Time's trek from sun to atom

What time is it?

A simple enough question; but throughout time, time has been difficult to measure. That's because time is entirely relative - a function of human need and perception, rather than an entity to itself. So need has pushed its development; as society grows more complex, so have the measurements of time.

At first we had only to look to the heavens. Regardless of how each culture defined them, time was easily and everywhere measured by the cyclical seasons and moon. Spring's planting, fall's harvests, winter's wait. In other climates, perhaps the animals' migrations or size of the catch.

Ice age hunters made notches in bones, believed to measure days by moon cycles. Five thousand years ago Sumerians (modern-day Iraq) measured 30-day months and 12, two-hour periods for each day. Yet it wasn't until around 3100 B.C.E. that Egyptians discovered that the planet Sirius returned to the sun every 365 days, so they are credited with the first modern calendar year. A millennium later the Mayans recorded a 356-day year.

Daily clocks also had a Middle East/North African birth, where bureaucracies and trading were more advanced and they developed needed clocks nearly 6,000 years ago. Egyptians in 3500 B.C.E. used obelisks - slender, tapering, four-sided monuments whose shadows formed a kind of sundial, breaking the day into morning and afternoon. Little marks around the base broke it down further (close to hours) and shadow length, the longest and shortest days of the year. Over time sundials became smaller, even portable.

After the sun came water, more practical on dark days. First found in the tomb of Pharoah Amenhotep I, who was buried in 1500 B.C.E., the Greeks later named water clocks "clepsydras" (water thieves). In 325 B.C.E. clepsydras were stone vessels with sloping sides, allowing water to drip from a small hole near the bottom; markings inside indicated smaller units like hours. These became more mechanized over centuries. By 500 C.E. some water clocks rang bells; others opened mini-doors to reveal little figures or moved pointers and dials.

Next came the Far East, where clock development became practically an art form. Third-century Chinese clepsydras illustrated astronomical phenomena. In 1088 C.E. Su Sung built a 30-foot clock tower with a water-driven escapement, automatically rotating celestial globe, and five front panels with doors permitting the viewing of changing manikins which rang gongs and held tablets indicating the hour of day.

But time's revolution was in Europe. After centuries of Europe's technological dormancy, weight-driven mechanical clocks suddenly appeared in Italy without known origin. Still, there was an accuracy problem which a Bavarian-designed spring helped to improve, but only slightly.

Finally in 1656 Dutch scientist Christiaan Huygens made the first pendulum clock, a concept invented by Galileo (who died before he could construct one). Huygens' clock had a record-breaking error of less than one minute per day. He later added the balance wheel and spring assembly still found in some wristwatches today.

By the late 1800s various improvements (mostly in Europe) led to clocks without pendulums, wrist chronometers which gave latitude and longitude, and daily accuracy to within a fraction of a second. One clock design was curiously named a "slave pendulum" - the W. H. Shortt clock in 1921 - because it had two pendulums, one pushing the other to maintain motion.

The more modern quartz timepieces arrived by the 1920s. Using quartz crystals and pressure to generate electricity, a circuit design meant accurate timekeeping at a constant frequency for far longer than previously imagined. Still, quartz has nothing on quantum mechanics.

Enter the atomic age of time: By the 1930s scientists knew that atoms resonate, absorbing and emitting electromagnetic radiation with stability, a potential "pendulum." As technological advances with high-frequency radio waves and microwaves developed, the notion of an atomic clock was born.

In 1949 the U.S. National Institute of Standards and Technology built the world's first atomic beam clock (based on ammonia). It didn't perform much better than anything else, so in 1955 scientists in England switched to cesium with better results. By 1967 NIST, in collaboration with the U.S. Naval Observatory, had established the parameters for a cesium "standard" to measure time about as accurately as the stars themselves.

So what is time? One second is exactly 9,192,631,770 oscillations, or cycles of the cesium atom's resonant frequency. Today, anyway. We are still off by 30 billionths of a second per year, so time will undoubtedly keep evolving.

Sholeh Patrick is a columnist and wanna-be scientist who just wasn't smart enough to make it. Email [email protected]