Key Takeaways:
- A 250-day experiment using a ring laser achieved unprecedented accuracy in measuring Earth's axial wobble (nutation and precession), surpassing previous gyroscope and ring laser technologies by a factor of 100.
- The ring laser directly and continuously measured various influences on Earth's axis, including its imperfect shape, solar and lunar gravitational forces, with sub-hourly temporal resolution and immediate data availability.
- Unlike previous methods relying on networks of radio telescopes, this single, relatively small, underground ring laser instrument independently performed these measurements.
- Future improvements in accuracy (a factor of 10 increase) could enable direct measurement of spacetime distortion caused by Earth's rotation, offering a direct test of Einstein's theory of relativity, including the Lense-Thirring effect.
Recently, the results of a 250-day experiment to refine a particular motion of our planet were published in the journal Science Advances. Lead author K. Ulrich Schreiber from the TUM Institute of Engineering for Astronomical and Physical Geodesy in Ottobrunn, Germany, said in a statement, “We have made great progress in measuring the 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.”
Yes, Earth wobbles
Earth’s axis — the imaginary line that stretches from the North Pole to the South Pole — is not firmly anchored in the sky, as you might think by looking at a globe. Various forces act on it, causing it to wobble. The strongest influence is Earth’s imperfectly shape: it bulges slightly at the equator compared to the poles. Also, the effect known as precession causes the extension of Earth’s axis to trace a circle in the sky. Currently, the northern extension points quite close to Polaris, the North Star. But where it points changes slowly during a cycle of approximately 26,000 years. So, in the future it will be aligned with other stars before returning to its current location.
But the gravitational forces exerted by the Sun and the 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 the Earth’s axis. There is a long-term nutation with a period of 18.6 years, but also many smaller ones with weekly or even 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 on different continents. The ring laser can do all this on its own as 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 other methods.
The team added that 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. This would give scientists a direct test of Einstein’s theory of relativity. This would allow, for example, the Lense-Thirring effect (the “dragging” of space by Earth’s rotation) to be tested directly at Earth’s surface.
