# Pol (geography)

In geography and astronomy, the pole is the point of intersection between the axis of rotation of a celestial body and its surface.

## Poles of the earth

Pole movement 2001-2005

On earth it is consequently the intersection of the earth's axis with the earth's surface , the geographic north and south pole . These poles are not suitable for determining positions on the earth's surface with the high accuracy in the centimeter range, which is sometimes required by geoscientific applications today. Because they are not fixed, but change a few meters a year due to the polar movement . This movement should not be confused with precession and nutation of the earth's axis. These are shifts in the axis of rotation of the earth's body. During the pole movement, however, the axis of rotation in the earth's body itself changes. A system of Cartesian coordinates that is implemented in the International Terrestrial Reference Frame (ITRF) is used today for high-precision position determination . Essentially, it is a system of high-precision measured points on the earth's surface, with the help of which slow changes such as continental drift , mountain elevations , elastic tidal deformation and even pole movements are compensated.

## Poles of an ellipsoid of revolution

An ellipsoid of revolution (e.g. the WGS 84 ellipsoid) can be related to this system of Cartesian coordinates , which is then the basis for cartographic representations. The poles of this ellipsoid of revolution are to be distinguished from the geographical poles. The north pole lies in the direction of the positive z-axis of the Cartesian coordinate system. In general, the direction of rotation of such an ellipsoid is called right-handed or prograde , if the rotation is counterclockwise when looking at the north pole , as with the earth. Otherwise, the rotation is called retrograde or retrograde .

## Poles of celestial bodies

Pole parameters of a celestial body

For the planets and moons of the solar system, according to a convention of the IAU from 1970, that pole of a celestial body is considered to be the north pole, which lies in the direction of the total angular momentum of the solar system. This roughly corresponds to the pole whose direction points north of the ecliptic (more precisely north of the Laplace plane of the solar system). This means that with some celestial bodies - e.g. B. Venus , Uranus and Pluto - according to the IAU convention, the rotation is retrograde , i.e. clockwise at the North Pole.

This convention was not always taken into account, especially for bodies whose axis of rotation is subject to considerable secular fluctuations such as asteroids and especially comets . In these, the pole with counterclockwise rotation was often referred to as the north pole, regardless of the direction. Accordingly, the Working Group on Cartographic Coordinates and Rotational Elements (WGCCRE), the responsible committee of the IAU, has developed a proposal according to which the poles of dwarf planets , minor planets and comets are determined in such a way that the rotation around the first main axis of inertia takes place in the right direction. To avoid confusion, however, the so-called poles should be referred to as the “positive” (north) or “negative” (south) pole.

The right ascension of the North Pole is indicated with and the declination with . The point of intersection (marked with ) of the equator of the celestial body with the celestial equator of the ICRF , which has a right ascension of + 90 °, is used as the starting point for the position of the prime meridian . The angle between the intersection of the prime meridian and the equator of the celestial body and the point is also specified. ${\ displaystyle \ alpha _ {0}}$${\ displaystyle \ delta _ {0}}$${\ displaystyle Q}$${\ displaystyle \ alpha _ {0}}$${\ displaystyle B}$${\ displaystyle Q}$${\ displaystyle W}$

## literature

• Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009. In: Celestial Mechanics and Dynamical Astronomy , Vol. 109, No. 2, pp. 101-135, doi : 10.1007 / s10569-010-9320-4