Retrograde and prograde motion

• 1. Introduction

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  Retrograde and prograde motion describes one of the most fundamental patterns in the universe: which direction things spin and orbit, and why a handful of worlds break from the rule. In our Solar System, every planet circles the Sun counterclockwise when viewed from above the Sun's north pole. That is the prograde direction, matching the way the Sun itself rotates. Yet two planets, Venus and Uranus, spin the wrong way. Triton, Neptune's largest moon, orbits backwards. And beyond our Solar System, astronomers have found entire planets circling their stars in the opposite direction to which those stars rotate. These exceptions are not quirks. They are clues to violent histories: collisions, captures, and gravitational tugs-of-war stretching back to the birth of solar systems. How do astronomers measure these directions? What decides whether a world spins one way or the other? And what does a backwards black hole do differently from a forward-spinning one?

• 2. Measuring Direction

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  Inclination is the angle between an object's orbital plane and a reference plane, such as the equatorial plane of whatever body it orbits. In the Solar System, inclination for the planets is measured from the ecliptic, which is the plane of Earth's orbit around the Sun. An inclination between 0 and 90 degrees means the object is orbiting in the same direction as its primary rotates. An inclination of exactly 90 degrees produces a perpendicular orbit that is neither prograde nor retrograde. An inclination between 90 and 180 degrees marks a retrograde orbit.

  Axial tilt follows the same logic for rotation rather than orbit. Axial tilt is the angle between a body's rotation axis and a line perpendicular to its orbital plane passing through the body's centre. An axial tilt up to 90 degrees means the body rotates in the same direction as its primary. An axial tilt between 90 and 180 degrees means rotation is reversed. Venus has an axial tilt of 177 degrees, placing it almost exactly backwards relative to its orbit. Uranus has an axial tilt of 97.77 degrees, tipping its rotation axis until it lies nearly parallel with the plane of the Solar System.

  The north pole of any planet or moon in the Solar System is defined by convention as the pole sitting in the same celestial hemisphere as Earth's north pole, regardless of which way the body rotates. That definition keeps astronomical coordinates consistent across worlds with very different histories.

• 3. Why Venus Spins Backwards

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  Venus almost certainly did not start out rotating in the slow retrograde direction it has today. Scientists believe it once spun quickly in the prograde direction, with a rotation period of several hours, much like most other planets. Two forces gradually reversed and slowed that spin over geological time.

  Gravitational tidal dissipation from the Sun works to pull Venus toward a tidally locked state, the same condition that keeps one face of the Moon permanently facing Earth. At the same time, Venus has an atmosphere thick enough to generate thermally driven atmospheric tides, which exert a retrograde torque on the planet. Venus's present slow rotation is an approximate equilibrium between these two competing forces. Its rotation takes 243 days, a figure confirmed by the Magellan spacecraft. Comparing Magellan data over a 500-day observation period with measurements taken during the 16-year gap between the Magellan and Venus Express missions reveals that Venus's rotation period is not perfectly stable; the two measurements differ by about 6.5 minutes.

  Mercury, though closer to the Sun than Venus, avoided tidal locking through a different mechanism. Its orbit is eccentric enough to pull it into a 3:2 spin-orbit resonance, meaning it completes three rotations for every two orbits. Near perihelion, Mercury's angular orbital velocity briefly exceeds its rotational velocity, causing the Sun to appear to move backwards in Mercury's sky. Earth and Mars are also subject to solar tidal forces, but both orbit far enough out that those forces are too weak to have brought them to equilibrium.

• 4. Captured Moons and Triton

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  A moon that forms within the gravitational field of its parent planet as that planet is still assembling will naturally orbit in the prograde direction; this is a regular moon. A body formed elsewhere and later pulled in by a planet's gravity can be captured into either a prograde or retrograde orbit depending on the geometry of the encounter. Such bodies are irregular moons, and the distinction carries long-term consequences.

  All retrograde satellites lose orbital energy through tidal deceleration. For most retrograde moons, which are small and distant, this effect is negligible. The one exception is Triton, the largest of Neptune's moons. Triton is large and orbits close to Neptune, making it the only retrograde satellite in the Solar System for which tidal deceleration is non-negligible. Because retrograde orbits inevitably decay under tidal forces at close range, Triton is expected to eventually spiral inward and be destroyed.

  Saturn's ring system offers another trace of retrograde capture. The particles in Saturn's Phoebe ring are thought to have retrograde orbits because they originate from Phoebe, an irregular moon with a retrograde orbit. The preponderance of retrograde moons around Jupiter has been linked to orbital stability: within the Hill sphere, the region of stability for retrograde orbits at large distances from the primary is actually larger than for prograde orbits. Saturn, however, has a more even mix of retrograde and prograde moons, suggesting the underlying dynamics are more complex than a single stability argument can explain.

• 5. Small Bodies and Comets

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  Among the smaller inhabitants of the Solar System, retrograde orbits are rare but not absent. Only roughly a hundred asteroids in retrograde orbits were known as of 2012, compared to the many thousands with prograde orbits. Some of these may be burnt-out comets; others may have acquired their unusual orbits through gravitational interactions with Jupiter. A small number of retrograde asteroids have been found in orbital resonance with Jupiter and Saturn, a configuration more commonly associated with prograde bodies.

  Data on asteroid rotation is sparse. As of 2012, rotation measurements existed for fewer than 200 asteroids, and the methods used to determine pole orientations often produce large discrepancies. The asteroid spin vector catalog at Poznan Observatory deliberately avoids the terms retrograde rotation and prograde rotation because asteroid coordinates are typically given relative to the ecliptic plane rather than each asteroid's individual orbital plane, making the label ambiguous.

  Comets from the Oort cloud are far more likely than asteroids to travel in retrograde orbits. Halley's Comet is the most famous example. Among the centaurs, the first known to have a retrograde orbit was 20461 Dioretsa. Among Kuiper belt objects, the first discovered with a retrograde orbit was followed by others including 471325 Taowu and 2011 MM4; all of these have highly tilted orbits, with inclinations in the 100-125 degree range. Meteoroids traveling in retrograde orbits hit Earth at higher relative speeds than prograde ones and tend to burn up in the atmosphere rather than surviving to reach the ground as meteorites.

• 6. Exoplanets Breaking the Rules

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  WASP-17b was the first exoplanet discovered to be orbiting its star in the opposite direction to the star's rotation. A second such planet, HAT-P-7b, was announced just one day later. Both discoveries pointed toward a broader pattern: in one study, more than half of all known hot Jupiters had orbits misaligned with their parent stars' rotation axes, and six were orbiting fully backwards.

  One explanation ties this to the environment where stars and planets form. Stars and their planets do not form in isolation but within star clusters that contain molecular clouds. When a protoplanetary disk collides with or steals material from one of those clouds, the disk can be tilted into a retrograde configuration, and the planets that form within it inherit that orientation. The Kozai mechanism, which involves gravitational interactions with other bodies in the same system, offers another path to retrograde planetary orbits, as does a near-collision with another planet or an early flip of the star itself driven by interactions between the star's magnetic field and the planet-forming disk.

  The accretion disk of the protostar IRAS 16293-2422 provides a concrete example: parts of that disk rotate in opposite directions, making it the first known counterrotating accretion disk. If planets form in that system, the inner planets will likely orbit in the opposite direction to the outer planets. For terrestrial planets in general, giant impacts during formation are the main factor setting rotation rate; since collisions during that stage are equally likely from any direction in three dimensions, prograde spin with small axial tilt is not actually the expected outcome for terrestrial planets across the universe.

• 7. Retrograde at Galactic Scale

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  Retrograde motion does not stop at the scale of solar systems. Stars in disk galaxies generally orbit the galactic centre in one direction, but not all of them. Stars with retrograde orbits relative to the galaxy's general rotation are more likely to be found in the galactic halo than in the galactic disk. The Milky Way's outer halo contains many globular clusters with retrograde orbits and retrograde or zero rotation.

  Several studies have proposed that the Milky Way's halo is divided into two components: an inner, more metal-rich region where stars orbit in the prograde direction on average, and an outer, metal-poor region where stars orbit in the retrograde direction. Other researchers have challenged this picture, arguing that the observational data can be explained without positing a dual halo once improved statistical analysis and measurement uncertainties are accounted for. The debate is ongoing.

  Kapteyn's Star, a nearby object with an unusually high-velocity retrograde orbit around the galaxy, is thought to have ended up in that trajectory after being stripped from a dwarf galaxy that later merged with the Milky Way. A galaxy called Complex H, which was orbiting the Milky Way in a retrograde direction relative to the Milky Way's own rotation, is currently in the process of colliding with it. And at the very centre of spiral galaxies, where at least one supermassive black hole resides, the direction of spin matters enormously: a retrograde black hole, spinning opposite to its accretion disk, produces jets far more powerful than those of a prograde black hole, which may produce no detectable jet at all.