Time is a concept that has long perplexed philosophers and scientists. Time, space, matter, and energy are all bound together to make the fabric of our universe. We are embedded in time, and cannot think or speak without reference to time. The exact definition of time may be elusive, but we are all interested in measuring and marking time. So how do we tell time? How did our ancestors mark time? The science of timekeeping is called horology.
Time has been measured, divided, and subdivided into many units - nanoseconds, microseconds, milliseconds, seconds, minutes, hours, days, weeks, lunar months, solar months, seasons, years, decades, centuries, millennia, stages, epochs, periods, eras, and eons. Time has been marked by the crowing of roosters, sunrise, sunset, and midday. Many clever inventions have been devised to keep time, including sandglasses, water clocks, and graduated oil lamps. Then there were pendula clocks; and spring, battery, and electric clocks or watches. Today, precise time is kept by a number of atomic clocks at governmental agencies such as the U.S. Naval Observatory in Washington, D.C. Since 1967, an atomic second has been defined as the interval of time it takes a cesium-133 atom to "vibrate" (i.e., release radiation during electron transitions between energy levels) 9,192,631,770 times.
Atomic clocks were invented in an attempt to control for the variations in solar time. But even with their precision and constancy, it is still necessary to periodically add "leap seconds" to atomic clocks to compensate for the Earth's gradually slowing rotation (spin) and lengthening of the day (due to tidal friction), so that our clocks and calendars remain close together. On average, one leap second is added about every year to year and a half.
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