A brief history of time

Timekeeping has changed dramatically since the Egyptian shadow clocks and the European pocket sundials.

Here's a quick look, courtesy of the National Institute of Standards and Technology (NIST), at how timekeeping has changed in the 5,000 years since Sumerians developed a calendar that divided the year into 30-day months, divided each day into 12 periods (each corresponding to 2 of our hours), and divided these periods into 30 parts (each like 4 of our minutes).

3500 B.C.– Egyptians refine primitive sundial clocks into more accurate shadow clocks that can divide the day into equal parts. By 1500 B.C., one such shadow clock divided a sunlit day into 10 parts and two twilight hours.

900 – Innovative time-watchers in medieval Europe design and produce several different types of pocket sundials. One English model identified tides and compensated for seasonal changes of the sun's altitude.

1300s – The first primitive tower clocks–mechanical devices regulated by weights and escapements–begin to appear on church steeples and public buildings in Italy.

1500 – Peter Henlein of Nuremberg, Germany, is credited with the invention of the spring-powered clock, which permitted development of smaller clocks and watches.

1656 – Dutch scientist Christiaan Huygens comes up with the world's first pendulum clock. It's so accurate that it loses only 10 seconds a day.

1676 – Huygens invents the balance wheel and spring assembly that will power watches until the modern quartz revolution.

1927 – The first quartz clocks are invented. Exploiting the piezoelectric qualities of certain crystals, an electric circuit is used to make the crystal vibrate, creating a constant electric signal. These clocks have no gears or escapements to disturb their regular frequency.

1949 – The world's first atomic clock makes its appearance. After further refinement during the next few years, a cesium-based atomic clock will set a new record for accuracy, by "losing" only one-millionth of a second per year.

For more timely information, see the NIST web site at <http://www.physics.nist.gov/GenInt/Time/ time.html>.