Japanese quake may shorten days

The Japanese earthquake may trigger shorter days, spurring changes in computer time-keeping

Japan's March 11 earthquake may eventually result in shorter days, according to NASA scientists.

Computers, however, should be well-prepared to handle this fluctuation once it is introduced into the official time-keeping systems, given the existing systems for reconciling computer time with solar time.

The 9.0-magnitude quake -- the fifth-largest since 1900 -- has possibly shifted the Earth's mass, thereby changing its rotation and shortening the days by up to 1.8 microseconds per day, argues research scientist Richard Gross of NASA's Jet Propulsion Laboratory in Pasadena, California.

A microsecond is one-millionth of a second. While humans won't notice the shortened days, any resulting changes will eventually affect computers, which get their times readjusted through the periodic injection of leap seconds.

Gross used a model from the United States Geological Survey to calculate how the earthquake may have shifted the Earth's mass. He estimated that the Earth's figure axis (the axis about which Earth's mass is balanced) has been shifted by about 6.5 inches (17 centimeters) toward 133 degrees east longitude. This new distribution of weight may cause the planet to wobble differently than it did before, thereby shortening the time it takes to complete a 24-hour cycle.

Gross expects to refine his calculations as more data from the earthquake is recorded.

This is far from the first time that day length has changed. On average the day length is about 86,400 seconds, but it fluctuates by small increments for a variety of reasons.

"Earth's rotation changes all the time as a result of not only earthquakes, but also the much larger effects of changes in atmospheric winds and oceanic currents," Gross said in a statement. "Over the course of a year, the length of the day increases and decreases by about a millisecond. The position of Earth's figure axis also changes all the time."

Previous quakes have resulted in minuscule adjustments in the length of days. Last year's magnitude 8.8 earthquake in Chile might have shortened the day by about 1.26 microseconds. A 2004 magnitude 9.1 Sumatran earthquake may have shortened the day by 6.8 microseconds, NASA estimates.

Reconciling computer time with this new Earth time will probably be done through the introduction of the next leap second. In 1971, the International Telecommunications Union added leap seconds from the time scale used by most computer systems, Coordinated Universal Time (UTC), to accommodate such fluctuations on an as-needed basis.

With leap seconds, a second is added to or subtracted from the official time to reconcile it with solar time. While such adjustments are rarely noticed by system administrators, they can occasionally be problematic for those systems that execute high-speed financial trading, record the results of in-depth research and execute other duties that require second-by-second resolution.

Since 1972, 24 leap seconds have been introduced into UTC. Most have been added to offset the gradual slowing of the Earth's rotation. The U.S. National Institute of Standards and Technology maintains a list of leap seconds that can be parsed by computers through the NTP (Network Time Protocol).

Still, it might be a while before the ITU must issue another leap second to reconcile the divide between digital and solar time that this earthquake has wrought.

"Two microseconds per day is under a [millisecond] per year. That's not very big on the scale of a whole second," one contributor of the Time Nuts mailing list noted.

Joab Jackson covers enterprise software and general technology breaking news for The IDG News Service. Follow Joab on Twitter at @Joab_Jackson. Joab's e-mail address is Joab_Jackson@idg.com

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Joab Jackson

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