Stutters in Earth's spin change day length

THREE times in the last decade Earth's spin has missed a beat. These seemingly random blips cause days to temporarily stretch and shrink. They have emerged from the clearest ever view of how long a day is.

Earth's spin fluctuates as the oceans and the atmosphere push and tug on the planet's spin But these small daily variations hide longer-term patterns, some well known, some not.

Richard Holme of the University of Liverpool, UK, looked at 50 years of GPS and astronomical data to see how day length varied during that time. The analysis threw up a well-known cycle due to slow changes at the Earth's core, which lengthen days by a few milliseconds over roughly a decade, then shrink them down again.

There's also a 5.9-year cycle, due to a persistent wobble between the fluid outer core and surrounding mantle, which changes day length by fractions of milliseconds a year.

When Holme stripped away both of these regular cycles, sudden unexpected jumps in day length emerged from the calculations. Three times in recent years ? in 2003, 2004, and 2007 ? our planet's spin has stuttered. The jumps interrupt the longer-term changes by a fraction of a millisecond, and last several months before going back to normal (Nature, DOI: 10.1038/nature12282).

Satellite readings of the planet's magnetic field over the last 20 years show that the field also undergoes sudden jerks, and Holmes found that they coincide with the jumps in the Earth's spin. He says the sudden changes probably occur when a patch of molten outer core temporarily sticks to the mantle, causing a step change in angular velocity.

Jon Mound of the University of Leeds, UK, says we need to rethink the dynamics of the Earth's core in the light of these findings.

This article appeared in print under the headline "The beat that Earth's heart skipped"

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