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The Earth spins around once every 24 hours on its axis, creating the continuous cycle of day and night. But this rotation isn’t as straight forward as it sounds. Scientists working with lasers and mirrors are refining a new system to track the Earth‘s rotation and its kinks. The pull of gravity from the sun and the moon contribute to the planet‘s wobble. So do variations in atmospheric pressure, ocean loading and the wind, which change the position of the Earth‘s axis relative to the surface. Together their effect is called the Chandler wobble, and it has a period of 435 days. Another force causes the rotational axis to move over a period of a year. This annual wobble is due to the Earth‘s elliptical orbit around the sun.
Between these two effects, the Earth‘s axis migrates irregularly along a circular path with a radius of up to 20 feet (6 meters). Pinning down the overall wobble of the planet‘s rotation is key to keeping certain tracking systems accurate. In the mid-1990s, scientists of Germany’s Technische Universitaet Muenchen and Federal Agency for Cartography joined forces with researchers at New Zealand’s University of Canterbury to develop a simpler method for tracking the Chandler wobble and annual wobble. The scientists had the idea of building a ring laser similar to ones used in aircraft guidance systems, only millions of times more exact. Yet at the end of the 1990s, work on the world’s most stable ring laser got under way at Wettzell Geodetic Observatory, in the Bavarian Forest of southeast Germany. The installation includes two counter-rotating laser beams that travel around a square path with mirrors in the corners, which form a closed beam path. With some tweaks to the system, the researchers have succeeded in corroborating the Chandler and annual wobble measurements made from the radio telescopes. They now aim to make the apparatus even more accurate, enabling them to determine changes in the Earth‘s rotational axis over a single day. The scientists also plan to make the ring laser capable of running continuously for a period of years.