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First ever direct measurement of the Earth’s rotation

Date: 2011-12-27

Geodesists are pinpointing the orientation of the Earth’s axis using the world’s most stable ring laser

A group with researchers at the Technical University of Munich (TUM) and the Federal Agency for Cartography and Geodesy (BKG) are the first to plot changes in the Earth’s axis through laboratory measurements. To do this, they constructed the world’s most stable ring laser in an underground lab and used it to determine changes in the Earth’s rotation. Previously, scientists were only able to track shifts in the polar axis indirectly by monitoring fixed objects in space. Capturing the tilt of the Earth’s axis and its rotational velocity is crucial for precise positional information on Earth – and thus for the accurate functioning of modern navigation systems, for instance. The scientists’ work has been recognized an Exceptional Research Spotlight by the American Physical Society.

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. “Locating a point to the exact centimeter for global positioning is an extremely dynamic process – after all, at our latitude, we are moving at around 350 meters to the east per second,” explains Prof. Karl Ulrich Schreiber, meanwhile as station director of the geodetic observatory Wettzell where the ring laser is settled. Karl Ulrich Schreiber had directed the project in TUM’s Research Section Satellite Geodesy. The geodetic observatory Wettzell is run together by TUM and BKG.

The researchers have succeeded in corroborating the Chandler and annual wobble measurements based on the data captured by radio telescopes. They now aim to make the apparatus more accurate, enabling them to determine changes in the Earth’s rotational axis over a single day. The scientists also plan to make the ring laser capable of continuous operation so that it can run for a period of years without any deviations. “In simple terms,” concludes Schreiber, “in future, we want to be able to just pop down into the basement and find out how fast the Earth is accurately turning right now."

Picture shwing the Zerodur glass apparatus

For more information please visit the TU München homepage http://portal.mytum.de/pressestelle/pressemitteilungen/NewsArticle_20111220_100621/newsarticle_view?.

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