Retrograde motion

Retrograde motion is 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. Six 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]}

Within the 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 Jupiter, however Saturn has a more even mix of retrograde/prograde moons so the reasons are more complicated^[8].

Asteroids, comets and Kuiper belt objects

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.^[9]

Comets from the Oort cloud are much more likely than asteroids to be retrograde.^[9] Halley's Comet has a retrograde orbit around the sun.^[10]

The first Kuiper belt object discovered to have a retrograde orbit is 2008 KV42.^[11] The dwarf planet Pluto has retrograde rotation. Pluto's axial tilt is approximately 120 degrees.^[12]

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.^[13]

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.^[14]

Stars

Stars with a retrograde orbit 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^[15] and with a retrograde or zero rotation.^[16] 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.^[17]

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 merged with the milky way.^[18]

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, probably as a result of infalling material.^[19]

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.^{[20] [21]}

See also

• Artificial satellites in retrograde orbit
• Yarkovsky effect

References

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. 
• 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. 

• 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.
• 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. 

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