The accurate measurement of time has been one of the most challenging problems in human history. We now tend to take for granted the civil time-keeping system in general use. This system has evolved over many centuries and from time to time has been substantially revised. The original definitions of day, month, and year depended, respectively, on observations of the periodic motions of the Earth, Moon, and Sun with respect to the celestial background as observed from Earth. Since all these motions have fluctuations in their periods, it is not possible to define a completely regular unit on which to base an accurate time measurement in terms of the day, month, or year.

The first time-keeping instrument that did not depend on celestial observation was the pendulum clock. It did, however, depend on the Earth's gravity, which varies with geographic location and the positions of the Sun and Moon. The second, originally defined as 1/(24 x 60 x 60) of a day, more recently has been redefined in terms of the microwave emissions of certain atoms (e.g., quartz crystals). This new definition provides a uniform standard with which to measure intervals of time.