By international agreement in 1967, the fundamental unit of time is the second. But the second is no longer fast enough. Pieces of the second, unimaginably short and beyond human perception, now make a difference in a variety of modern technologies—computers, cellular phone networks, radio and television broadcasting, and electric power transmissions. Billionths, even trillionths, of a second are put to work.

	Advertisement, March 1928; by the Synchronome Co., London, from the Horological Journal. William Shortt and F. Hope-Jones developed the first free-pendulum clock.

Traditionally, time has been defined in terms of Earth's rotation relative to the sun and other stars. Early in the 19th century, astronomers defined the second as 1/86,400 of a mean solar day. But in the 1920s, a new kind of mechanical clock—the free-pendulum clock, accurate to within ten seconds a year—enabled scientists to prove that Earth does not keep uniform time. It speeds and slows as it orbits the sun and wobbles on its axis as it spins-a day varies in length by tiny fractions of a second. Scientists began the search for a new standard for defining the second.

	Quartz crystal, an uncut sample; the cut of the quartz determines the number of times per second it will vibrate. Transfer from National Museum of Natural History, Department of Mineral Sciences

Twentieth-century advances in timekeeping resulted from work done by engineers seeking to improve telephone and radio communication. In 1927, Warren Marrison, a Canadian engineer working at Bell Telephone Laboratories, developed a clock that kept time using the vibrations of a quartz crystal. Quartz clocks were more accurate than any mechanical timekeeper, gaining or losing only a second in three years. In the 1940s and 1950s, quartz standards replaced mechanical ones in some astronomical observatories and scientific laboratories.

The quartz clock demonstrated that scientists did not have to rely on the traditional sources of astronomy and mechanical clocks to define tiny slices of time. Physicists began to use the oscillations of certain atoms as precision regulators. Since 1967, the length of a second has been defined by how long it takes the cesium 133 atom to vibrate 9,192,631,770 times when subjected to electromagnetic waves. Superprecise atomic clocks count these unthinkably tiny fragments of a second.

Your watch probably doesn't need to keep track of nanoseconds (billionths of a second), but your computer does. A variety of modern technologies—from cellular phones, radio and television broadcasts, and electric power transmissions to the Global Positioning System navigation aid—all depend on such infinitesimal subdivisions of the second. Superprecise quartz and atomic clocks count nanoseconds, even picoseconds (trillionths of a second).

Atomic clock chassis, early 1980s; test unit built by Frequency and Time Systems, Inc., Beverly, Massachusetts; identical to ones used in the U.S. military's GPS satellite system. Twenty-four GPS satellites orbit Earth and continuously transmit their exact location based on time derived from cesium and rubidium atomic clocks. Gift of Datum		Global Positioning System receiver, 1993–1997; built for the U.S. military, it compares signals from three orbiting satellites to calculate the user's time, velocity, latitude, longitude, and altitude. GPS receivers are now available for civilian use. Gift of Rockwell International, Collins Avionics and Communications Division

Battery-powered quartz wristwatches give just about everyone access to the split-second accuracy of the quartz time standard that once was available only to scientists and technicians. The first battery-powered quartz watches hit the American marketplace in the early 1970s. Since then, styles have diversified, prices have fallen, and quartz watches outsell mechanical ones by a huge margin.

Wristwatch, 1972; Pulsar, by HMW, Lancaster, Pennsylvania, in cooperation with Electro-Data, Inc., Garland, Texas; the first digital quartz watch with push-button-activated LED (light-emitting diode) display; sold for $2,100 Gift of John M. Bergey		Wristwatch, 1970; Accuquartz, with a Beta 21 module, by Bulova, Biel, Switzerland Loan from Butterfield Jewelers
Wristwatch module, 1969; Seiko Astron 35SQ, by Suwa Seikosha Co., Suwa, Japan, for the first commercially available quartz watch Gift of Seiko Corporation, Japan		Wristwatch module, 1972; Pulsar, by HMW, Lancaster, Pennsylvania, in cooperation with Electro-Data, Inc., Garland, Texas Gift of Richard Walton
Wristwatch, 1973; Teletime, by Gruen Industries, Inc., New York; the first quartz watch with digital twisted nematic LCD (liquid crystal display) by ILIXCO, Cleveland, Ohio Gift of Fred E. Whelan		Wristwatch, 1975; by Texas Instruments, Dallas; the first quartz watch marketed at a very low price–just $20 Gift of Hubert H. Myers
Wristwatch, 1993; Mega 1, by Junghans Uhren GmbH, Schramberg, Germany; with radio receiver that resets the watch to a time signal sent from Braunschweig, Germany Gift of Junghans Uhren GmbH