Tonight, long before the first New Year's toast hits the national gullet, scientists will already have ushered in 1991 by putting a stitch in time.

They'll insert a "leap second," doubling the interval from one second to two, at 23:59:60 Universal Time, formerly known as Greenwich Mean Time. That's 7 p.m. here.

It will be the 16th leap second added since 1972, when time as we know it began. In that year, civilization adopted a system whereby chronological intervals would no longer be based on the motions of the Earth, the sun or stars, but on the natural resonant frequencies of certain atoms -- the atomic clock. That method is now used to produce the international standard called Coordinated Universal Time, which regulates, with a precision unknown in nature, everything from navigation to naps.

The Earth, however, has remained indifferent to such meddling and obstinately persists in keeping its own hours, which are considerably less regular -- principally because the planet's rate of rotation varies constantly. During the last half of the 19th century, Earth sped up; in the present era, it's been slowing down by a couple millionths of a percent from one year to the next.

The clock-planet disparity, says Dennis McCarthy, chief of the Earth Orientation Parameters Division at the U.S. Naval Observatory, "causes a dilemma. We can have a uniform atomic time interval, but it'll eventually get out of sync with the Earth. Or we could use the Earth as a standard, but then we wouldn't have uniform seconds."

In order to have it both ways, chrono-czars wait until the atomic clock and the real world get about 0.9 seconds out of whack, and then stick in a leap second on the last day of June or December, as appropriate.

Not that anyone would notice if they didn't. It would take thousands of years for the accumulated difference between atomic time and "apparent" time -- the way a day appears to Earth's inhabitants -- to become a perceptible nuisance to average folks. So why reconcile the two systems at all?

One major reason is that many functions essential to modern life employ exacting measurements of both universal and solar time. For example, McCarthy says, "if you're navigating at sea, you have to use a clock to get your longitude. And if you're off by as much as a second, you'll miscalculate your position by as much as a quarter of a mile." Hence we need the leap second.

Uniformity Demanded

But old habits die hard. Even well into the 20th century, society kept trying to use the same old celestial guideposts that have served since the dawn of time. For a while, the standard second was calculated as one 86,400th of a "mean solar day" (the average time between two consecutive noons -- the instant when the sun reaches its highest point above a given place -- over 12 months). That solution worked well enough, but produced a second that varied in length -- a sort of original flex-time -- because the Earth itself varied. An alternative scheme temporarily defined the second as a fraction of one particular solar year, creating a "hard" interval but introducing other problems.

By the late 1950s, the increasingly fastidious requirements of modern physics were demanding a uniform time measure that wasn't tied to the ephemera of the heavens. Scientists found such a standard in the admirably constant rate at which a given atom will resonate if it is zapped with the right amount of energy. In 1967, the second was defined as the time it takes for an atom of cesium-133 to tick through exactly 9,192,631,770 resonant cycles after it has been excited by electromagnetic radiation. Since 1972, that "atomic second" has been the internationally accepted standard time interval.

Atomic clocks are highly accurate -- varying only by about 5 parts in 100 trillion. This, however, is not close enough for government work. To do better, the Naval Observatory takes measurements from a score of cesium clocks, averages them, and then uses the result to micro-adjust a set of "masers" that record the frequency of hydrogen atoms. The adjusted masers form the basis of the master clock, which is the authoritative standard for U.S. time.

Rotational Rate Varies

Meanwhile, the Earth is keeping a more erratic schedule.

There are a number of reasons why the planet's rotational rate varies, and some are fairly well understood. For one thing, when the moon's gravitational field pulls on the seas to create the tides, it also slightly distorts the shape of the Earth's crust, making it less spherical and thus changing the planet's "moment of inertia."

This phenomenon is well known to figure skaters, who find that they can spin faster if they tuck their arms in. But when the arms move away away from the body, the rotation slows down because some of the skater's body mass has to move through a larger arc. Overcoming this greater moment of inertia takes more energy. Hence the available energy is, in a sense, spread out over the bigger job to be done and can turn the skater only at a slower rate. Sometimes, however, the moon's constantly changing position relative to Earth can have the opposite effect, making the planet more spherical, and raising its speed.

In addition, Earth's rotation fluctuates seasonally: Depending on whether it's summer or winter, different amounts of solar radiation strike the land-rich Northern Hemisphere or land-poor Southern Hemisphere. This produces dramatic differences in wind patterns. They can speed rotation by blowing with the planet's motion, especially when they hit mountains, or slow it by blowing the other way. There are also decade- and century-long variations in spin rate that may be caused by sluggish shifts in the molten goop at the Earth's core.

As it happens, scientists can determine quite accurately how fast the Earth is spinning at any given moment by clocking its rotation against a fixed point, much as a rider on a carousel might use the brass ring to measure the time it takes to make one circuit. Astronomers use the radio waves emitted by quasars -- which are so enormously far away that their motion with respect to Earth is negligible -- as fixed-point references. A series of receiving antennas around the planet monitor the arriving waves; the difference in arrival times between one antenna and the next indicates how fast the planet is revolving.

McCarthy's division at the Naval Observatory -- like its counterparts in other countries -- constantly tracks the spin-rate changes and puts out weekly predictions. Data from about 60 such labs around the Earth are fed to the International Earth Rotation Service in Paris, which decides whether and when to add a leap second and issues the required three-month warning.

The horological bonus is inserted by altering the computer programs of the world's master clocks. Normally, the second counter runs from 0 to 59 and then starts again at 0. But at midnight UT, it will run to 60 -- making room for the second whose time has come.