Set Your Clocks Ahead … by One Second: Leap Second

The International Earth Rotation and Reference Systems Service was added a second to the clock on Saturday. The “leap second” will take into account the slowing of the Earth’s rotation.

This Saturday on June 30, a “leap second” was added to the clock at midnight universal time (4pm Pacific). After 11:59:59 PM, the clocks will roll to 11:59:60.

Why Do We Need Leap Second?

Leap second Setting

Leap second June 30 2012

The earth is spinning a little slower than it used to, courtesy of the moon’s gravitational tug, making each day about two milliseconds longer than it was a century ago. So every few years, the clocks are corrected to synchronize with official U.S. atomic clocks.

Leap seconds are not added based on a fixed and predictable schedule. Hard-to-predict climate changes and geological events, as well as the gravitational effects of the moon on the Earth, all affect the rotation speed of the Earth. As such, leap seconds are not generally announced until about six months before they occur.

There are no noticeable side effects from the change. The last leap second was tacked on to atomic clocks in January 2009. Another one shouldn’t be necessary until 2015.

Video: What is a leap second?

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