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