Earth's wobble in space measured from the ground with a ring laser

Space

Technological Innovation Website Editorial Team - September 22, 2025

Ring laser at the Wettzell Geodetic Observatory in Germany. [Image: Astrid Eckert/TUM]

Ring laser

The Earth rotates as it moves through space as if it were on a slightly tilted axis from pole to pole. But there isn't actually an axis there, so our planet wobbles slightly. Measuring these wobbles typically requires the use of complex radio astronomy devices.

Now, for the first time, German researchers have managed to measure these fluctuations in the Earth's axis using a completely new method, entirely locally and on the ground, using a ring laser.

"We have made great progress in measuring the Earth. What our ring laser can do is unique in the world. We are 100 times more accurate than was previously possible with gyroscopes or other ring lasers. Precisely measuring fluctuations helps us better understand and model the Earth system with high precision," said Ulrich Schreiber of the Technical University of Munich.

In addition to confirming the measurement capability of the new method, the 250-day experiment provided information that will allow a 10-fold increase in the accuracy and stability of ring laser measurements compared to this proof-of-concept.

With this, it will even be possible to measure the distortion of spacetime caused by the Earth's rotation, a direct test of the theory of relativity. This will allow, for example, testing the Lense-Thirring effect—the "dragging" of space by the Earth's rotation—and doing so directly from the Earth's surface, without the need for space technology.

Effect of Earth's precession and nutation on ring laser observations. [Image: K. Ulrich Schreiber et al. - 10.1126/sciadv.adx6634]

Precession and nutation

The Earth's axis is subject to various forces, causing it to wobble to varying degrees. The strongest influence is the Earth's imperfectly round shape, known as the geoid —our planet protrudes slightly at the equator compared to the poles.

The effect, known as precession , causes the length of Earth's axis to trace a circle in the sky. Currently, it is precisely aligned with the North Star. But in the future, it will align with other stars before returning to the North Star, completing a 26,000-year cycle.

But the gravitational forces of the Sun and Moon, which sometimes reinforce and sometimes weaken each other, also exert a pull on the Earth's axis. This effect, known as nutation, causes small wavelike movements in the Earth's axis' precession circle. 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 Earth's axis does not wobble uniformly, but at varying degrees of intensity.

The ring laser was able to measure all of these effects directly and continuously over 250 days with a level of precision never before seen for inertial sensors—sensors that operate independently of external signals. Unlike in the past, this does not require a network of multiple very large radio telescopes (VLBI) on different continents.

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

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