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, technically¹, a solar day. "The solar day is gradually getting longer because Earth's rotation² is slowing down ever so slightly," says Daniel MacMillan of NASA's Goddard Space Flight Center in Greenbelt, Md.

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（干涉法） (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 coordination³ of these measurements, as well as processing and archiving the data collected. And NASA is helping⁴ to lead the development of the next generation of VLBI system through the agency's Space Geodesy Project, led by Goddard.

From VLBI, scientists have learned that Earth is not the most reliable timekeeper. The planet's rotation is slowing down overall because of tidal forces between Earth and the moon. Roughly every 100 years, the day gets about 1.4 milliseconds, or 1.4 thousandths of a second, longer. Granted, that's about 100 or 200 times faster than the blink of an eye. But if you add up that small discrepancy⁵ every day for years and years, it can make a very big difference indeed.

"At the time of the dinosaurs⁶, Earth completed one rotation in about 23 hours," says MacMillan, who is a member of the VLBI team at NASA Goddard. "In the year 1820, a rotation took exactly 24 hours, or 86,400 standard seconds. Since 1820, the mean solar day has increased by about 2.5 milliseconds."

By the 1950s, scientists had already realized that some scientific measurements and technologies demanded more precise timekeeping than Earth's rotation could provide. So, in 1967, they officially changed the definition of a second. No longer was it based on the length of a day but on an extremely predictable measurement made of electromagnetic transitions in atoms of cesium（铯） . These "atomic clocks" based on cesium are accurate to one second in 1,400,000 years. Most people around the world rely on the time standard based on the cesium atom: Coordinated⁷ Universal Time (UTC).

Another time standard, called Universal Time 1 (UT1), is based on the rotation of Earth on its axis⁸ with respect to the sun. UT1 is officially computed⁹ from VLBI measurements, which rely on astronomical¹⁰ reference points and have a typical precision of 5 microseconds, or 5 millionths of a second, or better.

"These reference points are very distant astronomical objects called quasars（类星体） , which are essentially¹¹ motionless when viewed from Earth because they are located several billion light years away," says Goddard's Stephen Merkowitz, the Space Geodesy Project manager.