Published online 30 December 2004 | Nature | doi:10.1038/news041229-6

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Natural disaster shaved millionths of a second off planet's day.

The Earth has been rocked on its axis. © NASA

The devastating earthquake that struck the Indian Ocean on 26 December was so powerful that it has accelerated the Earth's rotation, geophysicists have declared. They estimate that the shockwave shortened the period of our planet's rotation by some three microseconds.

The change was caused by a shift of mass towards the planet's centre, as the Indian Ocean's heavy tectonic plate lurched underneath Indonesia's one, say researchers at NASA's Jet Propulsion Laboratory in Pasadena, California. This caused the globe to rotate faster, in the same way that a spinning figure-skater accelerates by tucking in her arms.

The blast literally rocked the world on its axis, add Richard Gross and his NASA colleagues. They estimate that Earth now tilts by an extra 2.5 centimetres in the wake of the jolt.

Day rate

The shortening of Earth's day is no cause for consternation, particularly in light of the huge humanitarian crisis sparked by Sunday's events. The death toll from the tsunami that lashed coasts across the Indian Ocean has now passed 100,000.

“I would be stunned if there was any change in the rotation rate that would necessitate addition or subtraction of a leap second.” Tom O'Brian

US National Institute of Standards and Technology

But the change will nonetheless be relevant to physicists charged with keeping the world's official time, which since 1967 has been based on a battery of around 250 highly accurate atomic clocks in 60 labs throughout the world. These labs report to the International Bureau of Weights and Measures near Paris, which sets the Coordinated Universal Time (UTC).

UTC has to be kept as close as possible to the time period of Earth's rotation, which can fluctuate in response to unpredictable events such as large earthquakes. Overall, the Earth's rotation tends to slow down as the Moon's gravity pulls on its seas and continents, causing bulges that give the opposite effect of Sunday's compacting quake.

Leap to it

Because of this trend, physicists have slipped in 22 separate 'leap seconds' since 1972, each one delaying UTC until the Earth's rotation catches up. They are inserted either as an extra final second on 31 December, or at the very end of June. The most recent was at the end of 1998.

The change caused by the Indian Ocean quake, at just a few millionths of a second, is too slight to need correcting, says Tom O'Brian, head of the US National Institute of Standards and Technology's Time and Frequency Division in Boulder, Colorado, which runs an atomic clock.

"I would be stunned if there was any change in the rotation rate that would necessitate addition or subtraction of a leap second," he told news@nature.com.

The latest change runs counter to Earth's general trend of slowing down, O'Brian explains. It has never been necessary to subtract a second from UTC to make up for a speedily spinning Earth, he says.

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This is partly because, when the atomic clock system was adopted in 1967, physicists chose 1900 as the year with the best average data on how fast the Earth spins. This meant that, because of Earth's natural slowing, the atomic clock was already running fast when it was set up.

On average, just over one leap second is needed every year, O'Brian says. That makes the fact that we haven't had one since 1998 something of a surprise. "Since the last leap second the Earth's rotation seems to have been changing at a very slow rate," he says.

US National Institute of Standards and Technology