Retrograde and prograde motion
The direction of rotation is determined by an inertial frame of reference, such as distant fixed stars.
When a galaxy or a planetary system forms, its material takes a shape similar to that of a disk.
In 2010 the discovery of several hot Jupiters with backward orbits called into question the theories about the formation of planetary systems.
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.
Venus probably began with a fast prograde rotation with a period of several hours much like most of the planets in the Solar System.
[7] 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.
[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.
The gas giants of the Solar System are too massive and too far from the Sun for tidal forces to slow down their rotations.
[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.
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.
[30] The Sun's motion about the centre of mass of the Solar System is complicated by perturbations from the planets.
[31] Retrograde motion, or retrogression, within the Earth's atmosphere is seen in weather systems whose motion is opposite the general regional direction of airflow, i.e. from east to west against the westerlies or from west to east through the trade wind easterlies.
Prograde motion with respect to planetary rotation is seen in the atmospheric super-rotation of the thermosphere of Earth and in the upper troposphere of Venus.
[32] Artificial satellites destined for low inclination orbits 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).
[3][4] Retrograde motion may also result from gravitational interactions with other celestial bodies in the same system (See Kozai mechanism) or a near-collision with another planet,[1] or it may be that the star itself flipped over early in their system's formation due to interactions between the star's magnetic field and the planet-forming disk.
[2] One proposed explanation is that hot Jupiters tend to form in dense clusters, where perturbations are more common and gravitational capture of planets by neighboring stars is possible.
[38] The last few giant impacts during planetary formation tend to be the main determiner of a terrestrial planet's rotation rate.
During the giant impact stage, the thickness of a protoplanetary disk is far larger than the size of planetary embryos so collisions are equally likely to come from any direction in three dimensions.
This results in the axial tilt of accreted planets ranging from 0 to 180 degrees with any direction as likely as any other with both prograde and retrograde spins equally probable.
[39] The pattern of stars appears fixed in the sky, insofar as human vision is concerned; this is because their massive distances relative to the Earth result in motion imperceptible to the naked eye.
These studies demonstrate that the observational data can be explained without a duality, when employing an improved statistical analysis and accounting for measurement uncertainties.
[49][50] 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.
