PARIS, France - When the very first second of 2000 ticks in the South Pacific and moves through the planet's 24 time zones, those snapshots in time will have been scientifically verified in Paris where researchers rely on the Global Positioning Satellite network and 30-year-old lunar reflectors to synchronize the world's atomic clocks.

It's a process that occurs regularly but carries extra meaning this New Year's Day as countries argue over which will be the first to experience the dawning of 2000.

The most recent adjustment to allow for Earths rotation was made a year ago, but no change will be necessary this year.

"This December 31st, we won't have 61 seconds in the last minute of the millennium as we did last year and our Web site will show the international time of the new year calculated by our Y2K compatible computers," said Arias.

Since January 1, 1961, just after the introduction of the first atomic clocks, a total of 36 leap seconds have been introduced to bring so-called Universal Time into compliance with real Earth's time.

Because of the slowing down of the Earth rotation, days get longer by several millionths of a second.

The minor change may not be apparent to the average clock-watcher, but its a big difference for atomic clocks (invented in 1955), which are able to mark a nanosecond, i.e. a billionth of a second and even one hundredth of a nanosecond.

"Approximately, every two years or a year and a half, we add a second on December 31 or on June 30, and we notify the national time laboratories around the world of the new official time called UTC, for Coordinated Universal Time," said Arias.

These leap seconds are inserted on the advice of the International Earth Rotation Service (IERS) to ensure that, on average over the years, the sun is overhead -- that is to say at the zenith -- within 0.9 seconds of 12:00:00 UTC on the meridian of Greenwich. An international conference in 1884 placed the zero meridian at Greenwich, the basis for calculating mean time all over the world.

Nowadays, time is measured by atomic clocks, making UTC the atomic-era successor of Greenwich Mean Time, GMT, which was used when the unit of time was the mean solar day.

But even atomic clocks can vary by a few billionths of a ond per day.

"Who can certify that one's atomic clock is more precise than an another one?" Arias said.

"And because there are time differences between atomic clocks, the best solution is the International Atomic Time (TAI) which is calculated by the BIPM from the mean time of more than 200 atomic clocks," says Arias.

This scientific solution also has a political advantage: no country has to be selected to be the world reference for exact time.

But comparing different atomic clocks and working out an average time for the entire world raises another problem: the transmission delays you get by looking at the time displayed, for example, in Washington, D.C., and at the Observatory of Paris.

Because the two sites are not in simultaneous view, time displayed by phone, by Internet or through computer software is seriously affected by transmission delays.

The solution for more than 10 years has been the GPS system, which offers the principal tool for national and international comparisons of atomic clocks located on different continents.

By using GPS observations, it is possible to determine clock differences with uncertainties well below one nanosecond (billionth of a second), comparable to the best time measurement techniques available.

"The GPS is an essential intermediary tool to compare, for example, the time between U.S. Naval Observatory (USNO) in Washington, D.C., with the one of the Observatory of Paris," said Wlodzimierz Lewandowski, physicist at the BIPM Time Lab.

"The two cannot see each other but their GPS receptors are in simultaneous view of the same satellite," he said. "This system provides an accuracy of 2 to 3 nanoseconds which is one billion times better than using the phone system or Internet."

Even if the GPS satellites are relatively far away in space, the position of the satellite and that of the ground antenna are very well known. More importantly, the signal transmitted through space is very pure because it remains undisturbed.

"These comparisons are made almost permanently; the difference of time between the pair of clocks is put into the TAI software which calculates the average time every five days," said Lewandowski, who expects space technologies to become essential to gain even more precision in time calculation.

GPS receptors antennas are the only exterior signs identifying the small 17^th century style pavilion building of the BIPM Time Lab which is situated in Saint Clouds park on the outskirts of Paris.

Since 1967, atomic clocks have revolutionized time measurement standards. Before them, noontime was calculated by measuring the sun at its zenith, Earth's revolution and Earth solar orbit. The second was the tiny subdivision of Earth revolution, that is to say the 1/31,556,925.9747 of this revolution.

In the last three decades, the atomic clock has altered the definition of time.

In 1967, the atomic second was defined as the interval of time taken to complete 9,192,631,770 oscillations of the atom of radioactive gas called Cesium-133.

The precision with which the atoms are "vibrating" makes them perfect natural clocks with a very accurate frequency. So each second is the duration of over 9 billion cycles of a particular hyperfine structure transition in the ground state of Cesium-133.

According to this Cesium standard, the atomic day of 24 hours becomes the sum of 86,400 atomic seconds. And the year was no longer 365.242,199 days but 290,091,200,500,000,000 atom oscillations, give or take one or two oscillations. The atomic second has become the new time standard measurement.

"In the future we could come back to an astronomic reference by using the spinning neutron stars called pulsars. But first, we must be sure they are precise on long term period," said Arias.

More studies may lead to a means of interpreting data from millisecond pulsars which are millions of light-years away so as to provide a time scale having outstanding stability in the very long term.

In the infinite human quest to track time, scientists are already using "Cesium fountain" clocks which are even more accurate by using a process that "cools" the Cesium atom and provides with a highly precise time measurement of picosecond (10 to the negative 12^th power seconds).

State of the art technologies, developed in France, are already working on even higher precision accuracy.

"Even though these time calculations and technologies don't seem to have any thing to do with our daily life and with our perception of time, this science of measuring time is very important for astronomy, for telescopes like Hubble, for GPS technology and even for measuring relativist effects of Einstein theory," said Arias.

The nanosecond-sensitive atomic clock development played an important role with the GPS satellite network.

Since it's based on the flight path of radio waves moving at the velocity of 300,000 kilometers per second, at this speed, the 30 centimeters -- equivalent to one billionth of a second -- translates into a major deviation if used, for example, to guide an aircraft during landing.

The practical effects are great. In a few years, new GPS networks will control most of the worlds air traffic. Last spring, the most accurate air attacks on Kosovo used GPS-navigated bombs dropped by U.S. Air Force B-2 stealth bombers.

And Einstein's theory of general relativity in 1916, according to which the movement of time and space are inextricably linked, is a daily reality for scientists who deal with time technology.

"With the nanosecond precision, the effects of Einstein's relativity are realities that we have to take into consideration in our calculation," said Arias. For example, a precision clock put in the center of Earth, one placed on its surface and another one located in the center of the solar system would produce three different times.

Already some European advanced project, like PHARAO (Projet d'Horloge Atomique par Refroidissement d'Atomes en Orbite), are planning to elaborate new generation atomic clock placed in orbit using atoms maintained at ultra-low temperatures to improve time measurement precision.