Measuring Time

It is now possible to buy a watch or clock that synchronizes itself with the United States Naval Observatory.  More than fifty clocks at the Naval Observatory measure the frequency emitted by atoms of the metallic element cesium.  These atomic clocks are accurate to within one second every 1,400,000 years.  In fact, our ability to measure time is more accurate than the stability of the earth.  A “leap second” is added to the clock in many years because ocean tides are causing the earth to turn slightly slower.

Humankind first measured the time of day with sundials, but sundials were usable only on sunny days.  In ancient Rome, lawyers would be scheduled to speak in the Forum ante medium, before the run reaches its highest point, or post medium, after the sun reaches its highest point.  Today the abbreviations a.m. and p.m. reflect this practice.

The first mechanical clocks in Europe had no numbers.  Most people couldn’t read, and the technology to create a clock face was not initially in place.  Clocks would count out the hour by ringing a large bell high in a tower.  The term o’clock is a contraction for “of the clock.”

The Romans borrowed the seven-day week from the Hebrews. The week is associated with the Old Testament, where God created the world in six days and rested on the seventh. The Romans once had an eight-day week, but shortened their week when they adopted Christianity.

Our months correspond with the phases of the moon.  A full moon occurs every 27 days, but in time we have adjusted the calendar in order make twelve months equal to the solar year. 

Leap Seconds

An extra second was added to the clock 59 seconds after 11:59 on the following dates:

December 31, 2008

December 31, 2006

December 31, 1998

June 30, 1997

December 31, 1995

Resources:

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