Gain some, lose some

by Brian Fisher Johnson
Tuesday, October 27, 2015

The magnitude-8.8 earthquake that struck Chile in February increased Earth's rate of rotation by 1.26 millionths of a second. Credit: Both: Thomas Lay/ShelterBox.

Scientists now know that Earth’s rotation is very slowly decelerating, such that the length of the rotational day is about two milliseconds longer than the 86,400 seconds it was nearly two centuries ago. But that doesn’t mean Earth’s rotation is always slowing down. The tidal force of the moon acts to slow Earth’s rotation by taking away some of the planet’s energy. But other factors, such as the expansion and contraction of the atmosphere with the seasons, the churning of material within Earth’s core, and the rebounding of Earth’s surface as the weight of glaciers is removed, can sometimes act to speed up Earth’s rotation by redistributing its mass, thereby altering Earth’s moment of inertia and thus its speed of rotation. 

“In some years, Earth’s length of day is closer to 86,400 seconds than 86,400.0025 seconds,” notes Robert Nelson, a physicist and president of the Satellite Engineering Research Corporation in Bethesda, Md. Even Chile’s magnitude-8.8 earthquake in February shortened Earth’s day, according to models by Richard Gross at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. — although by only 1.26 millionths of a second.

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