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Encyclopedia > Retrograde and direct motion

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. The eight planets and three dwarf planets of the Solar System. ... A sphere rotating around its axis. ... m. ...


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.

Contents

Inherent retrograde motion

The word retrograde derives from the Latin words retro, backwards, and gradus, step. Latin is an ancient Indo-European language originally spoken in Latium, the region immediately surrounding Rome. ...

Look up retrograde in Wiktionary, the free dictionary.

Inherent retrogradation is defined by motion relative to an axis of rotation or orbit. Wikipedia does not have an article with this exact name. ... Wiktionary (from wiki and dictionary) is a multilingual, Web-based project to create a free content dictionary, available in over 150 languages. ... The axis of rotation of a rotating body is a line such that the distance between any point on the line and any point of the body remains constant under the rotation. ...


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.) The orbital poles of the Solar system. ... The left-handed orientation is shown on the left, and the right-handed on the right. ... The axis of rotation of a rotating body is a line such that the distance between any point on the line and any point of the body remains constant under the rotation. ... Logo of the IAU The International Astronomical Union (French: Union astronomique internationale) unites national astronomical societies from around the world. ... Major features of the Solar System (not to scale; from left to right): Pluto, Neptune, Uranus, Saturn, Jupiter, the asteroid belt, the Sun, Mercury, Venus, Earth and its Moon, and Mars. ... 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 Neptune). ...


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. A geographical pole is either of two fixed points on the surface of a spinning body or planet, at 90 degrees from the equator, based on the axis around which a body spins. ... World map showing the equator in red In tourist areas, the equator is often marked on the sides of roads The equator marked as it crosses Ilhéu das Rolas, in São Tomé and Príncipe. ...


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). A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. ...


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. Major features of the Solar System (not to scale; from left to right): Pluto, Neptune, Uranus, Saturn, Jupiter, the asteroid belt, the Sun, Mercury, Venus, Earth and its Moon, and Mars. ... The Sun is the star at the center of the Solar System. ... A clockwise motion is one that proceeds like the clocks hands: from the top to the right, then down and then to the left, and back to the top. ... Comet Hale-Bopp Comet McNaught as seen from Swifts Creek, Victoria, Australia on 23 January 2007 A 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 â€” both primarily from the effects of...


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. A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. ... This article or section does not cite any references or sources. ... It has been suggested that minor planet be merged into this article or section. ... Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ... Adjectives: Jovian Atmosphere Surface pressure: 20–200 kPa[4] (cloud layer) Composition: ~86% H2 ~13% Helium 0. ... Adjectives: Saturnian Atmosphere Surface pressure: 140 kPa Composition: >93% hydrogen >5% helium 0. ... Adjectives: Uranian Atmosphere Surface pressure: 120 kPa (at the cloud level) Composition: 83% Hydrogen 15% Helium 1. ... For other meanings see Phoebe. ... Adjectives: Neptunian Atmosphere Surface pressure: (?)≫(?) 100 kPa : is asked the pressure at the cloud level. ... Triton (trye-tÉ™n, IPA: , Greek Τρίτων), or Neptune I, is the planet Neptunes largest moon. ...


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). 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.) Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... Adjectives: Venusian or (rarely) Cytherean Atmosphere Surface pressure: 9. ... Adjectives: Uranian Atmosphere Surface pressure: 120 kPa (at the cloud level) Composition: 83% Hydrogen 15% Helium 1. ... In astronomy, Axial tilt is the inclination angle of a planets rotational axis in relation to a perpendicular to its orbital plane. ...


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 the Earth (so-called diurnal motion). However, objects such as the Space Shuttle and many artificial satellites appear to move from west to east. These are direct satellites (they actually orbit the Earth in the same direction as the Moon), but they orbit the Earth faster than the 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 the 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 the Earth which paradoxically appear to move westward, in the same direction as the Moon. Image File history File links Retrogadation. ... Image File history File links Retrogadation. ... The Experimental Advanced Superconducting Tokamak (EAST, internally called HT-7U) is a project being undertaken to construct an experimental superconducting tokamak magnetic fusion energy reactor in Hefei, the capital city of Anhui Province, in eastern China. ... A compass rose with west highlighted This article refers to the cardinal direction; for other uses see West (disambiguation). ... Diurnal motion is an astronomical term referring to the apparent daily motion of stars in orbit around the Earth, caused by the Earths rotation around its axis. ... NASAs Space Shuttle, officially called Space Transportation System (STS), is the United States governments current manned launch vehicle. ... An Earth observation satellite, ERS 2 For other uses, see Satellite (disambiguation). ... Phobos (IPA or , Greek Φόβος: Fright), is the larger and innermost of Mars two moons (the other being Deimos), and is named after Phobos, son of Ares (Mars) from Greek Mythology. ...

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. Image File history File links Retrogadation1. ... Image File history File links Retrogadation1. ... Adjectives: Martian Atmosphere Surface pressure: 0. ... Adjectives: Jovian Atmosphere Surface pressure: 20–200 kPa[4] (cloud layer) Composition: ~86% H2 ~13% Helium 0. ... Adjectives: Saturnian Atmosphere Surface pressure: 140 kPa Composition: >93% hydrogen >5% helium 0. ... Adjectives: Uranian Atmosphere Surface pressure: 120 kPa (at the cloud level) Composition: 83% Hydrogen 15% Helium 1. ... Adjectives: Neptunian Atmosphere Surface pressure: (?)≫(?) 100 kPa : is asked the pressure at the cloud level. ... Asteroids is a popular vector-based video arcade game released in 1979 by Atari. ... Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ... Adjectives: Plutonian Atmosphere Surface pressure: 0. ...


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. The orbital period is the time it takes a planet (or another object) to make one full orbit. ...


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 geocentric model (in Greek: geo = earth and centron = centre) of the universe is a paradigm which places the Earth at its center. ... In the Ptolemaic system of astronomy, the epicycle (literally: on the cycle in Greek) was a geometric model to explain the variations in speed and direction of the apparent motion of the Moon, Sun, and planets. ... Nicolaus Copernicus (in Latin; Polish Mikołaj Kopernik, German Nikolaus Kopernikus - February 19, 1473 – May 24, 1543) was a Polish astronomer, mathematician and economist who developed a heliocentric (Sun-centered) theory of the solar system in a form detailed enough to make it scientifically useful. ...


In Astrology when planets are in their period of retrograde motion as seen from Earth, it is considered to be a time of the negative traits & a general duration of the inauspicious qualities of that planet. For example, when Mercury is in retrograde, it is considered to be a time of problems with interaction, travel and even the simple basic workings of things. [1] It is also thought that the general averse aspects of a retrograde planetary motion carry over into the lives of the people born under them as a part of their lifetime astrological chart. [2] Hand-coloured version of the anonymous Flammarion woodcut. ...


Examples

Some significant examples of retrograde motion in the solar system: Major features of the Solar System (not to scale; from left to right): Pluto, Neptune, Uranus, Saturn, Jupiter, the asteroid belt, the Sun, Mercury, Venus, Earth and its Moon, and Mars. ...

  • 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.

(*min temperature refers to cloud tops only) Atmospheric characteristics Atmospheric pressure 9. ... A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. ... Atmospheric pressure 0 kPa Ananke (a-nang-kee, IPA ; Greek Ανάγκη) is one of Jupiters moons. ... Atmospheric pressure 0 kPa Carme (IPA: , kar-mee, Greek Κάρμη) is one of Jupiters moons. ... Pasiphaë (IPA: , pa-sif-a-ee, Greek Πασιφάη) is a moon of Jupiter. ... Atmospheric pressure 0 kPa Sinope (IPA: , si-noe-pee, Greek Σινώπη) is a moon of Jupiter discovered by Seth Barnes Nicholson at Lick Observatory in 1914, and is named after Sinope of Greek mythology. ... For other meanings see Phoebe. ... Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ... Triton (trye-tÉ™n, IPA: , Greek Τρίτων), or Neptune I, is the planet Neptunes largest moon. ... Atmospheric characteristics Surface pressure ≫100 MPa Hydrogen - H2 80% ±3. ... Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ... Atmospheric characteristics Atmospheric pressure 120 kPa Hydrogen 83% Helium 15% Methane 1. ...

See also


 
 

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