Poles of astronomical bodies
Encyclopedia
The poles of astronomical bodies are determined based on their axis of rotation in relation to the celestial pole
s of the celestial sphere
.
(IAU) defines the geographic north pole of a planet
or any of its satellites in the solar system
as the planetary pole that is in the same celestial hemisphere relative to the invariable plane
of the solar system as Earth's North pole. This definition means that an object's direction of rotation may be negative (retrograde
rotation) — in other words, it rotates clockwise when viewed from above its north pole, rather than the "normal" counterclockwise direction exhibited by Earth's north pole. Venus
rotates in the opposite direction to the other planets, and Uranus
has been knocked on its side and rotates almost perpendicular to the rest of the solar system. The ecliptic
remains within 3° of the invariable plane over five million years, but is now inclined about 23.44° to Earth's celestial equator
used for the coordinates of poles. This large inclination means that the declination of a pole relative to Earth's celestial equator could be negative even though a planet's north pole (such as Uranus) is north of the invariable plane.
In 2009 the responsible IAU Working Group decided to define the poles of dwarf planets, minor planets, their satellites, and comets according to the right-hand rule
. To avoid confusion with the "north" and "south" definitions relative to the invariable plane, "positive" is the pole toward which the thumb points when the fingers are curled in its direction of rotation ("negative" for the opposite pole). This change was needed because the poles of some asteroids and comets precess rapidly enough for their north and south poles to swap within a few decades using the invariable plane definition.
The projection of a planet's geographic north pole onto the celestial sphere
gives its north celestial pole
. The location of the celestial poles of some selected solar system objects is shown in the following table. The coordinates are given relatve to Earth's celestial equator and the vernal equinox as they existed at J2000 (2000 January 1 12:00:00 TT
) which is a plane fixed in inertial space now called the International Celestial Reference Frame
(ICRF). Many poles precess or otherwise move relative to the ICRF, so their coordinates will change.
Some bodies in the solar system, including Saturn
's moon
Hyperion
and the asteroid 4179 Toutatis
, lack a stable geographic north pole. They rotate chaotically
because of their irregular shape and gravitational influences from nearby planets and moons, and as a result the instantaneous pole wanders over their surface, and may momentarily vanish altogether (when the object comes to a standstill with respect to the distant stars).
: they are the locations on the planet's surface at which the planet's magnetic field lines are vertical. The direction of the field determines whether the pole is a magnetic north or south pole, exactly as on Earth. The Earth's magnetic axis is approximately aligned with its rotational axis, meaning that the magnetic poles are relatively close to the geographic poles. However, this is not necessarily the case for other planets; the magnetic axis of Uranus
, for example, is inclined by as much as 60°.
satellites, four more poles can be defined. They are the near, far, leading, and trailing poles. Take Io
for example; this moon of Jupiter
rotates synchronously, so its orientation with respect to Jupiter stays constant. There will be a single, unmoving point of its surface where Jupiter is at the zenith
, exactly overhead — this is the near pole, also called the sub- or pro-Jovian point. At the antipode
of this point is the far pole, where Jupiter lies at the nadir
; it is also called the anti-Jovian point. There will also be a single unmoving point which is furthest along Io's orbit (best defined as the point most removed from the plane formed by the north-south and near-far axes, on the leading side) —this is the leading pole. At its antipode lies the trailing pole. Io can thus be divided into north and south hemispheres, into pro- and anti-Jovian hemispheres, and into leading and trailing hemispheres. Note that these poles are mean poles because the points are not, strictly speaking, unmoving: there is continuous libration
about the mean orientation, because Io's orbit is slightly eccentric and the gravity of the other moons disturbs it regularly.
These poles also apply to planets that are rotating synchronously with their primary stars, as is likely the case with many hot Jupiter
s and as was once thought to be the case with Mercury
. Other synchronously-rotating objects, such as Pluto
and some asteroids with large asteroid moon
s, can also be described as having "near" and "far" poles - though "leading" and "trailing" may not be as significant in these cases.
Celestial pole
The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars called the celestial sphere...
s of the celestial sphere
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...
.
Geographic poles
The International Astronomical UnionInternational Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
(IAU) defines the geographic north pole of a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
or any of its satellites in the solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
as the planetary pole that is in the same celestial hemisphere relative to the invariable plane
Invariable plane
The invariable plane of a planetary system, also called Laplace's invariable plane, is the plane passing through its barycenter perpendicular to its angular momentum vector. In the Solar System, about 98% of this effect is contributed by the orbital angular momenta of the four jovian planets...
of the solar system as Earth's North pole. This definition means that an object's direction of rotation may be negative (retrograde
Retrograde motion
Retrograde motion is motion in the direction opposite to the movement of something else, and is the contrary of direct or prograde motion. This motion can be the orbit of one body about another body or about some other point, or the rotation of a single body about its axis, or other phenomena such...
rotation) — in other words, it rotates clockwise when viewed from above its north pole, rather than the "normal" counterclockwise direction exhibited by Earth's north pole. Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
rotates in the opposite direction to the other planets, and Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
has been knocked on its side and rotates almost perpendicular to the rest of the solar system. The ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...
remains within 3° of the invariable plane over five million years, but is now inclined about 23.44° to Earth's celestial equator
Celestial equator
The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space...
used for the coordinates of poles. This large inclination means that the declination of a pole relative to Earth's celestial equator could be negative even though a planet's north pole (such as Uranus) is north of the invariable plane.
In 2009 the responsible IAU Working Group decided to define the poles of dwarf planets, minor planets, their satellites, and comets according to the right-hand rule
Right-hand rule
In mathematics and physics, the right-hand rule is a common mnemonic for understanding notation conventions for vectors in 3 dimensions. It was invented for use in electromagnetism by British physicist John Ambrose Fleming in the late 19th century....
. To avoid confusion with the "north" and "south" definitions relative to the invariable plane, "positive" is the pole toward which the thumb points when the fingers are curled in its direction of rotation ("negative" for the opposite pole). This change was needed because the poles of some asteroids and comets precess rapidly enough for their north and south poles to swap within a few decades using the invariable plane definition.
The projection of a planet's geographic north pole onto the celestial sphere
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...
gives its north celestial pole
Celestial pole
The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars called the celestial sphere...
. The location of the celestial poles of some selected solar system objects is shown in the following table. The coordinates are given relatve to Earth's celestial equator and the vernal equinox as they existed at J2000 (2000 January 1 12:00:00 TT
Terrestrial Time
Terrestrial Time is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of the Earth....
) which is a plane fixed in inertial space now called the International Celestial Reference Frame
International Celestial Reference Frame
The International Celestial Reference Frame is a quasi-inertial reference frame centered at the barycenter of the Solar System, defined by the measured positions of 212 extragalactic sources . Although relativity implies that there is no true inertial frame, the extragalactic sources used to...
(ICRF). Many poles precess or otherwise move relative to the ICRF, so their coordinates will change.
| Object | North pole | South pole | ||
|---|---|---|---|---|
| RA Right ascension Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:... |
Dec Declination In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and... |
RA | Dec | |
| Sun | 286.13 | +63.87 | 106.13 | −63.87 |
| Mercury | 281.01 | +61.41 | 101.01 | −61.41 |
| Venus | 272.76 | +67.16 | 92.76 | −67.16 |
| Earth | — | +90.00 | — | −90.00 |
| Moon | 269.99 | +66.54 | 89.99 | −66.54 |
| Mars | 317.68 | +52.89 | 137.68 | −52.89 |
| Jupiter | 268.06 | +64.50 | 88.05 | −64.50 |
| Saturn | 40.60 | +83.54 | 220.60 | −83.54 |
| Uranus | 257.31 | −15.18 | 77.31 | +15.18 |
| Neptune | 299.36 | +43.46 | 119.36 | −43.46 |
| | Positive pole | | Negative pole | |||
| Pluto | 132.99 | −6.16 | 312.99 | +6.16 |
Some bodies in the solar system, including Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
's moon
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
Hyperion
Hyperion (moon)
Hyperion , also known as Saturn VII, is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. It is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance...
and the asteroid 4179 Toutatis
4179 Toutatis
4179 Toutatis/1989 AC is an Apollo, Alinda, and Mars-crosser asteroid with a chaotic orbit produced by a 3:1 resonance with the planet Jupiter, a 1:4 resonance with the planet Earth, and frequent close approaches to the terrestrial planets...
, lack a stable geographic north pole. They rotate chaotically
Chaos theory
Chaos theory is a field of study in mathematics, with applications in several disciplines including physics, economics, biology, and philosophy. Chaos theory studies the behavior of dynamical systems that are highly sensitive to initial conditions, an effect which is popularly referred to as the...
because of their irregular shape and gravitational influences from nearby planets and moons, and as a result the instantaneous pole wanders over their surface, and may momentarily vanish altogether (when the object comes to a standstill with respect to the distant stars).
Magnetic poles
Planetary magnetic poles are defined analogously to the Earth's magnetic polesMagnetic pole
Magnetic pole may refer to:* One of the two ends of a magnet* The magnetic poles of astronomical bodies, a special case of magnets, two special cases of which are the Geomagnetic poles:...
: they are the locations on the planet's surface at which the planet's magnetic field lines are vertical. The direction of the field determines whether the pole is a magnetic north or south pole, exactly as on Earth. The Earth's magnetic axis is approximately aligned with its rotational axis, meaning that the magnetic poles are relatively close to the geographic poles. However, this is not necessarily the case for other planets; the magnetic axis of Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
, for example, is inclined by as much as 60°.
Near, far, leading and trailing poles
In the particular (but frequent) case of synchronousSynchronous rotation
In astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...
satellites, four more poles can be defined. They are the near, far, leading, and trailing poles. Take Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....
for example; this moon of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
rotates synchronously, so its orientation with respect to Jupiter stays constant. There will be a single, unmoving point of its surface where Jupiter is at the zenith
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...
, exactly overhead — this is the near pole, also called the sub- or pro-Jovian point. At the antipode
Antipode
Antipode, Antipodes, or Antipodeans may refer to:* Antipodal point, the diametrically opposite point on a sphere* Antipodes Water Company, a premium bottled water brand...
of this point is the far pole, where Jupiter lies at the nadir
Nadir
The nadir is the direction pointing directly below a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface there. Since the concept of being below is itself somewhat vague, scientists define the nadir in more rigorous...
; it is also called the anti-Jovian point. There will also be a single unmoving point which is furthest along Io's orbit (best defined as the point most removed from the plane formed by the north-south and near-far axes, on the leading side) —this is the leading pole. At its antipode lies the trailing pole. Io can thus be divided into north and south hemispheres, into pro- and anti-Jovian hemispheres, and into leading and trailing hemispheres. Note that these poles are mean poles because the points are not, strictly speaking, unmoving: there is continuous libration
Libration
In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.-Lunar libration:...
about the mean orientation, because Io's orbit is slightly eccentric and the gravity of the other moons disturbs it regularly.
These poles also apply to planets that are rotating synchronously with their primary stars, as is likely the case with many hot Jupiter
Hot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...
s and as was once thought to be the case with Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
. Other synchronously-rotating objects, such as Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
and some asteroids with large asteroid moon
Asteroid moon
A minor planet moon is an astronomical body that orbits a minor planet as its natural satellite. It is thought that many asteroids and Kuiper belt objects may possess moons, in some cases quite substantial in size...
s, can also be described as having "near" and "far" poles - though "leading" and "trailing" may not be as significant in these cases.