Greeting another new year without a leap second

December 19, 2003

Does it seem like the world is speeding up? That the pace of life is increasing?

If you feel that way, there's scientific evidence to prove your point. The world has sped up over the last few years. Timekeepers at the National Institute of Standards and Technology (NIST) note that they have not had to insert an extra second (called a leap second) into their time scale for five years. Why? Because the rate of the Earth's rotation has sped up since 1999.

From 1972 (when the world went to the current system of atomic timekeeping) until 1999, 22 seconds were added to the world's time in order to keep atomic time synchronized with Earth's time, as measured by the Earth's spin. Since then, none, nada. Scientists are not sure why this is so, but they do offer some thoughts.

Tom O'Brian, a physicist and chief of NIST's Time and Frequency Division in Boulder, Colo., suggests changes in motion of the Earth's core, the effect of ocean tides and weather, and changes in the shape of the Earth may all be affecting the spin of Mother Earth. In general, he notes, the long-term trend has been for the Earth's rotation to slow down, but not in the last five years.

O'Brian said most scientists expect the Earth to continue slowing down again in the future. So maybe there is hope for those feeling particularly harried.

"The Earth's rotation rate has been the primary clock for nearly all of human history," he says, adding that "only in the last 50 years have we had clocks accurate enough to measure changes in the Earth's spin." NIST introduced the world's first atomic clock in 1949.
For more information about leap seconds, see

National Institute of Standards and Technology (NIST)

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