Tick tock ... tick - Extra second added to 2008

By Jim Wolf

WASHINGTON (Reuters) - Those eager to put 2008 behind them will have to hold their good-byes for just a moment this New Year's Eve.

The world's official timekeepers have added a "leap second" to the last day of the year on Wednesday, to help match clocks to the Earth's slowing spin on its axis, which takes place at ever-changing rates affected by tides and other factors.

The U.S. Naval Observatory, keeper of the Pentagon's master clock, said it would add the extra second on Wednesday in coordination with the world's atomic clocks at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time, or UTC.

That corresponds to 6:59:59 p.m. EST (23:59:59 GMT), when an extra second will tick by -- the 24th to be added to UTC since 1972, when the practice began.

UTC is the time scale kept by highly precise atomic clocks around the world, accurate to about a billionth of a second per day, the Naval Observatory says. For those with a need for precision timing, it has replaced Greenwich Mean Time, or GMT.

The decision to add or remove a second is the responsibility of the International Earth Rotation and Reference Systems Service, based on its monitoring of the Earth's rotation.

The goal is to make sure clocks vary from the Earth's rotational time by no more than 0.9 seconds before an adjustment. That keeps UTC in sync with the position of the sun above the Earth.

Mechanisms such as the Internet-based Network Time Protocol and the satellite-based Global Positioning System depend on precision timing.