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As Earth moves through space, it wobbles slightly. A team of researchers from the Technical University of Munich (TUM) and the University of Bonn has now succeeded in measuring these fluctuations in Earth's axis using a completely new method—until now, possible only through complex radio astronomy. The team used the high-precision ring laser at TUM's geodetic observatory in Wettzell, Bavaria.

The results of the 250-day experiment were in Science Advances. Lead author Prof. K. Ulrich Schreiber from the TUM Institute of Engineering for Astronomical and Â鶹ÒùÔºical Geodesy emphasizes, "We have made great progress in measuring Earth. What our ring laser can do is unique worldwide. We are 100 times more accurate than previously possible with gyroscopes or other ring lasers. The precise measurement of the fluctuations helps us better understand and model the Earth system with high accuracy."

The wobbling Earth

In reality, Earth's axis is not firmly anchored in the sky, as it appears on a globe. Various forces act on it, causing it to wobble to varying degrees. The strongest influence is Earth's imperfectly round shape; it bulges slightly at the equator compared to the poles. The effect known as precession causes the extension of Earth's axis to trace a circle in the sky. Currently, it is aligned precisely with the North Star. But in the future, it will be aligned with other stars before returning to the North Star in a cycle of 26,000 years.

But the gravitational forces of the sun and moon, which sometimes reinforce or weaken each other, also pull on Earth's axis. This effect, known as nutation, causes small wave movements in the precession circle of Earth's axis. There is a distinct nutation with a period of 18.6 years, but also many smaller ones with weekly or daily fluctuations. As a result, the axis does not wobble evenly, but with varying degrees of intensity.

Unprecedented precision

The ring laser was able to measure all these effects directly and continuously over 250 days with a level of accuracy previously unheard of for inertial sensors, i.e., sensors that operate independently of external signals. Unlike in the past, this does not require a network of several large radio telescopes (VLBI) on different continents.

The ring laser can do all this on its own in a relatively small instrument located in an underground facility in Wettzell. In addition, the temporal resolution of the fluctuations is less than an hour instead of a day—and the results are available immediately, rather than after days or weeks, as is the case with VLBI.

With a further increase in the measurement accuracy and stability of the ring laser by a factor of 10 in the future, it would even be possible to measure the spacetime distortion caused by Earth's rotation—a direct test of the theory of relativity. This would allow, for example, the Lense-Thirring effect, i.e., the "dragging" of space by Earth's rotation, to be tested directly at Earth's surface.