Poles of astronomical bodies
The poles of astronomical bodies are determined based on their
.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.[1] 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,[2] 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.[1] 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
Object | North pole | South pole | ||||
---|---|---|---|---|---|---|
RA | Dec. | Constellation [3] | 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
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
See also
References
- ^ DTIC ADA538254.
- .
- ^ Moews, David (2008); Finding the constellation which contains given sky coordinates