# retrogradation

## Information about retrogradation

Direct motion is the motion of a planetary body in a direction similar to that of other bodies within its system, and is sometimes called prograde motion. Retrograde motion is motion in the contrary direction. In the case of celestial bodies, such motion may be real, defined by the inherent rotation or orbit of the body, or apparent, as seen in the skies from Earth.

While the terms direct and prograde are equivalent in this context, the former is the traditional term in astronomy. Prograde was first seen in an abstract of an astronomy-related professional article in 1963 (J. Geophys. Res. 68, 4979). It probably originated with "rocket scientists" who did not know the correct word and therefore invented a plausible substitute.

## Inherent retrograde motion

The word retrograde derives from the Latin words retro, backwards, and gradus, step.

Inherent retrogradation is defined by motion relative to an axis of rotation or orbit.

The north orbital pole of a celestial body is defined by the right-hand rule: If one curves the fingers of the right hand along the direction of orbital motion, with the thumb extended parallel to the orbital axis, the direction the thumb points is defined to be north. (The International Astronomical Union has defined a different convention for planetary bodies in the solar system. According to this definition, the north pole is the one that points north of the invariable plane.)

Similarly, the north rotational pole of a body is defined by the direction of the thumb if one were to wrap the fingers around the body's equator in the direction it spins.

There are two notations for retrograde motion that are mathematically equivalent: The body can be considered to orbit backwards, or it can be considered to orbit forwards, but with its orbit upside-down. For example, a moon in a retrograde orbit that is inclined from the pole of its planet by 10°, and with a 6-hour orbital period, could be said to have the orbital parameters of:
• 10° (rightside-up) and −6 h (backwards),
in which case no inclination would ever exceed 90° (anything more than 90° would be upside-down), or of:
• 170° (upside-down) and +6 h (forwards), in which case no period would ever be negative.
Similarly, a moon spinning backwards on an axis inclined by 10° from the axis of its orbit can instead be described as being flipped upside-down and spinning forwards.

The choice between these two notations is largely arbitrary. It is more common to keep the period positive and let the inclination vary between 90° and 180° for retrograde motion, and between 0° and 90° for direct motion, but when this inclination is not listed, a negative period is the only indication that an orbit or rotation is retrograde. Thus it is common to see negative periods in tables of data (See natural satellite).

### Retrograde orbits

In the Solar system, most bodies orbit in a similar (direct) direction to the rotation of the Sun. All planets and most smaller bodies orbit the Sun counterclockwise as seen from a position above the Sun's north pole. The exceptions are mostly long-period and nonperiodic comets, which can have any inclination.

Similarly, the larger and closer moons orbit their planets in the same direction as the planets' rotation, and so are also direct. However, the gas giant planets have large numbers of small "irregular" moons in highly inclined or elliptical orbits, thought to be captured asteroids or Kuiper belt objects (or fragments thereof), and the majority of these are instead retrograde: 48 retrograde to 7 direct for Jupiter, 18 to 8 for Saturn, and 8 to 1 for Uranus. One of the largest of these is the Saturnian moon Phoebe. Neptune is somewhat different: It seems to have captured its only surviving large moon, the retrograde but otherwise regular Triton, from the Kuiper Belt. The six irregular moons beyond Triton's orbit are evenly divided between direct and retrograde; some of these may be original Neptunian moons whose orbits were disturbed by Triton's capture, rather than being captured bodies themselves.

### Retrograde rotation

Most planets, including Earth, spin in the direct sense: they spin in the same direction as they orbit the Sun (that is, their north rotational pole and north orbital pole point in similar directions, more or less in the direction of the Solar north pole). Interestingly, because of this, on these planets, if they didn't rotate, the sun would obviously rise once each year and set once each year, even outside the poles, but the sun would rise in the west and set in the east. The exceptions are Venus and Uranus. Uranus rotates nearly on its side relative to its orbit. It has been described as having an axial tilt of 82° and a negative rotation of −17 hours, or, equivalently, of having an axis tilted at 98° and a positive rotation. Since current speculation is that Uranus started off with a typical direct orientation and was knocked on its side by a large impact early in its history, it is most commonly described as having the higher axial tilt and positive rotation. (Since Uranus' moons are considered relative to Uranus itself, their description is unaffected by the choice made for the planet.)

Retrograde Venus, on the other hand, has an axial tilt of less than 3°, and a very slow rotation of 243 days. Perhaps because it is easier to conceive of Venus as rotating slowly backwards than being 'upside down' relative to its near-twin Earth, but also because it is thought that an early massive impact may have resulted in Venus' current rotation while leaving its axis more or less unaffected, Venus is nearly always described as having its axis at 3° and a rotation of −243 days, rather than 177° and +243 days.

## Apparent retrograde motion

T1, T2, ..., T5 - positions of Terra
P1, P2, ..., P5 - positions of a planet
A1, A2, ..., A5 - projection to celestial sphere

When we observe the sky, the Sun, Moon, and stars appear to move from east to west because of the rotation of Earth (so-called diurnal motion). However, objects such as the orbiter of the Space Shuttle and many artificial satellites appear to move from west to east. These are direct satellites (they actually orbit Earth in the same direction as the Moon), but they orbit Earth faster than Earth itself rotates, and so appear to move in the opposite direction. Mars has a natural moon, Phobos, with a similar orbit. From the surface of Mars it appears to move in the opposite direction to Earth's moon (Luna), even though both Phobos and Luna have direct orbits, because its orbital period is less than a Martian day, whereas Luna's orbital period (one month) is longer than a Terrestrial day. There are also smaller numbers of truly retrograde artificial satellites orbiting Earth which paradoxically appear to move westward, in the same direction as the Moon.

Apparent path
As seen from Earth, the planets beyond Earth's orbit (Mars, Jupiter, Saturn, Uranus, and Neptune) appear to periodically switch direction as they cross the sky. Though all stars and planets appear to move from east to west on a nightly basis in response to the rotation of Earth, the planets generally drift slowly eastward relative to the stars. This motion is normal for the planets, and so is considered direct motion. However, since Earth completes its orbit in a shorter period of time than the planets outside its orbit, we periodically overtake them, like a faster car on a multi-lane highway. When this occurs, the planet we are passing will first appear to stop its eastward drift, and then drift back toward the west. Then, as Earth swings past the planet in its orbit, it appears to resume its normal motion west to east. Asteroids and Kuiper Belt Objects (including Pluto) also exhibit apparent retrogradation.

Mars goes through apparent retrogradation every 25.7 months. The more distant outer planets retrograde more frequently. The period between such retrogradations is the synodic period of the planet.

This apparent retrogradation puzzled ancient astronomers, and was one reason they named these bodies 'planets' in the first place: 'Planet' comes from the Greek word for 'wanderer'. In the geocentric model of the solar system, retrograde motion was explained by having the planets travel in deferents and epicycles. It was not understood to be an illusion until the time of Copernicus. The accompanying map shows the retrograde motion of Mars for the year 2009-2010, which occurs against the background of the constellation Cancer.

## Examples

Some significant examples of retrograde motion in the solar system:
• Venus rotates slowly in the retrograde direction.
• The moons Ananke, Carme, Pasiphaë and Sinope all orbit Jupiter in a retrograde direction. Many other minor moons of Jupiter orbit retrograde.
• The moon Phoebe orbits Saturn in a retrograde direction, and is thought to be a captured Kuiper belt object.
• The moon Triton orbits Neptune in a retrograde direction, and is also thought to be a captured Kuiper belt object.
• The planet Uranus has an axial tilt of 98°, which is near 90°, and can be considered to be rotating in a retrograde direction depending on one's interpretation.

## Retrograde motion in astrology

In astrology, the apparent retrograde motion of the planets () was traditionally thought to be unlucky or inauspicious, as it went against the 'natural' order of movement, and a planet which was retrograde at the time of birth was considered a weak spot in the natal chart.[1] Most modern astrologers still consider the retrograde movement of a planet to be indicative of stress or difficulty. For example, the retrograde movement of Mercury is commonly thought to signify difficulties in communication, such as post or emails going astray, verbal misunderstandings, and travel delays and frustrations.[2] However, many astrologers do not consider retrograde movement to be of any particular significance, especially given that the outer planets are retrograde for over 40% of the time.[3] The claims of astrology are not accepted by modern astronomers, or by modern scientists in general.

## References

1. ^ [1]
2. ^ [2]
3. ^ Sasha Fenton, Understanding Astrology, p73, p145, Aquarian Press, London 1991

## External links

planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion in its core, and has cleared its neighbouring region of
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A rotation is a movement of an object in a circular motion.
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ORBit is a CORBA compliant Object Request Broker (ORB). The current version is called ORBit2 and is compliant with CORBA version 2.4. It is developed under the GPL license and is used as middleware for the GNOME project.
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Latin}}}
Official status
Official language of: Vatican City
Used for official purposes, but not spoken in everyday speech
Regulated by: Opus Fundatum Latinitas
Roman Catholic Church
Language codes
ISO 639-1: la
ISO 639-2: lat
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A rotation is a movement of an object in a circular motion.
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orbital pole is either end of an imaginary line running through the center of an orbit perpendicular to the orbital plane, projected onto the celestial sphere. It is similar in concept to a geographical pole but based on the planet's orbit instead of the planet's rotation.
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right hand grip rule.
In mathematics and physics, the right-hand rule is a common mnemonic for understanding notation conventions for vectors in 3-D.
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A rotation is a movement of an object in a circular motion.
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International Astronomical Union (IAU) unites national astronomical societies from around the world. It also acts as the internationally recognized authority for assigning designations to celestial bodies (stars, planets, asteroids, etc.
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Solar System or solar system[a] consists of the Sun and the other celestial objects gravitationally bound to it: the eight planets, their 166 known moons,[1]
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The invariable plane of the solar system is the plane passing through its barycenter (center of mass) which is perpendicular to its angular momentum vector, about 98% of which is contributed by the orbital angular momenta of the four jovian planets (Jupiter, Saturn, Uranus, and
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equator is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole. It thus divides the Earth into a Northern Hemisphere and a Southern Hemisphere. The equators of other planets and astronomical bodies are defined analogously.
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A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. Such objects are often called moons. Technically, the term could also refer to a planet orbiting a star, or even to a star orbiting a galactic center, but these
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Solar System or solar system[a] consists of the Sun and the other celestial objects gravitationally bound to it: the eight planets, their 166 known moons,[1]
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The Sun

Observation data
Mean distance
from Earth 1.4961011 m
(8.31 min at light speed)
Visual brightness (V) −26.74m [1]
Absolute magnitude 4.
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clockwise motion is one that proceeds 'like the clock's hands': from the top to the right, then down and then to the left, and back to the top. In a mathematical sense, a circle defined parametrically in a positive Cartesian plane by the equations x = sin t
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comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and
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A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. Such objects are often called moons. Technically, the term could also refer to a planet orbiting a star, or even to a star orbiting a galactic center, but these
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gas giant (sometimes also known as a Jovian planet after the planet Jupiter) is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System; Jupiter, Saturn, Uranus, and Neptune.
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Asteroids, also called minor planets or planetoids, are a class of astronomical objects. The term asteroid is generally used to indicate a diverse group of small celestial bodies in the solar system that orbit around the Sun.
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Kuiper belt (pronounced IPA: /ˈkaɪpɚ/, to rhyme with "viper"),[1] sometimes called the Edgeworth-Kuiper belt
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Jupiter

This processed color image of Jupiter was produced in 1990 by the U.S. Geological Survey from a Voyager image captured in 1979. The colors have been enhanced to bring out detail.
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Saturn

Saturn, as seen by Cassini
Orbital characteristics[1][2]
Epoch J2000
Aphelion distance: 1,513,325,783 km
10.11595804 AU
Perihelion distance: 1,353,572,956 km
9.
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Uranus

Uranus, as seen by Voyager 2
Discovery
Discovered by: William Herschel
Discovery date: March 13, 1781
Orbital characteristics[1][2]
Epoch J2000
Aphelion distance: 3,004,419,704 km
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For other meanings see Phoebe.

Phoebe

Discovery
Discovered by: W.H. Pickering
Discovery date: March 17, 1899 / August 16, 1898
Orbital characteristics [1]
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NEPTUNE is an acronym for North-East Pacific Time-series Undersea Networked Experiments. The NEPTUNE Canada project will lay approximately 800 km of power and fibre optic cables over the northern part of the Juan de Fuca tectonic plate off the west coast of Vancouver Island in
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Triton

Discovery
Discovered by: William Lassell
Discovery date: October 10, 1846
Orbital characteristics
Semi-major axis: 354,800 km
Eccentricity: 0.0000
Orbital period: −5.877 d
(retrograde)
Inclination: 130.
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EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001. Their greatest hit, their debut single "time after time", peaked at #13 in the Oricon singles chart.
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VENUS is an acronym for the Victoria Experimental Network Under the Sea . The VENUS project is operated out of the University of Victoria and is an advanced cabled sea floor observatory, consisting of fibre optic cables connecting oceanographic instruments on the sea floor of the
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