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Retrograde and prograde motion

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This article is about retrograde motions of celestial bodies relative to a gravitationally central object. For the apparent motion as seen from a particular vantage point, see Apparent retrograde motion.

Retrograde motion is movement in the direction opposite to the movement of something else, and is the contrary of direct or prograde motion. The idea of retrograde or prograde motion is useful in three contexts: (1) for describing the orbits of celestial bodies, (2) for describing the rotations of celestial bodies, and (3) for explaining the backtracking by planets which is visible to observers on Earth (See the article: Apparent retrograde motion).[1]

Orbit or rotation

In reference to celestial bodies, 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 to the 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 hub.

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.[2] The nature of the collapse is explained by the principle called conservation of angular momentum.

Inclination

A celestial object's 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 ecliptic plane, which is the plane of Earth's orbit around the sun. [3] The inclination of moons is measured from the equator of the planet they orbit. An object with an inclination up to 90 degrees is orbiting or revolving in the same direction as the primary is rotating. An object with an inclination of 90 degrees has an 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

A celestial object's axial tilt indicates whether the object's rotation is direct or retrograde. Axial tilt is the angle between an object's axis and a line which passes through the object's centre, and which is perpendicular to its orbital plane. 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 90 degrees has a 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.[3]

Earth and the planets

All eight planets in our solar system orbit the sun in the direction that the sun is rotating. Most of the planets rotate in the same direction, which is counterclockwise when viewed from above the Earth's north pole. The exceptions—the planets with retrograde rotation—are Venus and Uranus. Venus's axial tilt is 177 degrees (or -3 degrees), which means it is spinning almost exactly in the opposite direction to its orbit.[4][1] Uranus's axial tilt is 98 degrees.[1]

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.[5] In our solar system, all the large moons except Triton (the largest of Neptune's moons), have regular orbits.[6] Of Jupiter's 55 moons, 48 have retrograde orbits. Of Saturn's 26 moons, 18 have retrograde orbits, and the particles in Saturn's Phoebe ring are thought to have a retrograde orbit because they originate from the irregular moon Phoebe. Of Neptune's 9 moons, 8 have a retrograde orbit.[4] [7]

Asteroids

Asteroids usually have a direct orbit. By 1 May 2009, astronomers had identified a mere 20 asteroids in retrograde orbits. The retrograde asteroids may be burnt-out comets.[8]

Kuiper belt objects

The first Kuiper belt object discovered to have a retrograde orbit is 2008 KV42.[9] The dwarf planet Pluto has retrograde rotation. Pluto's axial tilt is approximately 120 degrees.[10]

Comets

Comets from the Oort cloud are much more likely than asteroids to be retrograde.[8] Halley's Comet has a retrograde orbit around the sun.[11]

The sun

The sun's motion about the 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.[12]

Exoplanets

Astronomers have discovered some exoplanets with retrograde orbits. WASP-17b is the first exoplanet that was discovered to be orbiting its star opposite to the direction the star is rotating.[2] HAT-P-7b also has a retrograde orbit. The retrograde motion may be the result of gravitational interactions with other celestial bodies (See Kozai mechanism.) or a collision with another planet.[13]

Stars

Stars with a retrograde orbit, Kapteyn's Star, for example, 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[14] and with a retrograde or zero rotation.[15] Stars in the inner halo favour a prograde rotation while stars in the outer halo favour a retrograde rotation.[16]

Galaxies

In spiral galaxies, the central bulge typically co-rotates with the disk. 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.[17]

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.[18] [19]

See also

References

  1. ^ a b c http://cseligman.com/text/sky/retrograde.htm
  2. ^ a b Grossman, Lisa (13 August 2009). "Planet found orbiting its star backwards for first time". NewScientist. Retrieved 10 October 2009.
  3. ^ a b http://www.newuniverse.co.uk/Axial_tilt.html
  4. ^ a b "Prograde and retrograde motion". Space Wiki. Undated. Retrieved 10 October 2009. {{cite web}}: Check date values in: |date= (help)
  5. ^ Encyclopedia of the solar system. Academic Press. 2007. {{cite encyclopedia}}: Missing or empty |title= (help)
  6. ^ Mason, John (22 July 1989). "Science: Neptune's new moon baffles the astronomers". NewScientist. Retrieved 10 October 2009.
  7. ^ Young, Kelly (5 May 2005). "Twelve new moons discovered around Saturn". NewScientist. Retrieved 10 October 2009.
  8. ^ a b Hecht, Jeff (1 May 2009). "Nearby asteroid found orbiting sun backwards". NewScientist. Retrieved 10 October 2009.
  9. ^ Hecht, Jeff (5 September 2008). "Distant object found orbiting Sun backwards". NewScientist. Retrieved 10 October 2009.
  10. ^ http://www.daviddarling.info/encyclopedia/P/Pluto.html
  11. ^ Halley's Comet
  12. ^ Javaraiah, J. (12 July 2005). "Sun's retrograde motion and violation of even-odd cycle rule in sunspot activity". Royal Astronomical Society, Monthly Notices. 362 (2005). Royal Astronomical Society: 1311–1318. Retrieved 11 October 2009.
  13. ^ Grossman, Lisa (13 August 2009). "Second backwards planet found, a day after the first". NewScientist. Retrieved 10 October 2009.
  14. ^ Kravtsov, V. V. (1 June 2001). "Globular clusters and dwarf spheroidal galaxies of the outer galactic halo: On the putative scenario of their formation" (PDF). Astronomical and Astrophysical Transactions. 20:1 (2001): 89–92. doi:10.1080/10556790108208191. Retrieved 13 October 2009.
  15. ^ Kravtsov, Valery V. (28 August 2002). "Second parameter globulars and dwarf spheroidals around the Local Group massive galaxies: What can they evidence?". Astronomy & Astrophysics. 396 (2002). EDP Sciences: 117–123. doi:10.1051/0004-6361:20021404. Retrieved 13 October 2009.
  16. ^ Carollo, Daniela (13 December 2007). "Two stellar components in the halo of the Milky Way" (PDF). Nature. 450. doi:10.1038/nature06460. Retrieved 13 October 2009. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  17. ^ Prada, F. (14 March 1996). A Counter-rotating Bulge in the Sb Galaxy NGC 7331. arXiv.org. Retrieved 10 October 2009. {{cite conference}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  18. ^ Cain, Fraser (22 May 2003). "Galaxy Orbiting Milky Way in the Wrong Direction". Universe Today. Retrieved 13 October 2009.
  19. ^ Lockman, Felix J. (2 June 2003). "High-velocity cloud Complex H: a satellite of the Milky Way in a retrograde orbit?" (PDF). The Astrophysical Journal. 591 (1 July 2003). The American Astronomical Society: L33 – L36. Retrieved 13 October 2009.

Further reading

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