Except for one thing: Those micromoments don't actually vanish, and in an era of intense technology, they now matter a whole lot. "We have become critically dependent on incredibly precise timekeeping," O'Brian says. Technologies such as smartphones, GPS devices and the power grid rely on thousands of separated elements - such as satellites, cell towers, generating stations, computers, electrical switches and countless computers - that cannot get more than a millionth of a second out of sync with one another before bad stuff happens. Consider GPS signals between satellites and receivers on the ground. Those are radio signals that move at the speed of light, which means they travel about one foot every billionth of a second (which is a nanosecond). So if the clocks in GPS satellites and your GPS receiver drift just one millionth of a second - a thousand nanoseconds - out of sync with each other, the system will not pinpoint your location more precisely than within about two-fifths of a mile. If the synchronisation drifts off by one thousandth of a second, the system couldn't tell you for sure what city you're in. The moment-to-moment monitoring and management by which electrical engineers maintain the flows of current in power grids, whose interconnected components can span thousands of miles, are possible only because of precisely synchronised clocks and high-speed communication by which even the most distant parts of the grid can keep track of each other's status. And forget about talking and texting on mobile phones or Googling on your computer without superlatively timed handoffs of billions of signals between cellphone towers and perfectly timed transmissions of data packets crisscrossing the planet at lightning speed only to miraculously reassemble everywhere into coherent Web pages. "If we relied on the Earth's length of day, we could not have any of this," says O'Brian, whose group at NIST develops, maintains and improves the supremely regular atomic clocks on which all other timekeeping ultimately is based.

An atomic second is defined, in techspeak, as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133 atoms." Translation: The cesium atoms behave like magnificently fast pendulums that never, ever waver the way the Earth's rotation does. It is because of those more than 9 billion oscillations per second that it is possible to synchronise clocks with better than millionths-of-a-second precision. An Earth second, on the other hand, has been defined since early in the 19th century as 1/86,400th of a 24-hour day (60 seconds times 60 minutes times 24 hours = 86,400). The trouble for modern technologies, O'Brian says, is that the planet's length of day "is wandering unpredictably every day." What's slowing the Earth down? The moon is the biggest and steadiest drag on the Earth's rotation. Think tides here. As the Earth turns, the moon's gravitation pulls on the Earth's oceans and some of the crust below. As the planet rotates, this tidal interaction with the moon acts like the rubber damper on a carnival wheel that slowly brings the rotating wheel to a halt. This accounts for the bulk of the roughly two-millisecond slowdown of Earth's rotation for each century's worth of this tidal drag. But there are many other planetary processes that either bog down or rev up the Earth's spin. Any large-scale interaction between parts of the Earth, such as ocean winds or movements of the planet's molten iron core against the solid mineral mantle, subtly speeds up or slows down the Earth's rotation. Any process that redistributes lots of mass upward or downward will slow or speed the rotation, much as a spinning ice skater slows or quickens her rotation by drawing in or extending her arms. Evaporation, precipitation, melting, changing wind patterns, earthquakes and volcanic eruptions are among the processes that can do this. "In recent decades, the Earth's rotation has been speeding up," O'Brian points out. "This is almost certainly a temporary effect, based on weather, climate changes, changes in the crust or flow of magma."

For geophysicist Richard Gross of the Jet Propulsion Laboratory in Pasadena, California, even the tiniest changes in Earth's rotation are more than merely interesting. "To navigate a spacecraft through space and get it to Mars, you need to precisely know how Earth is oriented with respect to Mars," says Gross, whose works contributes to JPL's interplanetary navigation. He noted that even microsecond changes in Earth's rotation produce enough of a shift in the Earth-Mars orientation to throw the navigational precision off by a mile or more. And that could mean the difference between putting a Martian lander down successfully on a predetermined flat surface or sending it tumbling into a ravine. To help Gross get better at mastering these subtleties, he and a colleague calculated a few years ago that the Sumatran earthquake of December 26, 2004, repositioned enough of the Earth's mass to snip 6.8 microseconds from the length of the day. "We have come full circle," O'Brian says. The rotation of the Earth had long been the most accurate measure of time for humanity, but now such technologies as atomic clocks and GPS devices make it possible to measure tiny variations in Earth's rotation. And the scientific reverberations are not just for space junkies. In a July paper in the journal Nature, for instance, researchers in England and France argued that sub-millisecond-scale variations in Earth's rotation that occur on a 5.9-year cycle are probably linked to motions and interactions within the planet's molten core where no one has ever been to take a look. Washington Post