﻿ Axial tilt#Obliquity of the ecliptic .28Earth.27s axial tilt.29

# Axial tilt#Obliquity of the ecliptic .28Earth.27s axial tilt.29

The earth's axis is the axis of rotation of the earth. It runs through the center of mass of the Earth, the so-called Geozentrum. As part of the equatorial coordinate system they are called sometimes celestial axis.

• Their intersections with the surface lay the geographic north and south poles fixed. The magnetic poles, however, not directly related to the position of Earth's axis.
• Their intersections with the ( fictional) celestial sphere form the northern and southern celestial pole. The north celestial pole currently falls almost together with the Polarstern.
• The Earth's axis is defined as normal at the center of the earth a layer, it is called equatorial plane.

As a result, the axis of inclination of approximately 66.56 ° relative to the Ekliptikalebene ( the mean plane of the path of the earth ), - the complementary angle to the lines ( also referred to as Erdneigung ) 23.44 ° between the equatorial and ecliptic - there are the four seasons.

## Precession

The axis of rotation of the earth remains parallel to itself deviations thereof are described in the following on their orbit around the sun during a year in a good approximation.

Because the earth deviates from the spherical shape ( Earth flattening 1:298 or 21 miles), it is subject to tipping forces of the moon and sun. As a result, the earth's axis shifted long - and short- periodic with respect to the fundamental system ( sky -fixed coordinate system ), which is called precession and nutation.

The precession causes in 25,800 years, a revolution of the Earth's axis on a cone around the pole of the ecliptic. The cone has a half angle of 23.44 ° ( obliquity ), the slow cycle of precession is also called Platonic year. The astronomers and surveyors take account of this cycle in the form of a change of the vernal equinox, which travels slowly through the constellations of the zodiac. Since ancient times it has shifted from Aries into Pisces, and in 13,000 years is not the pole star above the north pole stand, but the bright star Vega.

However, the precession cone is not smooth, but slightly waved, because of precession nutation superimposed. It is caused by the obliquity of the Moon's orbit, which varies in 18.5 years between 18 ° and 29 °. Thus, the Lunisolar precession changes periodically, and the 23 ° cone " bends " by about ± 20 ". In addition, there influences of the planets and small, rapidly varying effects come with periods between 1 month and 5 days.

## Shift in the earth system

Compared to a connected to the earth coordinate system in the North and South Poles have Invariant coordinates, the true Earth's axis also takes periodic motions. However, they are relatively small, are called and monitored by international services Polbewegungen. They are mainly caused by mass displacements inside the earth, in the oceans and in the atmosphere and be up to 0.25 seconds of arc, which corresponds to the earth's surface about 9 meters. The two largest signal components periods of 435 days ( Chandler oscillation ) and 365 days (annual oscillation) with amplitudes of about 6.3 and 3.2 meters. This variation is superimposed by smaller oscillations with periods of a few hours to a few decades. Spontaneous displacements with amplitudes of a few centimeters have been triggered for example by the earthquake, which took place on 26 December 2004 in the Indian Ocean off the Indonesian island of Sumatra. The earthquake in Chile in 2010 may have had a shift in the Earth's axis by about 8 centimeters a result, the Tōhoku Earthquake of 2011, perhaps by as much as 10 cm.

Also, throughout the earth, the earth's axis (continuous) has shifted: during the Ordovician, this shifted, for example as follows:

Changes in the Earth's rotation axis influence - as well as those of the ecliptic angle or shape changes of the Earth's orbit (eccentricity ) around the sun - the absorbed solar radiation from the earth and their geographical distribution. This influences are possible on the course of the seasons and on the global climate, which can contribute to the initiation or strengthening of hot or ice ages (see Milankovitch cycles).

The exact knowledge of the position of the Earth's rotation axis is indispensable Besides Astronomy for navigation with satellite and for space travel. If the current pole position is not taken into account, positioning error would be of about ten meters, the result that would, for example, to a multiple zoom during space flights it. The observation of the Earth's rotation and the provision of the so-called Earth rotation parameters is a central task of geodesy and geophysics also allows conclusions on the interior of the Earth.

311104
de