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Japanese quake shortened day just a smidgen

It's not as big a change as what the wind and ocean currents cause, but last week's massive quake changed how the Earth spins.

Earth, as seen from space. NASA

Friday's magnitude 9.0 earthquake in Japan was big enough to shorten the length of Earth's day by 1.8 millionths of a second, a NASA scientist has calculated.

Richard Gross of NASA's Jet Propulsion Laboratory projected the change based on calculations of how the distribution of Earth's mass changed. Moving mass toward the north or south poles, and thus closer to Earth's axis of rotation, can make the planet spin faster in much the same way an ice skater can spin faster by bringing arms and legs closer toward the body.

Gross also calculated that Earth's figure axis shifted by about 17 centimeters, or 6.5 inches. The figure axis is the axis about which the Earth's mass is balanced, and moving it changes how Earth wobbles as it rotates. The north-south axis around which Earth rotates isn't changed by internal forces such as earthquakes, and it's about 10 meters, or 33 feet, away from the figure axis.

Gross' figures are preliminary and likely will change as new measurements emerge, NASA said.

Although it takes a big earthquake to make such changes to Earth's behavior--and last week's was the fifth largest since 1900--changing the planet's rotational speed actually happens constantly through less dramatic forces. Effects from the wind and ocean currents lengthen and shorten the length of the day by a full millisecond from one year to the next, an effect 550 times greater than the Japanese earthquake.