The world, like many of us, isn't quite as fast as it used to be, so astronomers last night declared a brief time out.

It only lasted a second, but about 40 people in the Time Service Division of the United States Naval Observatory worked on it off and on all year. In the world of navigation, where time is distance, these things matter. Get your time wrong and your missiles fall on the wrong people. Among other difficulties, of course.

The problem, according to astronomer Alice Babcock (who deals in Earth Orientation Parameters) is that what you see, timewise, isn't exactly what you get.

It's all very well to say, as Webster does, that a day is how long it takes the world to spin once around. But what if that spin takes longer one day than it does another? How do you deal with that? Who times the timer?

Which, of course, is just what's happening. So sluggish has the earth become over the years that our days are probably four hours longer than those experienced by, say, a stegosaurus.

Enter the atomic clock, a suitcase-sized, 170-pound electronic whiz box that looks like James Bond's stereo set. The observatory has about 30 of them up there on Massachusetts Avenue, stowed around the grounds in various vaults.

While some 60 telescopes and observatories around the world and some two dozen satellites monitor the earth's rotation to calibrate universal astronomical time (which used to be good enough for everybody), atomic clocks calculate atomic time, which ignores the earth, sun and moon altogether.

The standard interval of atomic time is the International Second, defined in October 1967 by the 13th General Conference of Weights and Measures as the resonant frequency of the cesium atom.

Atomic clocks operate by cooking atoms of cesium, a semi-liquid metal like mercury, until they emit an optimum number of electrons. By keeping those electrons coming and counting the oscillations of the microwaves necessary to do so, the atomic clock can tell when a second has passed. By the laws of physics, cesium gives off electrons best when fed an electromagnetic frequency of 9,192,631,770 cycles per second.

Atomic time is so uniform it varies less than one billionth of an atomic second per day. But it tends to get ahead of astronomical time over the long run, which confuses things almost as much as Daylight Saving Time.

By international agreement, scientists have decreed the two time systems can never be more than 8/10ths of a second apart. "Leap seconds" are added when necessary to bring the atomic clock in line with real time. Astronomers prefer to add them either at the year's end or the half-year mark. Leap seconds have been added for the past two years on June 30. The last one needed before that was on Jan. 1, 1980.

Last night, the world was running 3/10ths of a second behind the atomic clock. The second was officially inserted at the end of the final minute of June 30th at the Greenwich Observatory in England, home of Greenwich Mean Time where all longitude begins.

Here in Washington that meant 8 p.m. EDT was actually 7:59 p.m. plus 61 seconds.

For those who knew what was happening, the moment was traceable electronically in beeps and graphs, but for the benefit of media types who require visuals, it was chronicled on a specially prepared digital readout that had been practicing on the same 61-second minute all afternoon.

After insertion of the leap second, atomic time was 7/10ths of a second ahead of the world, but that was expected to correct itself slowly during the coming months as the earth wound down.

Exactly why the earth is running down is as complicated as the atomic clock. The overall net slowdown, says astronomer Babcock, is the inevitable and mathematically predictable result of "tidal friction"--the gentle pull of the moon's gravity over the millenia.

Other variations, some fast, some slow, come from relatively predictable seasonal conditions--heating and cooling of land surfaces and winds on the mountain ranges.

But that leaves a third set of variables over which astronomers and geophysicists puzzle--fluctuations in the rate of the earth's spin that can't yet be accounted for or predicted and that operate within the net slowdown.

Some hypothesize they come from volcanic upheavals within the earth's molten core--shifts in mass that accelerate or retard the planet's rotation on its axis.

When they will necessitate another leap second, no one can say.