If the day seems a little longer than usual on Saturday, June 30, 2012, that\'s because it will be. An extra second, or "leap" second, will be added at midnight to account for the fact that it is taking Earth longer and longer to complete one full turn-a day-or, technically, a solar day.
"The solar day is gradually getting longer because Earth\'s rotation is slowing down ever so slightly," says Daniel MacMillan of NASA\'s Goddard Space Flight Center in Greenbelt, Md.
digital clock readout showing 23:59:60
June 30 will be one second longer than the typical day. Rather than changing from 23:59:59 on June 30 to 00:00:00 on July 1, the official time will get an extra second at 23:59:60. Credit: NASA
A radio telescope used in ICRF2
With this antenna at Kokee Park on the Hawaiian island of Kauai, NASA makes regular VLBI (Very Long Baseline Interferometry) measurements that are used in the time standard called UT1 (Universal Time 1). Credit: U.S. Navy/PMRF
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Scientists know exactly how long it takes Earth to rotate because they have been making that measurement for decades using an extremely precise technique called Very Long Baseline Interferometry (VLBI). VLBI measurements are made daily by an international network of stations that team up to conduct observations at the same time and correlate the results. NASA Goddard provides essential coordination of these measurements, as well as processing and archiving the data collected. And NASA is helping to lead the development of the next generation of VLBI system through the agency\'s Space Geodesy Project, led by Goddard.
http://www.nasa.gov/topics/solarsystem/features/extra-second.html 
