NIST-F1 Cesium Fountain Atomic ClockFor most of human history we have defined time through the movements of planets and stars. One day is the time it takes the Earth to rotate about its axis, one year the duration of a single orbit about the sun. But in January 2012 the way we think of time may change.

In order to keep the time determined by Earth's motion in line with the seconds measured by atomic clocks, the International Earth Rotation and Reference Systems Service inserts "leap seconds" into the calendar. But leap seconds may fall out of favor after next year's World Radiocommunication Conference, run by the UN's International Telecommunication Union (ITU). A group of astronomical scientists and engineers, led by David Finkleman of the Center for Space Standards & Innovation, drew attention to these deliberations in an article published in the July-August issue of American Scientist. The essay is also available on, a pre-print blog for the mathematics and physical sciences.

Most societies require a universal definition of time—one minute in Texas must last the same length as one minute in New York. If there is one very accurate clock, the time it keeps can be broadcast at regular intervals to help keep all watches ticking in time. In the U.S., for example, the National Institute of Standards and Technology (NIST) uses the resonant frequency of cesium-133 atoms in the NIST-F1 Cesium Fountain Atomic Clock to keep time so accurately that even if it ran for 60 million years, NIST-F1 wouldn't drop or add a single second.

NIST-F1 is one of several international atomic clocks used to define international civil time (dubbed Coordinated Universal Time, or UTC), a job they perform a little too well. In fact, atomic clocks are actually more stable than Earth's orbit—to keep clocks here synched up with the motion of celestial bodies, timekeepers have to add leap seconds. The use of a leap year, adding a day to February every four years, locks the seasons, which result from Earth's orbit about the sun and the planet's tilt as it orbits, into set places in the civil calendar. Similarly, leap seconds ensure that the time it takes Earth to spin 360 degrees is equal to one day as defined by humans and their atomic clocks. Most recently, an extra second was tacked on to universal time on December 31, 2008.

However, since 1999, the Radiocommunication Sector of the ITU has been proposing the elimination of leap seconds from the measurement of UTC. Although the organization did not participate in the creation of the current leap second system, the radio waves it regulates are used to transmit UTC, giving it some influence.

Getting rid of leap seconds would certainly make it easier to calculate UTC, but this measure would also decouple astronomical time from civil time: The time measured by atomic clocks would gradually diverge from the time counted out by the movement of Earth through space. Eventually, one year will no longer be the length of Earth's orbit around the sun. Instead, it will be equivalent to a certain number of cycles of radiation from the cesium-133 atom (almost a billion billion cycles, to be precise).

These discrepancies will be extremely relevant to astronomers, who will need to keep track of two different times if the leap seconds proposal is adopted. In addition, keeping time based only on atomic clocks' measurements will give time itself a different meaning. After hundreds of years of letting planetary and lunar motion define time, we will shrink our scale, and let atoms determine it instead.