WASHINGTON (CNN) -- It's almost time to reset clocks, not by an hour or a minute but by a second.

Before the clock strikes midnight Thursday in Greenwich, England (7 p.m. EST) the world will receive an extra moment -- a leap second of time.

Astronomers say the addition is needed to keep the world's clocks in sync with the rotation of the Earth.

"We now know that the rotation of the Earth is slowing down very slightly ... and that's why we have a leap second," Steven Dick of the U.S. Naval Observatory says.

The observatory maps the whereabouts of celestial objects and measures the rotation of the Earth.

Ocean tides are the major culprit in the slowdown of the Earth, making it necessary to adjust the world's timepieces on a regular basis.

"About every year or year and a half, we have to add a leap second to take into account the slowing down of the Earth and to synchronize atomic time with astronomical time," Dick says.

Astronomical clocks keep time according to the Earth's rotation, while atomic clocks keep time according to the unchanging vibrations or oscillations of a radioactive atom.

The atomic clock, the world's official timekeeper, has had 22 leap second adjustments since 1972. Without them, those lost seconds would add up -- thousands of years from now sunrise would occur at noon, and sunset at midnight.

The Naval Observatory makes sure each extra second is programmed into computers on global positioning satellites, which send the correct time back to Earth.

And for people to whom seconds matter, there's a high-tech watch that automatically adds leap seconds. It has a tiny receiver that picks up radio signals timed to the official atomic clock.

If your timepiece isn't that precise, just remember: to keep in synch while toasting the New Year, wait just a second before raising your glass.