Retrograde and prograde motion

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Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is the central object (right figure). It may also describe other motions such as precession or nutation of the object's rotational axis. Prograde or direct motion is motion in the same direction as the primary rotates. Rotation is determined by an inertial frame of reference, such as distant fixed stars. However, retrograde and prograde can also refer to an object other than the primary if so described.

In our Solar System, the orbits about the Sun of all planets and most other objects, except many comets, are prograde, i.e. in the same direction as the Sun rotates. The rotations of most planets, except Venus and Uranus, are also prograde. Most natural satellites have prograde orbits about their planets. Prograde satellites of Uranus orbit in the direction Uranus rotates, which is retrograde to the Sun. Retrograde satellites are generally small and distant from their planets, except Neptune's satellite Triton, which is large and close. All retrograde satellites are thought to have formed separately before being captured by their planets.

Formation of celestial systems[edit]


Orbital parameters[edit]



Axial tilt[edit]

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 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 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.


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 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 during the formation of the Solar System, an Earth-sized protoplanet collided with Uranus, causing the skewed orientation.[6]

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 dissipation, and also has a thick enough atmosphere to create thermally driven atmospheric tides that create a retrograde torque. Venus' present slow retrograde rotation is in equilibrium balance between gravitational tides trying to tidally lock Venus to the Sun and atmospheric tides trying to spin Venus in a retrograde direction. In addition to maintaining this present day equilibrium, tides are also sufficient to account for evolution of Venus's rotation from a primordial fast prograde direction to its present-day slow retrograde rotation.[7] In the past, various alternative hypotheses have been proposed to explain Venus' retrograde rotation, such as collisions or it having originally formed that way.[a]

Despite being closer to the Sun than Venus, Mercury is not tidally locked because it has entered a 3:2 spin–orbit resonance due to the 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.[8] The rotations of Earth and Mars are also affected by tidal forces 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 giants of the Solar System are too massive and too far from the Sun for tidal forces to slow down their rotations.[7]

Dwarf planets[edit]

All known dwarf planets and 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.[9] Pluto and its moon Charon are both tidally locked to each other. It is suspected that the Plutonian satellite system was created by a massive collision.[10][11]

Earth's atmosphere[edit]

Retrograde motion, or retrogression, within the Earth's atmosphere is seen in weather systems whose motion is opposite the general direction of airflow, i.e. from east to west against the westerlies or from west to east through the trade wind easterlies.

Artificial satellites[edit]

Artificial satellites 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).

Natural satellites and rings[edit]

The orange moon is in a retrograde orbit.


In the Solar System, many of the asteroid-sized moons have retrograde orbits, whereas all the large moons except Triton (the largest of Neptune's moons) have prograde orbits.[13] The particles in Saturn's Phoebe ring are thought to have a retrograde orbit because they originate from the irregular moon Phoebe.



With the exception of Hyperion all the known regular planetary natural satellites in the Solar System are tidally locked to their host planet, so they have zero rotation relative to their host planet, but have the same type of rotation relative to the Sun as their host planet, because they have prograde orbits around their host planet. That is to say, 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, although Haumea's rotation direction is not known.[15]

Small Solar System bodies[edit]



Some asteroids with retrograde orbits may be burnt-out comets,[16] but some may acquire their retrograde orbit due to gravitational interactions with Jupiter.[17]

Due to their small size and their large distance from Earth it is difficult to telescopically analyse the rotation of most asteroids. As of 2012, data is available for less than 200 asteroids and the different methods of determining the orientation of poles often result in large discrepancies.[18] The asteroid spin vector catalog at Poznan Observatory[19] 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.[20]

Asteroids with satellites, also known as binary asteroids, make up about 15% of all asteroids less than 10 km in diameter in the main belt and 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.[21] As of 2012, and where the rotation is known, all satellites of asteroids orbit the asteroid in the same direction as the asteroid is rotating.[22]



Comets from the Oort cloud are much more likely than asteroids to be retrograde.[16] Halley's Comet has a retrograde orbit around the Sun.[24]

Kuiper belt[edit]

Most Kuiper belt objects have prograde orbits around the Sun. The first Kuiper belt object discovered to have a retrograde orbit was 2008 KV42.[25] Other Kuiper belt objects with retrograde orbits are (471325) 2011 KT19,[26] (342842) 2008 YB3, (468861) 2013 LU28 and 2011 MM4.[27] All of these orbits are highly tilted, with inclinations in the 100° - 125° range.


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.[28]

The Sun[edit]




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.[32]

WASP-17b was the first exoplanet that was discovered to be orbiting its star opposite to the direction the star is rotating.[33] A second such planet was announced just a day later: HAT-P-7b.[34]




The pattern of stars appears fixed in the sky, but that is only because they are so far away that their motion isn't visible to the naked eye; actually, they are orbiting the centre of the galaxy. 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[36] and with a retrograde or zero rotation.[37] The structure of the halo is topic of an ongoing debate. Several studies claimed to find a halo consisting of two distinct components.[38][39][40] 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. However, these findings have been challenged by other studies,[41][42] 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.



Satellite galaxies[edit]


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.[45][46]

Counter-rotating bulges[edit]

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.[47]

Central black holes[edit]

The center of a spiral galaxy contains at least one supermassive black hole.[48] 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.[49][50]

See also[edit]




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Further reading[edit]