Seen from the North Pole, the rotation of the earth around its axis from west to east is counterclockwise, and from the South Pole it is clockwise. The earth's rotation axis forms an angle of 66.34 degrees with the ecliptic plane and is perpendicular to the equatorial plane.

The rotation of the earth is an important movement form of the earth. The average rotation angular velocity is 4. 167× 10 negative cubic degree/second, and the rotation linear velocity on the equator of the earth is 465 meters/second. It takes 23 hours and 56 minutes for the earth to rotate once, increasing or decreasing by three thousandths to four thousandths of a second every 10 year.

The position of the earth's rotation axis on the earth is constantly changing, which is called the pole shift. 1765 L Euler proved that if there is no external force, the rotation axis of the rigid earth will swing freely around the shape axis with a period of 305 sidereal day. It was not until 1888 that people confirmed the existence of the pole shift from the observation of latitude changes.

In 189 1, S.C. Zhang Dele of the United States further pointed out that the pole shift includes two main periodic components: one is a free swing with a period of about 14 months, also known as Zhang Dele swing; The other is forced oscillation with a period of 12 months.

The actual observed Chandler swing is the free swing predicted by Euler. But because the earth is not an absolute rigid body, Zhang Dele's swing period is about 40% longer than predicted by Euler. Zhang Dele's swing amplitude changes slowly from 0.06 "to 0.25", and its period changes from 4 10 to 440 days.

Another major component of the pole shift is the annual forced oscillation with an amplitude of about 0.09 ",which is relatively stable, mainly due to the seasonal changes of the atmosphere and polar ice and snow.

Secular polar motion can be obtained by removing the periodic components in the pole shift. The average speed in secular polar motion is about 0.003 "/ year, and the direction is about 70 west longitude.

Baidu Encyclopedia-Earth Rotation