At a moment that could be called 6:59:60 p.m. today, the U.S. Naval Observatory and the National Bureau of Standards are going to give everyone a little extra time to get ready for New Year's Eve celebrations. One second.

Under ordinary conditions, 6:59:60 would be 7:00:00 on the clock. Today, it won't.

Computers at the two institutions, keepers of the nation's official clocks, have been programmed to insert one extra second, a "leap second," at that time. When ordinary clocks say it is one second after 7 p.m., the official clocks will say it is still 7 p.m. exactly. As a result, 1988 will arrive one second later than previously planned.

The two agencies are taking the step to bring the highly regular atomic clocks that are the world's time standard back into synchronization with the slowing rotation of Earth. Earth's daily rotation slowed enough during 1987 that the official clocks are now about one second ahead of the planet. The action, effectively stopping the clock for one second, allows Earth to rotate just enough to catch up with the clock.

While New Year's Eve revelers in time zones around the globe are likely to insert their own leap seconds at their local midnights -- as will happen in New York's Times Square celebration -- the United States and the 32 other countries that belong to the International Earth Orientation Service based in Sevres, France, have chosen to do it simultaneously at midnight on the Universal Time clock, which is 7 p.m. Eastern Standard Time.

"Universal Time (Coordinated)" is the official name of what until 1975 was called Greenwich Mean Time, named for the now defunct royal observatory in Greenwich, England, the world's original keeper of time for use in celestial navigation.

Because many ships still rely on celestial navigation, the relationship between time and the position of stars must be kept precise. As Earth turns, the stars' positions relative to the horizon change. By relating star positions to exact time, navigators can tell exactly where they are. If Earth has turned less than the clock indicates -- the very situation that had developed -- navigators would be misled into assuming they were miles away from where they were.

Leap seconds are necessary every few years because official atomic clocks, which rely on highly regular oscillations of cesium atoms (exactly 9,192,631,770 per second), are much steadier than the rotation of Earth. The clocks are set to click off days every 24 atomic clock hours. Earth's rotation, however, has been slowing, taking a tad longer than 24 hours to complete one turn.

Leap seconds are added at irregular intervals -- typically every two or three years -- and not always at year's end, because Earth's slowdown is irregular. The main reason it is slowing is that some of the momentum that keeps it spinning is lost as friction. Bulging sea levels that make for high tide, for example, tend to stay in one position with respect to the moon as Earth rotates underneath. This causes tidal friction, which steals energy from rotation at a fairly even rate.

Sometimes the slowdown is less than otherwise because of changes in the distribution of Earth's mass as crustal plates plunge under one another and as magma of different densities moves about within the planet.

The changes in the speed of Earth's rotation are analogous to changes that figure skaters make in the speed at which they spin.

A skater can spin faster by pulling arms and legs close into the axis of spin or slow the spinning by extending arms and one leg. With the arms and leg out, part of the skater's mass must be moved through a greater distance than when limbs are pulled in. The available spinning energy is the same in both cases, but if the mass must move a greater distance, the spin slows, and vice versa.