The axis of the earth is the axis of rotation of the earth . It runs through the center of mass of the earth's body, the geocenter . In the context of the equatorial coordinate system , it is sometimes called the celestial axis.
- Their intersections with the earth's surface define the geographic north and south poles. The magnetic poles , on the other hand, are not directly related to the position of the earth's axis.
- Their intersections with the (fictional) celestial sphere form the north and south celestial poles . The north celestial pole currently almost coincides with the North Star .
- The earth axis defines a plane as the normal in the center of the earth , this is called the equatorial plane .
As a result of the axial inclination of 66.56 ° in relation to the ecliptical plane (the earth's mean orbit plane ) - this is the complementary angle to the earth's inclination of 23.44 ° between the equator and ecliptic - there are the seasons .
The earth's axis of rotation remains parallel to itself to a good approximation on its orbit around the sun for a year. Deviations from this are described below.
Because the earth deviates from the spherical shape ( earth flattening 1: 298 or 21 kilometers), it is subject to tilting forces from the moon and sun. As a result, the earth's axis shifts in long and short periods compared to the fundamental system (sky-fixed coordinate system), which is reflected in precession and nutation .
In 25,800 years, the precession causes the earth's axis to revolve on a cone around the pole of the ecliptic . The cone has half an opening angle of 23.44 ° ( ecliptic obliquity ), the slow cycle of precession is also called the Platonic year . Astronomers and geodesists take this cycle into account in the form of a change in the vernal equinox , which slowly moves through the constellations of the zodiac . Since ancient times, it has shifted from Aries into Pisces, and 13,000 years is not Polarstern over the North Pole are but the bright star Vega .
However, the precession cone is not smooth, but rather wavy, because the precession is superimposed on the nutation. This is caused by the inclination of the lunar orbit (about 5 ° to the ecliptic, the intersection points are called lunar nodes ), which makes one lunisolar precession orbit in 18.5 years . The declination amplitude of the moon varies between 18 ° and 29 °. As a result, the lunar node of the 23 ° cone "bends" by around ± 20 ″ during one orbit . In addition, there are influences of the planets and small, rapidly changing effects with periods between 1 month and 5 days.
Shift in the earth's body
Compared to a coordinate system connected to the earth , in which the north and south poles have unchangeable coordinates, the true earth axis also makes periodic movements. But they are relatively small, are called pole movements and are monitored by international services. They are mainly caused by mass displacements in the interior of the earth, in the oceans and in the atmosphere and amount to up to 0.25 arc seconds, which corresponds to about 9 meters on the earth's surface. The two largest signal components have periods of 435 days ( Chandler oscillation) and 365 days (annual oscillation) with amplitudes of approximately 6.3 and 3.2 meters, respectively. These variations are superimposed by smaller oscillations with periods from a few hours to decades. Spontaneous displacements with amplitudes of a few centimeters were triggered, for example, by the seaquake that occurred on December 26, 2004 in the Indian Ocean off the Indonesian island of Sumatra . The earthquake in Chile in 2010 possibly shifted the earth's axis by approx. 8 centimeters, the Tōhoku earthquake of 2011 perhaps even by 10 cm.
The earth's axis has also shifted (continuously) in the course of the earth's history.
Changes in the Earth's axis of rotation - just like those in the obliqueness of the ecliptic or changes in shape ( eccentricity ) of the earth's orbit around the sun - affect the solar radiation absorbed by the earth and its geographical distribution. This means that influences on the course of the seasons and the global climate are possible, which can contribute to the triggering or intensification of warm or ice ages (see Milanković cycles ).
The exact knowledge of the position of the earth's axis of rotation is indispensable for astronomy as well as for navigation with satellites and for space travel . If the current pole position were not taken into account, positioning errors of over ten meters would result. B. would increase in space flights many times that. The observation of the earth's rotation and the provision of the so-called earth rotation parameters are central tasks of geodesy and also allow geophysical conclusions to be drawn about the interior of the earth.