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Encyclopedia > Retrogradation

Prograde motion is the rotational or orbital motion of a body in a direction similar to that of other bodies within a given system, and is sometimes called direct motion. Retrograde motion is in the contrary direction. The word 'retrograde' derives from the Latin words retro, backwards, and gradus, step. Rotation of a plane, seen as the rotation of the terrain relative to the plane (exposure time 1. ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... Latin is an ancient Indo-European language originally spoken in the region around Rome called Latium. ...

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Retrograde motion

Retrograde motion is the apparent 'backwards' motion of the outer planets (Mars, Jupiter, Saturn, Neptune, Uranus, and Pluto) observed from Earth with respect to the background stars. It is caused by Earth's the shorter orbital period compared to these planets, and is best explaned with an animation (see links below). A planet is generally considered to be a relatively large mass of accreted matter in orbit around a star that is not a star itself. ... For the Roman god, see Mars (mythology). ... Adjective Jovian Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Adjective Saturnian Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... Adjective Neptunian Atmospheric characteristics Surface pressure ≫100 MPa Hydrogen - H2 80% ±3. ... Adjective Uranian Atmospheric characteristics Atmospheric pressure 120 kPa (at the cloud level) Hydrogen 83% Helium 15% Methane 1. ... Adjective Plutonian Atmospheric characteristics Atmospheric pressure 0. ... Earth is the third planet from the Sun. ... The orbital period is the time it takes a planet (or another object) to make one full orbit. ...


Although these planets appear to move from east to west on a nightly basis due to the spin of Earth, they are in fact (most of the time) moving eastward with respect to the background of stars. This can be observed by noting the position of these planets compared to nearby stars for several nights in a row. This eastward motion is normal for these planets, so it is called direct motion, or prograde motion. However, since Earth completes its orbit in a shorter period of time than these outer planets, we occasionally overtake an outer planet, like a faster car on a multiple-lane race track. When this occurs, the planet we are passing will first appear to stop its eastward motion, move briefly westwards, and the continue its eastward motion. The westward motion is referred to as retrograde motion. Earth is the third planet from the Sun. ...


Mars goes through retrograde motion about every 25.7 months. The more distant outer planets retrograde more frequently.


This retrograde motion puzzled ancient astronomers, and was one reason why they named these bodies 'planets' which in Greek means 'wanderers'. In the geocentric model of the solar system, this motion was accounted for by having the planets travel in deferents and epicycles. In modern astronomy, the term retrograde motion usually refers to objects which are actually moving in a direction opposite to what is normal within a given system (i.e. usually comets). 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. ...


Retrograde orbits

In the Solar system, most bodies orbit in a similar (prograde) 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 which have retrograde orbits are mostly comets, which generally have highly disturbed orbits. Presentation of the solar system (not to scale) The solar system comprises the Earths Sun and the retinue of celestial objects gravitationally bound to it. ... For other uses, see Sun (disambiguation). ... 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 For other uses, see Comet (disambiguation). ...


Similarly, the larger and closer moons orbit their planets in the same direction as the planets' rotation, and so are also prograde. 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 prograde 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 prograde and retrograde; some of these may be original Neptunian moons whose orbits were disturbed by Triton's capture, rather than being captured bodies themselves. Moons of solar system scaled to Earths Moon The common noun moon (not capitalized) is used to mean any natural satellite of the other planets. ... A gas giant is a large planet that is not composed mostly of rock or other solid matter. ... An asteroid is a small, solid object in our Solar System, orbiting the Sun. ... Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ... Adjective Jovian Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Adjective Saturnian Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... Adjective Uranian Atmospheric characteristics Atmospheric pressure 120 kPa (at the cloud level) Hydrogen 83% Helium 15% Methane 1. ... For other meanings see Phoebe. ... Adjective Neptunian Atmospheric characteristics Surface pressure ≫100 MPa Hydrogen - H2 80% ±3. ... Triton (trye-tÉ™n, IPA , Greek Τρίτων), or Neptune I, is the planet Neptunes largest moon. ... Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ...


Retrograde rotation

Most planets, including Earth, spin in the prograde 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, Uranus, and Pluto. 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 prograde 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.) Earth is the third planet from the Sun. ... Adjective Venusian or (rarely) Cytherean (*min temperature refers to cloud tops only) Atmospheric characteristics Atmospheric pressure 9. ... Adjective Uranian Atmospheric characteristics Atmospheric pressure 120 kPa (at the cloud level) Hydrogen 83% Helium 15% Methane 1. ... Adjective Plutonian Atmospheric characteristics Atmospheric pressure 0. ... Axial tilt is an astronomical term regarding the inclination angle of a planets rotational axis in relation 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.


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. 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 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. To meet Wikipedias quality standards, this article or section may require cleanup. ... 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. ... The Space Shuttle Columbia seconds after engine ignition, 1981 (NASA). ... A satellite is any object that orbits another object (which is known as its primary). ... Phobos (IPA , Greek Φόβος: Fright), is the larger and innermost of Mars two moons, and is named after Phobos, son of Ares (Mars) from Greek Mythology. ...


Examples

Some significant examples of retrograde motion in the solar system: Presentation of the solar system (not to scale) The solar system comprises the Earths Sun and the retinue of celestial objects gravitationally bound to it. ...

  • 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 to 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. ... Moons of solar system scaled to Earths Moon The common noun moon (not capitalized) is used to mean any natural satellite of the other planets. ... 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

Hipparchus (Greek Ἳππαρχος) (ca. ... Positional astronomy is the study of the positions of celestial objects. ... Claudius Ptolemaeus (Greek: ; ca. ...

Reference

This article originated from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml Screenshot of KStars showing the night sky from Hanover. ... Tux is the official Linux mascot. ... KDE (K Desktop Environment) is a free desktop environment and development platform built with Trolltechs Qt toolkit. ...


External link

  • [1] - Animations of retrograde motion

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