Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

First ever direct measurement of the Earth's rotation

23.12.2011
Pinpointing the orientation of the Earth's axis using the world's most stable ring laser

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 who directed the project in TUM's Research Section Satellite Geodesy. The orientation of the Earth's axis relative to space and its rotational velocity are currently established in a complicated process that involves 30 radio telescopes around the globe. Every Monday and Thursday, eight to twelve of these telescopes alternately measure the direction between Earth and specific quasars. Scientists assume that these galaxy nuclei never change their position and can therefore be used as reference points. The geodetic observatory Wettzell, which is run by TUM and Germany's Federal Agency for Cartography (BKG), is also part of this process.

In the mid-1990s, scientists of TUM and BKG joined forces with researchers at New Zealand's University of Canterbury to develop a simpler method that would be capable of continuously tracking the Chandler wobble and annual wobble. "We also wanted to develop an alternative that would enable us to eliminate any systematic errors," continues Schreiber. "After all, there was always a possibility that the reference points in space were not actually stationary." The scientists had the idea of building a ring laser similar to ones used in aircraft guidance systems – only millions of times more exact. "At the time, we were almost laughed off. Hardly anyone thought that our project was feasible," says Schreiber.

Yet at the end of the 1990s, work on the world's most stable ring laser got underway at the Wettzell observatory. The installation comprises two counter-rotating laser beams that travel around a square path with mirrors in the corners, which form a closed beam path (hence the name ring laser). When the assembly rotates, the co-rotating light has farther to travel than the counter-rotating light. The beams adjust their wavelengths, causing the optical frequency to change. The scientists can use this difference to calculate the rotational velocity the instrumentation experiences. In Wettzell, it is the Earth that rotates, not the ring laser. To ensure that only the Earth's rotation influences the laser beams, the four-by-four-meter assembly is anchored in a solid concrete pillar, which extends six meters down into the solid rock of the Earth's crust.

The Earth's rotation affects light in different ways, depending on the laser's location. "If we were at one of the poles, the Earth and the laser's rotational axes would be in complete synch and their rotational velocity would map 1:1," details Schreiber. "At the equator, however, the light beam wouldn't even notice that the Earth is turning." The scientists therefore have to factor in the position of the Wettzell laser at the 49th degree of latitude. Any change in the Earth's rotational axis is reflected in the indicators for rotational velocity. The light's behavior therefore reveals shifts in the Earth's axis.

"The principle is simple," adds Schreiber. "The biggest challenge was ensuring that the laser remains stable enough for us to measure the weak geophysical signal without interference – especially over a period of several months." In other words, the scientists had to eliminate any changes in frequency that do not come from the Earth's rotation. These include environmental factors such as atmospheric pressure and temperature. They relied predominantly on a ceramic glass plate and a pressurized cabin to achieve this. The researchers mounted the ring laser on a nine-ton Zerodur base plate, also using Zerodur for the supporting beams. They chose Zerodur as it is extremely resistant to changes in temperature. The installation is housed in a pressurized cabin, which registers changes in atmospheric pressure and temperature (12 degrees) and automatically compensates for these. The scientists sunk the lab five meters below ground level to keep these kinds of ambient influences to a minimum. It is insulated from above with layers of Styrodur and clay, and topped by a four-meter high mound of Earth. Scientists have to pass through a twenty-meter tunnel with five cold storage doors and a lock to get to the laser.

Under these conditions, 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."

Publication:

Schreiber, K. U.; Klügel, T.; Wells, J.-P. R.; Hurst, R. B.; Gebauer, A.: How to detect the Chandler and the annual wobble of the Earth with a large ring laser gyroscope; Physical Review Letters, Vol. 107, Nr. 17, EID 173904, American Physical Society, ISSN 0031-9007, DOI: 10.1103/PhysRevLett.107.173904, 2011

Exceptional Research Spotlight recognized by the American Physical Society:
http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.107.173904
Contact:
Prof. Karl Ulrich Schreiber
Technical University Munich / Federal Agency for Cartography
Research Section Satellite Geodesy
Phone: 49-9941-603-113
E-Mail: schreiber@fs.wettzell.de
Further information:
http://www.fs.wettzell.de/

Klaus Becker | EurekAlert!
Further information:
http://www.fs.wettzell.de/
http://www.tum.de

More articles from Earth Sciences:

nachricht New research calculates capacity of North American forests to sequester carbon
16.07.2018 | University of California - Santa Cruz

nachricht Scientists discover Earth's youngest banded iron formation in western China
12.07.2018 | University of Alberta

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: First evidence on the source of extragalactic particles

For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.

To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...

Im Focus: Magnetic vortices: Two independent magnetic skyrmion phases discovered in a single material

For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.

Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...

Im Focus: Breaking the bond: To take part or not?

Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.

A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...

Im Focus: New 2D Spectroscopy Methods

Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.

"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....

Im Focus: Chemical reactions in the light of ultrashort X-ray pulses from free-electron lasers

Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.

Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Leading experts in Diabetes, Metabolism and Biomedical Engineering discuss Precision Medicine

13.07.2018 | Event News

Conference on Laser Polishing – LaP: Fine Tuning for Surfaces

12.07.2018 | Event News

11th European Wood-based Panel Symposium 2018: Meeting point for the wood-based materials industry

03.07.2018 | Event News

 
Latest News

NYSCF researchers develop novel bioengineering technique for personalized bone grafts

18.07.2018 | Life Sciences

Machine-learning predicted a superhard and high-energy-density tungsten nitride

18.07.2018 | Materials Sciences

Why might reading make myopic?

18.07.2018 | Health and Medicine

VideoLinks
Science & Research
Overview of more VideoLinks >>>