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Encyclopedia > Prograde and retrograde motion

Prograde motion is the motion of a planetary body in a direction similar to that of other bodies within its system, and is sometimes called direct motion, especially in astrology. 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, an optical illusion as seen from Earth. Astrology refers to any of several systems, traditions or beliefs in which knowledge of the apparent positions of celestial bodies is held to be useful in understanding, interpreting, and organizing knowledge about human affairs and events on Earth. ... 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. ...

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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 the region around Rome called Latium. ...

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 logo Wiktionary is a Wikimedia Foundation project intended to be a free wiki dictionary (including thesaurus and lexicon) in almost every language. ... 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 orbital poles of the Solar system. ... In mathematics and physics, the right-hand rule is a convention for determining relative directions of certain vectors. ... The word axis has several meanings: In mathematics, axis can mean: A straight line around which a geometric figure can be rotated. ...


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. For other uses of the word pole, see Pole (disambiguation). ... The equator is an imaginary circle drawn around a planet (or other astronomical object) at a distance halfway between the poles. ...


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 prograde motion, but when this inclination isn't 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.) Moons of solar system scaled to Earths Moon A natural satellite is a moon (not capitalized), that is, any natural object that orbits a planet. ...


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 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. ... The Sun is the spectral type G2V yellow star at the center of Earths 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 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 A natural satellite is a moon (not capitalized), that is, any natural object that orbits a planet. ... 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. ...


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 in the Solar system. ... 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.


Apparent retrograde motion

How do we see a retrograde motion ? T1, T2, ..., T5 - positions of Terra P1, P2, ..., P5 - positions of a planet A1, A2, ..., A5 - projection to celestial sphere
How do we see a 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. ... 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. ...

Like that ..
Like that ..

As seen from Earth, the planets beyond Earth's orbit (Mars, Jupiter, Saturn, Neptune, Uranus, and Pluto) 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. Image File history File links Retrogadation1. ... Image File history File links Retrogadation1. ... 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. ...


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 optical 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. ...


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 A natural satellite is a moon (not capitalized), that is, any natural object that orbits a planet. ... 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

The astrological aspects are noted in the central circle of this natal chart, where the different colors and symbols distinguish betwixt the different aspects, such as the square (red) or trine (blue) In astrology, an aspect is the relative angle between two heavenly bodies. ... Hipparchus (Greek ) (ca. ... Positional astronomy is the study of the positions of celestial objects. ... Claudius Ptolemaeus (Greek: ; ca. ...

External link


  Results from FactBites:
 
Prograde and retrograde motion - Wikipedia, the free encyclopedia (978 words)
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 the apparent 'backwards' motion of the outer planets (Mars, Jupiter, Saturn, Neptune, Uranus, and Pluto) observed from Earth with respect to the background stars.
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).
  More results at FactBites »

 
 

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