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.
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 .
Encyclopedia
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 . 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.
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:
in which case no inclination would ever exceed 90° , or of:
- 170° and +6 h , 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.
Retrograde orbits
In the
Solar system, most bodies orbit in a similar 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 , 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 . 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.
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
When we observe the sky, the Sun, Moon, and stars appear to move from
east to
west because of the rotation of the Earth . However, objects such as the
Space Shuttle and many artificial
satellites appear to move from west to east. These are direct satellites , 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 , 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.
As seen from Earth, the planets beyond Earth's orbit 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 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 optical illusion until the time of
Copernicus.
Examples
Some significant examples of retrograde motion in the
solar system:
- Venus rotates slowly in the retrograde direction.
- The moons Ananke, Carme, Pasiphaė 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.
See also
