If 1987 seems to be racing to its end too fast, relax -- a little. The U.S. Naval Observatory, official keeper of the nation's time, has decided to make this year longer than an ordinary year -- exactly one second longer.

The extra second, according to the observatory's Gail S. Cleere, will be tacked onto the last minute of Dec. 31. Thus, if the people who make the ball drop at Times Square are on the ball, the cheers will not erupt until a full second after ordinary clocks strike midnight.

The observatory is adding the second because the Earth's rotation slowed a tiny bit in 1987. But since the observatory's highly accurate clocks did not slow, they have been clicking off days infinitesimally faster than Earth has been taking to complete rotations. Stopping the clocks for one second will let Earth catch up.

The observatory can tell that Earth slowed by comparing the positions of stars observed from the same place at the same clock time on different days. If Earth and clocks are out of synch, the angle of view changes.

Resetting the official clock brings them back close enough together that people who rely on celestial navigation -- astronomers and space engineers, for example -- do not go off course.