Poles of astronomical bodies
The International Astronomical Union (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, 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 it 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 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 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.
|Object||North pole||South pole|
|Positive pole||Negative pole|
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).
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 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°.
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.
These poles also apply to planets that are rotating synchronously with their primary stars, as is likely the case with many hot Jupiters and as was once thought to be the case with Mercury. Other synchronously rotating objects, such as Pluto and some asteroids with large asteroid moons, can also be described as having "near" and "far" poles – though "leading" and "trailing" may not be as significant in these cases.
- Report of the IAU Working Group on cartographic coordinates and rotational elements: 2009
- J. Laskar, "Secular evolution of the Solar System over 10 million years", Astronomy and astrophysics 198 (1988) 341–362, p.351.