# Synopsis: Earth Wobble Rings True

A laser gyroscope measures extremely low-frequency wobbles in the Earth’s rotation.

The direction of Earth’s axis is not fixed, but instead wobbles by a tiny fraction of a degree. Astronomers track this change by continuously monitoring the position of distant quasars in the sky. But now Ulrich Schreiber of the Technical University of Munich and his colleagues, reporting in Physical Review Letters, have, for the first time, measured the wobble in a lab with a ring laser.

A ring laser is basically a laser cavity that has been bent around into a square or triangle loop, with mirrors at each corner. Laser light will travel around the ring in both directions. However, if the ring is rotating, then light moving in the same direction will have farther to go to complete a loop than light moving in the opposite direction. This travel difference causes a measurable frequency shift between the counterpropagating beams.

Ring laser gyroscopes are commonly used in aircraft, but the systems typically are not stable enough with respect to environmental fluctuations to measure the long-period changes in the Earth’s axis. To address this instability, the authors constructed a $4$-meter by $4$-meter square ring—the “Gross Ring”—out of zerodur, a ceramic glass with very low thermal expansion. Using data from spring 2010, the team extracted the signal of the dominant Chandler wobble, which is a $435$-day free oscillation of the Earth due to pressure fluctuations at the sea floor and wind activities around the Earth. This shows that ring lasers could provide an alternative to costly astronomical methods of studying the Earth’s rotation. – Michael Schirber

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