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Every six years, Earth spins slightly faster and then slower

Cyclic changes in day length overlie decades-long trends

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The world turns slightly faster and slower on a regular 5.9-year cycle, a new study suggests. Researchers also found small speed changes that happen at the same time as sudden alterations in Earth’s magnetic field.

The world’s rotation speed can change slightly, by up to milliseconds per day, because of shifts in winds or the movement of fluid in Earth’s interior. Scientists can measure how fast the Earth spins by observing distant objects in space and timing how long they take to come back into view — that is one day length.

The new study, published in the July 11 Nature, found trends in day length after subtracting the effect of weather, allowing researchers to home in on the effect of Earth’s fluid core.

Scientists have previously found hints of six-year oscillations in day length, which occur at the same time as larger, slower changes. But the new analysis revealed that the cycle is remarkably regular, with the maximum change in day length occurring once every 5.9 years. Using decades’ worth of data, the researchers found that the oscillations maintained this precise timing and strength for half a century. “That’s got to be saying something important,” says geophysicist Bruce Buffett of the University of California, Berkeley, who was not involved in the study. It’s too early to say exactly what causes the oscillations, he adds.

This regularity undercuts one hypothesis for the cause of the cycles: fluctuations in the sun’s energy, which are more variable, says study author Richard Holme of the University of Liverpool in England. Instead, the cycle must be caused by something inside the Earth.

Holme’s team also detected sudden, tiny increases and decreases in the Earth’s rotation speed that coincided with abrupt changes in the behavior of Earth’s magnetic field, known as geomagnetic “jerks.” The new day-length data could help scientists understand what causes the mysterious jerks, Buffett says.

Along with hinting at what’s going on in Earth’s core, the research may help improve geomagnetic forecasts, which are crucial in mining exploration and drilling.

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