [https://en.wikipedia.org/wiki/Exoplanets exoplanets] with retrograde orbits. [https://en.wikipedia.org/wiki/WASP-17b WASP-17b] is the first exoplanet that was discovered to be orbiting its [[star]] opposite to the direction the star is [[rotating]]. HAT-P-7b also has a ''retrograde'' [[orbit]]. The retrograde [[motion]] may be the result of [[gravitational]] interactions with other [[celestial]] bodies (See [https://en.wikipedia.org/wiki/Kozai_mechanism Kozai mechanism].) or a collision with another [[planet]]. It has been found that several [https://en.wikipedia.org/wiki/Hot_jupiter hot jupiters] have backward [[orbits]] and this calls into question the [[theories]] about the formation of [[planetary systems]]. By combining new [[observations]] with the old [[data]] it was found that more than half of all the [https://en.wikipedia.org/wiki/Hot_jupiter hot Jupiters] [[studied]] have orbits that are misaligned with the [[rotation]] [[axis]] of their [[parent]] [[stars]], and six exoplanets in this study have retrograde motion.
==Stars==
==Stars==
[[Stars]] with a retrograde orbit are more likely to be found in the [http://en.wikipedia.org/wiki/Galactic_halo galactic halo] than in the [http://en.wikipedia.org/wiki/Galactic_disk galactic disk]. The [[Milky Way]]'s outer halo has many [http://en.wikipedia.org/wiki/Globular_clusters globular clusters] with a retrograde orbit and with a retrograde or zero [[rotation]]. The halo consists of two distinct components. The [[stars]] in the inner halo mostly have prograde [[orbits]] around the [[galaxy]], while stars in the outer halo favour retrograde orbits.
[[Stars]] with a retrograde orbit are more likely to be found in the [https://en.wikipedia.org/wiki/Galactic_halo galactic halo] than in the [https://en.wikipedia.org/wiki/Galactic_disk galactic disk]. The [[Milky Way]]'s outer halo has many [https://en.wikipedia.org/wiki/Globular_clusters globular clusters] with a retrograde orbit and with a retrograde or zero [[rotation]]. The halo consists of two distinct components. The [[stars]] in the inner halo mostly have prograde [[orbits]] around the [[galaxy]], while stars in the outer halo favour retrograde orbits.
The nearby [http://en.wikipedia.org/wiki/Kapteyn%27s_Star 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 merged with the [[Milky Way]].[16]
The nearby [https://en.wikipedia.org/wiki/Kapteyn%27s_Star 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 merged with the [[Milky Way]].[16]
==Galaxies==
==Galaxies==
The [[center]] of a [http://en.wikipedia.org/wiki/Spiral_galaxy spiral galaxy] is at least one [http://en.wikipedia.org/wiki/Supermassive_black_hole supermassive black hole]. A prograde [[black hole]] spins in the same direction as its disk. A retrograde black hole spins in the [[opposite]] direction to the disk. A retrograde black hole spews jets that are much more [[powerful]] than the jets from a prograde black hole, which may have no jet at all. The retrograde [[black hole]] shoots more [[powerful]] jets because the gap between it and the inner edge of its disk is greater than the gap between the prograde black hole and the inner edge of its disk. The greater gap is presumed to provide more room for the build-up of the magnetic fields which fuel the jets. (This presumption is known as the "Reynold's conjecture" after the theoretical astrophysicist Chris Reynolds of the University of Maryland, College Park.)
The [[center]] of a [https://en.wikipedia.org/wiki/Spiral_galaxy spiral galaxy] is at least one [https://en.wikipedia.org/wiki/Supermassive_black_hole supermassive black hole]. A prograde [[black hole]] spins in the same direction as its disk. A retrograde black hole spins in the [[opposite]] direction to the disk. A retrograde black hole spews jets that are much more [[powerful]] than the jets from a prograde black hole, which may have no jet at all. The retrograde [[black hole]] shoots more [[powerful]] jets because the gap between it and the inner edge of its disk is greater than the gap between the prograde black hole and the inner edge of its disk. The greater gap is presumed to provide more room for the build-up of the magnetic fields which fuel the jets. (This presumption is known as the "Reynold's conjecture" after the theoretical astrophysicist Chris Reynolds of the University of Maryland, College Park.)
[http://en.wikipedia.org/wiki/NGC_7331 NGC 7331] is an example of a [[galaxy]] which has a bulge that is [[rotating]] in the [[opposite]] direction to the rest of the disk, probably as a result of infalling [[material]].
[https://en.wikipedia.org/wiki/NGC_7331 NGC 7331] is an example of a [[galaxy]] which has a bulge that is [[rotating]] in the [[opposite]] direction to the rest of the disk, probably as a result of infalling [[material]].
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.[21][22]
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.[21][22]
==References==
==References==
# Grossman, Lisa (13 August 2009). "Planet found orbiting its star backwards for first time". NewScientist. http://www.newscientist.com/article/dn17603-planet-found-orbiting-its-star-backwards-for-first-time.html. Retrieved 10 October 2009.
# Grossman, Lisa (13 August 2009). "Planet found orbiting its star backwards for first time". NewScientist. https://www.newscientist.com/article/dn17603-planet-found-orbiting-its-star-backwards-for-first-time.html. Retrieved 10 October 2009.
# Turning planetary theory upside down
# Turning planetary theory upside down
# http://www.newuniverse.co.uk/Axial_tilt.html
# https://www.newuniverse.co.uk/Axial_tilt.html
# Encyclopedia of the solar system. Academic Press. 2007.
# Encyclopedia of the solar system. Academic Press. 2007.
# Mason, John (22 July 1989). "Science: Neptune's new moon baffles the astronomers". NewScientist. http://www.newscientist.com/article/mg12316742.600-science-neptunes-new-moon-baffles-the-astronomers.html. Retrieved 10 October 2009.
# Mason, John (22 July 1989). "Science: Neptune's new moon baffles the astronomers". NewScientist. https://www.newscientist.com/article/mg12316742.600-science-neptunes-new-moon-baffles-the-astronomers.html. Retrieved 10 October 2009.
# Chaos-assisted capture of irregular moons, Sergey A. Astakhov, Andrew D. Burbanks, Stephen Wiggins & David Farrelly, NATURE |VOL 423 | 15 MAY 2003
# Chaos-assisted capture of irregular moons, Sergey A. Astakhov, Andrew D. Burbanks, Stephen Wiggins & David Farrelly, NATURE |VOL 423 | 15 MAY 2003
# Hecht, Jeff (1 May 2009). "Nearby asteroid found orbiting sun backwards". NewScientist. http://www.newscientist.com/article/dn17073-nearby-asteroid-found-orbiting-sun-backwards.html. Retrieved 10 October 2009.
# Hecht, Jeff (1 May 2009). "Nearby asteroid found orbiting sun backwards". NewScientist. https://www.newscientist.com/article/dn17073-nearby-asteroid-found-orbiting-sun-backwards.html. Retrieved 10 October 2009.
# Halley's Comet
# Halley's Comet
# Hecht, Jeff (5 September 2008). "Distant object found orbiting Sun backwards". NewScientist. http://www.newscientist.com/article/dn14669-distant-object-found-orbiting-sun-backwards.html. Retrieved 10 October 2009.
# Hecht, Jeff (5 September 2008). "Distant object found orbiting Sun backwards". NewScientist. https://www.newscientist.com/article/dn14669-distant-object-found-orbiting-sun-backwards.html. Retrieved 10 October 2009.
# http://www.daviddarling.info/encyclopedia/P/Pluto.html
# https://www.daviddarling.info/encyclopedia/P/Pluto.html
# Javaraiah, J. (12 July 2005). "Sun's retrograde motion and violation of even-odd cycle rule in sunspot activity". Royal Astronomical Society, Monthly Notices (Royal Astronomical Society) 362 (2005): 1311–1318. http://arxiv.org/abs/astro-ph/0507269v1. Retrieved 11 October 2009.
# Javaraiah, J. (12 July 2005). "Sun's retrograde motion and violation of even-odd cycle rule in sunspot activity". Royal Astronomical Society, Monthly Notices (Royal Astronomical Society) 362 (2005): 1311–1318. https://arxiv.org/abs/astro-ph/0507269v1. Retrieved 11 October 2009.
# Grossman, Lisa (13 August 2009). "Second backwards planet found, a day after the first". NewScientist. http://www.newscientist.com/article/dn17613-second-backwards-planet-found-a-day-after-the-first.html. Retrieved 10 October 2009.
# Grossman, Lisa (13 August 2009). "Second backwards planet found, a day after the first". NewScientist. https://www.newscientist.com/article/dn17613-second-backwards-planet-found-a-day-after-the-first.html. Retrieved 10 October 2009.
# Kravtsov, V. V. (1 June 2001). "Globular clusters and dwarf spheroidal galaxies of the outer galactic halo: On the putative scenario of their formation". Astronomical and Astrophysical Transactions 20:1 (2001): 89–92. doi:10.1080/10556790108208191. http://images.astronet.ru/pubd/2008/09/28/0001230622/89-92.pdf. Retrieved 13 October 2009.
# Kravtsov, V. V. (1 June 2001). "Globular clusters and dwarf spheroidal galaxies of the outer galactic halo: On the putative scenario of their formation". Astronomical and Astrophysical Transactions 20:1 (2001): 89–92. doi:10.1080/10556790108208191. https://images.astronet.ru/pubd/2008/09/28/0001230622/89-92.pdf. Retrieved 13 October 2009.
# Kravtsov, Valery V. (28 August 2002). "Second parameter globulars and dwarf spheroidals around the Local Group massive galaxies: What can they evidence?". Astronomy & Astrophysics (EDP Sciences) 396 (2002): 117–123. doi:10.1051/0004-6361:20021404. http://www.aanda.org/articles/aa/full/2002/46/aa2635/aa2635.html. Retrieved 13 October 2009.
# Kravtsov, Valery V. (28 August 2002). "Second parameter globulars and dwarf spheroidals around the Local Group massive galaxies: What can they evidence?". Astronomy & Astrophysics (EDP Sciences) 396 (2002): 117–123. doi:10.1051/0004-6361:20021404. https://www.aanda.org/articles/aa/full/2002/46/aa2635/aa2635.html. Retrieved 13 October 2009.
# Carollo, Daniela; Timothy C. Beers, Young Sun Lee, Masashi Chiba, John E. Norris, Ronald Wilhelm, Thirupathi Sivarani, Brian Marsteller, Jeffrey A. Munn, Coryn A. L. Bailer-Jones, Paola Re Fiorentin, Donald G. York (13 December 2007). "Two stellar components in the halo of the Milky Way". Nature 450. doi:10.1038/nature06460. http://stromlo.anu.edu.au/news/media_releases/nature06460.pdf. Retrieved 13 October 2009.
# Carollo, Daniela; Timothy C. Beers, Young Sun Lee, Masashi Chiba, John E. Norris, Ronald Wilhelm, Thirupathi Sivarani, Brian Marsteller, Jeffrey A. Munn, Coryn A. L. Bailer-Jones, Paola Re Fiorentin, Donald G. York (13 December 2007). "Two stellar components in the halo of the Milky Way". Nature 450. doi:10.1038/nature06460. https://stromlo.anu.edu.au/news/media_releases/nature06460.pdf. Retrieved 13 October 2009.
# http://www.newscientist.com/article/mg20427334.100-backward-star-aint-from-round-here.html
# https://www.newscientist.com/article/mg20427334.100-backward-star-aint-from-round-here.html
# D. Merritt and M. Milosavljevic (2005). "Massive Black Hole Binary Evolution." http://relativity.livingreviews.org/Articles/lrr-2005-8/
# D. Merritt and M. Milosavljevic (2005). "Massive Black Hole Binary Evolution." https://relativity.livingreviews.org/Articles/lrr-2005-8/
# "Some black holes make stronger jets of gas". UPI.com. 1 June 2010. http://www.upi.com/Science_News/2010/06/01/Some-black-holes-make-stronger-jets-of-gas/UPI-80711275420355/. Retrieved 1 June 2010.
# "Some black holes make stronger jets of gas". UPI.com. 1 June 2010. https://www.upi.com/Science_News/2010/06/01/Some-black-holes-make-stronger-jets-of-gas/UPI-80711275420355/. Retrieved 1 June 2010.
# Atkinson, Nancy (1 June 2010). "What's more powerful than a supermassive black hole? A supermassive black hole that spins backwards.". The Christian Science Monitor. http://www.csmonitor.com/Science/Cool-Astronomy/2010/0601/What-s-more-powerful-than-a-supermassive-black-hole-A-supermassive-black-hole-that-spins-backwards. Retrieved 1 June 2010.
# Atkinson, Nancy (1 June 2010). "What's more powerful than a supermassive black hole? A supermassive black hole that spins backwards.". The Christian Science Monitor. 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. Retrieved 1 June 2010.
# Prada, F.; C. Gutierrez, R. F. Peletier, C. D. McKeith (14 March 1996). "A Counter-rotating Bulge in the Sb Galaxy NGC 7331". arXiv.org. http://arxiv.org/abs/astro-ph/9602142v2. Retrieved 10 October 2009.
# Prada, F.; C. Gutierrez, R. F. Peletier, C. D. McKeith (14 March 1996). "A Counter-rotating Bulge in the Sb Galaxy NGC 7331". arXiv.org. https://arxiv.org/abs/astro-ph/9602142v2. Retrieved 10 October 2009.
# Cain, Fraser (22 May 2003). "Galaxy Orbiting Milky Way in the Wrong Direction". Universe Today. http://www.universetoday.com/2003/05/22/galaxy-orbiting-milky-way-in-the-wrong-direction/. Retrieved 13 October 2009.
# Cain, Fraser (22 May 2003). "Galaxy Orbiting Milky Way in the Wrong Direction". Universe Today. https://www.universetoday.com/2003/05/22/galaxy-orbiting-milky-way-in-the-wrong-direction/. Retrieved 13 October 2009.
# Lockman, Felix J. (2 June 2003). "High-velocity cloud Complex H: a satellite of the Milky Way in a retrograde orbit?". The Astrophysical Journal (The American Astronomical Society) 591 (1 July 2003): L33-L36. http://www.iop.org/EJ/article/1538-4357/591/1/L33/17178.web.pdf. Retrieved 13 October 2009.
# Lockman, Felix J. (2 June 2003). "High-velocity cloud Complex H: a satellite of the Milky Way in a retrograde orbit?". The Astrophysical Journal (The American Astronomical Society) 591 (1 July 2003): L33-L36. https://www.iop.org/EJ/article/1538-4357/591/1/L33/17178.web.pdf. Retrieved 13 October 2009.
==Further reading==
==Further reading==
* Gayon, Julie; Eric Bois (21 April 2008). "Are retrograde resonances possible in multi-planet systems?". Nice Sophia-Antipolis University, CNRS, Observatoire de la Cote d'Azur, Laboratoire Cassiopee, France: arXiv.org. doi:10.1051/0004-6361:3A20078460. http://arxiv.org/abs/0801.1089. Retrieved 10 October 2009.
* Gayon, Julie; Eric Bois (21 April 2008). "Are retrograde resonances possible in multi-planet systems?". Nice Sophia-Antipolis University, CNRS, Observatoire de la Cote d'Azur, Laboratoire Cassiopee, France: arXiv.org. doi:10.1051/0004-6361:3A20078460. https://arxiv.org/abs/0801.1089. Retrieved 10 October 2009.
* Kalvouridis, T. J. (May 2003). "Retrograde Orbits in Ring Configurations of N Bodies". Astrophysics and Space Science (Springer Netherlands) 284 (3): 1013–1033. doi:10.1023/A:1023332226388. ISSN (Print) 1572-946X (Online) 0004-640X (Print) 1572-946X (Online). http://www.springerlink.com/content/q4r26k4905335371/. Retrieved 11 October 2009.
* Kalvouridis, T. J. (May 2003). "Retrograde Orbits in Ring Configurations of N Bodies". Astrophysics and Space Science (Springer Netherlands) 284 (3): 1013–1033. doi:10.1023/A:1023332226388. ISSN (Print) 1572-946X (Online) 0004-640X (Print) 1572-946X (Online). https://www.springerlink.com/content/q4r26k4905335371/. Retrieved 11 October 2009.
* Orbital Evolution of Retrograde Interplanetary Dust Particles and Their Distribution in the Solar System, Jer-Chyi Liou, Herbert A. Zook and A. A. Jackson.
* Orbital Evolution of Retrograde Interplanetary Dust Particles and Their Distribution in the Solar System, Jer-Chyi Liou, Herbert A. Zook and A. A. Jackson.
* How large is the retrograde annual wobble?, N. E. King, Duncan Carr Agnew, 1991.
* How large is the retrograde annual wobble?, N. E. King, Duncan Carr Agnew, 1991.
* Fernandez, Julio A. (11 February 1981). "On the observed excess of retrograde orbits among long-period comets". Royal Astronomical Society, Monthly Notices (Royal Astronomical Society) 197 (Oct. 1981): 265–273. http://adsabs.harvard.edu/full/1981MNRAS.197..265F. Retrieved 11 October 2009.
* Fernandez, Julio A. (11 February 1981). "On the observed excess of retrograde orbits among long-period comets". Royal Astronomical Society, Monthly Notices (Royal Astronomical Society) 197 (Oct. 1981): 265–273. https://adsabs.harvard.edu/full/1981MNRAS.197..265F. Retrieved 11 October 2009.
[[Category: Astronomy]]
[[Category: Astronomy]]
[[Category: Physics]]
[[Category: Physics]]
