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This article is about geographic, magnetic and other poles on planets and other astronomical bodies. For the Earth's poles, see North Pole, South Pole, North Magnetic Pole, and South Magnetic Pole. See also Pole of inaccessibility.

The poles of astronomical bodies are determined based on their axis of rotation in relation to the celestial poles of the celestial sphere.

Contents

Geographic poles

The International Astronomical Union defines the geographic north pole of a planet or other object in the solar system as the planetary pole that is in the same ecliptic hemisphere as the Earth's North Pole. More accurately, "The north pole is that pole of rotation that lies on the north side of the invariable plane of the solar system".[1] This definition means that an object's axial tilt is always 90° or less, but its rotation period 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 the Earth. 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.

Another common definition uses the right-hand rule to define an object's north pole: it is then the pole around which the object rotates counterclockwise.[2] With this definition, axial tilts may be greater than 90° but rotation periods are always positive.

The projection of a planet's geographic north pole onto the celestial sphere gives its north celestial 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).

Magnetic poles

Planetary magnetic poles are defined analogously to the Earth's 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 orientated in approximately the same direction as its rotational axis, meaning that the magnetic poles are reasonably 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. 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 Jupiters and as was once thought to be the case with Mercury. Other synchronously-rotating objects, such Pluto or 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.

References

  1. ^ Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2000
  2. ^ Planetary Fact Sheet, NASA

See also

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