by Staff Writers, Space Daily
Munich, Germany (SPX) Dec 28, 2011

http://www.spacedaily.com/reports/First_ever_direct_measurement_of_the_Earths_rotation_999.html

The Earth wobbles. Like a spinning top touched in mid-spin, its rotational axis fluctuates in relation to space. This is partly caused by gravitation from the sun and the moon. At the same time, the Earth's rotational axis constantly changes relative to the Earth's surface. On the one hand, this is caused by variation in atmospheric pressure, ocean loading and wind.

These elements combine in an effect known as the Chandler wobble to create polar motion. Named after the scientist who discovered it, this phenomenon has a period of around 435 days. On the other hand, an event known as the "annual wobble" causes the rotational axis to move over a period of a year.

This is due to the Earth's elliptical orbit around the sun. These two effects cause the Earth's axis to migrate irregularly along a circular path with a radius of up to six meters.

Capturing these movements is crucial to create a reliable coordinate system that can feed navigation systems or project trajectory paths in space travel.