英语文摘：Japan Quake Shortens Earth Day

Japan’s devastating 8.9-magnitude earthquake last Friday was so powerful that experts say it slightly increased the Earth’s rotation speed, making the day a fraction of a second shorter.

U.S. space agency scientist Richard Gross said the quake shifted the way the Earth's mass is distributed, which made the planet spin a little faster, cutting the 24-hour day by an estimated 1.8 microseconds. That is less than two millionths of one second. 

The scenario is like that of a figure skater who draws her arms into her body during a spin, in order to turn faster on the ice.

The shift in the earth's mass was also seen in surface changes to the Japanese coastline, caused by the sudden slippage of colliding plates of the earth's crust far below.  Global positioning stations closest to the epicenter moved eastward by up to 4 meters during the quake.

Last year’s 8.8-magnitude earthquake in Chile, and the 9.1-magnitude quake that struck Indonesia in 2004 each cut several microseconds from the length of the Earth’s day.

It is normal for the length of a day on Earth to vary by about 1,000 microseconds (one millisecond) throughout the year.

The U.S. Geological Survey says the earthquake in Japan was the world’s fifth-largest since 1900.