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Eager for this year to end? Bad news: you'll have to wait an extra second for 2009. On December 31, the planet's official timekeepers will add a “leap second” to the coordinated universal time scale (UTC) followed around the world. The additional second makes up for the difference in two clocks – one based on Earth’s rotation and the other on the more precise atomic time of the UTC.

In the U.S., the extra second will be added by the U.S. Naval Observatory at 6:59:59 p.m. Eastern Standard Time (11:59:59 p.m. Universal time). It will be the 24th “leap second” tacked on to the universal time scale since 1972.

Universal time is based on atomic energy, with one second defined as the length of 9,192,631,770 energy transitions of the cesium atom. Before the era of atomic time, seconds were based on the speed of Earth’s rotation – but that’s been slowing by 2 milliseconds per day per century because of tidal friction.

To keep the time scales within 0.9 seconds of each other, the International Earth Rotation Reference Systems Service, which tracks the differences between the clocks, periodically inserts or subtracts a second to Universal time. The last one was added on New Year’s Eve three years ago.

The “leap second” is different from “leap year,” which occurs every four years on February 29. Leap years are based on the fact that it takes Earth 365 days plus six hours to completely circle the sun, according to Reuters.

Image © iStockphoto/Susan Trigg

5 Comments

1. 1. Stefen 04:53 PM 12/29/08

   If the Earth's rotation slows 24 seconds every 36 years, how many years will it take for the Earth to stop rotating?

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2. 2. Stefen 05:00 PM 12/29/08

   If the Earth's rotation slows 24 seconds every 36 years due to tidal friction, how many years will it take for the Earth to stop rotating?

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3. 3. Fred D 12:03 PM 12/30/08

   The Earth wil not stop rotating. Tidal friction will eventually cause earth to rotate once per year thus keeping one side facing the sun permanently, just as Mercury does, and the moon faces the earth.

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4. 4. jh443 in reply to Fred D 02:08 AM 12/31/08

   That depends on how you define rotating.
   
   If I place a ball on the table in such a way that it appears motionless, is it rotating? Most people would say that it isn't (and I'd agree). However, as seen from the polar perspective, it is rotating in the same way the moon is: In 24 hrs it rotates once, thereby keeping the same side facing the center of the earth - whereas the moon accomplishes this same feat about every 29 days.

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5. 5. invu in reply to Stefen 08:37 PM 1/2/09

   Stefen, why do you think the Earth's rotation slows "24 seconds every 36 years" due to tidal friction? The article says it slows with decceleration of 2 ms/day every century.

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