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The poles of astronomical bodies are determined based on their axis of rotation in relation to the celestial poles of the celestial sphere. Astronomical bodies include stars, planets, dwarf planets and small Solar System bodies such as comets and minor planets (e.g., asteroids), as well as natural satellites and minor-planet moons.
Poles of rotation
The International Astronomical Union (IAU) defines the 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 is independent of the object's direction of rotation about its axis. This implies that an object's direction of rotation, when viewed from above its north pole, may be either clockwise or counterclockwise. The direction of rotation exhibited by most objects in the solar system (including Sun and Earth) is counterclockwise. Venus rotates clockwise, 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's) 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, the poles are called "positive" and "negative." The positive pole is the pole toward which the thumb points when the fingers of the right hand are curled in its direction of rotation. The negative pole is the pole toward which the thumb points when the fingers of the left hand are curled in its direction of rotation. 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 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 relative 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. The Moon's poles are particularly mobile.
| Object | North pole | South pole | ||||
|---|---|---|---|---|---|---|
| RA | Dec. | Constellation | RA | Dec. | Constellation | |
| Sun | 286.13 | +63.87 | Draco | 106.13 | −63.87 | Carina |
| Mercury | 281.01 | +61.41 | Draco | 101.01 | −61.41 | Pictor |
| Venus | 272.76 | +67.16 | Draco | 92.76 | −67.16 | Dorado |
| Earth | — | +90.00 | Ursa Minor | — | −90.00 | Octans |
| Moon | 266.86 | +65.64 | Draco | 86.86 | −65.64 | Dorado |
| Mars | 317.68 | +52.89 | Cygnus | 137.68 | −52.89 | Vela |
| Jupiter | 268.06 | +64.50 | Draco | 88.06 | −64.50 | Dorado |
| Saturn | 40.59 | +83.54 | Cepheus | 220.59 | −83.54 | Octans |
| Uranus | 257.31 | −15.18 | Ophiuchus | 77.31 | +15.18 | Orion |
| Neptune | 299.33 | +42.95 | Cygnus | 119.33 | −42.95 | Puppis |
| Positive pole | Negative pole | |||||
| Pluto | 132.99 | −6.16 | Hydra | 312.99 | +6.16 | Delphinus |
Some bodies in the Solar System, including Saturn's moon Hyperion and the asteroid 4179 Toutatis, lack a stable 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).
Magnetic poles
Planetary magnetic poles are defined analogously to the Earth's North and South magnetic 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 geomagnetic 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°.
Orbital pole
In addition to the rotational pole, a planet's orbit also has a defined direction in space. The direction of the angular momentum vector of that orbit can be defined as an orbital pole. Earth's orbital pole, i.e. the ecliptic pole, points in the direction of the constellation Draco.
Near, far, leading and trailing poles
In the particular (but frequent) case of synchronous satellites, four more poles can be defined. They are the near, far, leading, and trailing poles. For example, Io, one of the moons 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 farthest 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. 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.
See also
- Galactic coordinate system
- Planetary coordinate system
References
- Archinal, B. A.; A’Hearn, M. F.; Bowell, E.; Conrad, A.; Consolmagno, G. J.; Courtin, R.; Fukushima, T.; Hestroffer, D.; Hilton, J. L.; Krasinsky, G. A.; Neumann, G.; Oberst, J.; Seidelmann, P. K.; Stooke, P.; Tholen, D. J.; Thomas, P. C.; Williams, I. P. (February 2011). "Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009". Celestial Mechanics and Dynamical Astronomy. 109 (2): 101–135. Bibcode:2011CeMDA.109..101A. doi:10.1007/s10569-010-9320-4. S2CID 189842666. DTIC ADA538254.
- Laskar, J. (1 June 1988). "Secular evolution of the solar system over 10 million years". Astronomy and Astrophysics. 198 (1–2): 341–362. Bibcode:1988A&A...198..341L. INIST 7705622.
- Moews, David (2008); Finding the constellation which contains given sky coordinates
This article needs additional citations for verification Please help improve this article by adding citations to reliable sources Unsourced material may be challenged and removed Find sources Poles of astronomical bodies news newspapers books scholar JSTOR June 2024 Learn how and when to remove this message The poles of astronomical bodies are determined based on their axis of rotation in relation to the celestial poles of the celestial sphere Astronomical bodies include stars planets dwarf planets and small Solar System bodies such as comets and minor planets e g asteroids as well as natural satellites and minor planet moons Poles of rotationThe International Astronomical Union IAU defines the 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 is independent of the object s direction of rotation about its axis This implies that an object s direction of rotation when viewed from above its north pole may be either clockwise or counterclockwise The direction of rotation exhibited by most objects in the solar system including Sun and Earth is counterclockwise Venus rotates clockwise 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 s 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 the poles are called positive and negative The positive pole is the pole toward which the thumb points when the fingers of the right hand are curled in its direction of rotation The negative pole is the pole toward which the thumb points when the fingers of the left hand are curled in its direction of rotation 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 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 relative 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 The Moon s poles are particularly mobile Object North pole South poleRA Dec Constellation RA Dec ConstellationSun 286 13 63 87 Draco 106 13 63 87 CarinaMercury 281 01 61 41 Draco 101 01 61 41 PictorVenus 272 76 67 16 Draco 92 76 67 16 DoradoEarth 90 00 Ursa Minor 90 00 OctansMoon 266 86 65 64 Draco 86 86 65 64 DoradoMars 317 68 52 89 Cygnus 137 68 52 89 VelaJupiter 268 06 64 50 Draco 88 06 64 50 DoradoSaturn 40 59 83 54 Cepheus 220 59 83 54 OctansUranus 257 31 15 18 Ophiuchus 77 31 15 18 OrionNeptune 299 33 42 95 Cygnus 119 33 42 95 PuppisPositive pole Negative polePluto 132 99 6 16 Hydra 312 99 6 16 Delphinus Some bodies in the Solar System including Saturn s moon Hyperion and the asteroid 4179 Toutatis lack a stable 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 Magnetic polesPlanetary magnetic poles are defined analogously to the Earth s North and South magnetic 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 geomagnetic 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 Orbital poleIn addition to the rotational pole a planet s orbit also has a defined direction in space The direction of the angular momentum vector of that orbit can be defined as an orbital pole Earth s orbital pole i e the ecliptic pole points in the direction of the constellation Draco Near far leading and trailing polesIn the particular but frequent case of synchronous satellites four more poles can be defined They are the near far leading and trailing poles For example Io one of the moons 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 farthest 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 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 See alsoGalactic coordinate system Planetary coordinate systemReferencesArchinal B A A Hearn M F Bowell E Conrad A Consolmagno G J Courtin R Fukushima T Hestroffer D Hilton J L Krasinsky G A Neumann G Oberst J Seidelmann P K Stooke P Tholen D J Thomas P C Williams I P February 2011 Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements 2009 Celestial Mechanics and Dynamical Astronomy 109 2 101 135 Bibcode 2011CeMDA 109 101A doi 10 1007 s10569 010 9320 4 S2CID 189842666 DTIC ADA538254 Laskar J 1 June 1988 Secular evolution of the solar system over 10 million years Astronomy and Astrophysics 198 1 2 341 362 Bibcode 1988A amp A 198 341L INIST 7705622 Moews David 2008 Finding the constellation which contains given sky coordinates Portals AstronomyStarsSpaceflightOuter spaceSolar System
