Scientists in Germany have used a highly sensitive underground ring laser to track Earth’s axial wobble without relying on telescopes, satellites, or external reference signals.
The researchers from the Technical University of Munich (TUM) and the University of Bonn successfully recorded the planet’s subtle rotational fluctuations using the custom-built ring laser, which until now was only possible through complex radio astronomy.
Housed at the Geodetic Observatory in Wettzell, Bavaria, the unique instrument delivered a level of precision 100 times greater than that of any previous ring laser or gyroscope.
“We have made great progress in measuring the Earth,” Ulrich Schreiber, PhD, a professor at the department of physics and astronomy at TUM, and lead author of the study, said. “What our ring laser can do is unique worldwide.”
The team’s findings are reportedly the result of a 250-day continuous experiment in which the laser measured complex motions of the planet’s axis. These also included precession and nutation, phenomena usually observed only through global networks of large radio telescopes.
Wobbling through space
The Earth’s axis, which is an imaginary line running through the North and South Poles, is in constant motion, even though it is commonly depicted as a fixed line. To break it down, as the Earth rotates on its axis it gradually drifts and wobbles.
These tiny shifts are caused by several overlapping factors, including gravitational pulls from the Moon and Sun, as well as the planet’s slightly flattened shape at the equator.
One of the most prominent ones, precession, is a slow circular movement of the Earth’s axis that takes approximately 26,000 years to complete. At the moment, the axis points almost directly at the North Star, but it will eventually return to its starting point in the future.
Superimposed on this slow drift are smaller and more frequent oscillations known as nutations. One dominant nutation cycle lasts about 18.6 years. Still, there are many shorter ones with weekly or even daily fluctuations. As a result, the Earth’s axis doesn’t wobble evenly but with varying degrees of intensity.
Until now, tracking these fluctuations required a network of Very Long Baseline Interferometry (VLBI) stations located on multiple continents. These observatories triangulate cosmic radio signals to determine Earth’s orientation in space. Still, the process is complex, slow and expensive.
From days to hours
But unlike VLBI, which often takes days or even weeks to process data, the new ring laser was able to capture the axial shifts in near real time and deliver high-resolution measurements with updates every hour or less.
The ring laser measured all these effects directly and continuously over 250 days, delivering a level of accuracy previously unheard of for inertial sensors that operate independently of external signals.
The team believes that if they can make the ring laser 10 times more accurate and stable, it could lead to even more precise measurements in the future, and even detect spacetime distortions caused by Earth’s rotation.
This would be a direct test of Einstein’s theory of relativity. It would also make it possible to observe the Lense-Thirring effect, also known as the ‘dragging’ of space, directly from the Earth’s surface.
“We are 100 times more accurate than previously possible with gyroscopes or other ring lasers,” Schreiber said in a press release. “The precise measurement of the fluctuations helps us better understand and model the Earth system with high accuracy.”
The study has been published in the journal Science Advances.