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
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. 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's present slow retrograde rotation is an approximate
equilibrium between gravitational tides trying to
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, 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 6.5minutes. 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. 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. 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. Pluto and its moon
Charon are tidally locked to each other. It is suspected that the Plutonian satellite system was created by a
massive collision.
Natural satellites 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 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. 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. The particles in Saturn's
Phoebe ring are thought to have a retrograde orbit because they originate from the irregular moon
Phoebe. All retrograde satellites experience
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. 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 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, although Haumea's rotation direction is not known.
Small Solar System bodies Asteroids Many
asteroids have a prograde orbit around the Sun. Only approximately a hundred
asteroids in retrograde orbits are known. Some asteroids with retrograde orbits may be burnt-out comets, but some may acquire their retrograde orbit due to gravitational interactions with
Jupiter. 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 fewer than 200 asteroids and the different methods of determining the orientation of
poles often result in large discrepancies. The asteroid spin vector catalog at Poznan Observatory 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. 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. 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. 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.
Comets Comets from the
Oort cloud are much more likely than asteroids to be retrograde.
Centaurs Most
centaurs have a prograde orbit around the Sun. The first centaur with a retrograde orbit to be discovered was
20461 Dioretsa. Other known centaurs with retrograde orbits include , , , , and . All of these orbits are highly inclined, with inclinations in the range of 160 to 180°.
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 . Other Kuiper belt objects with retrograde orbits are
471325 Taowu, , and
2011 MM4. All of these orbits are highly tilted, with
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.
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. == Planetary atmospheres ==