clock
clock Instrument for measuring time. The earliest timekeeping instruments had no moving parts, being designed to measure the movements of the Sun, Moon and stars. Examples include neolithic stone columns, and ancient Egyptian sundials and water clocks. Candle clocks and sandglasses were later types of non-mechanical clocks. The central feature of all mechanical clocks is an escapement mechanism, which enables a clock to tick off time at discrete intervals. This movement is transmitted through a series of gears to the hands, which are pushed forward a small distance with every escapement movement. Motive power for mechanical clocks has been provided variously by falling weights, pendulums and coiled springs. Other modern clocks include those using an electrically oscillated quartz crystal as the basis of time-division. Even more accurate are atomic clocks, which rely upon the natural oscillations of atoms (usually those of the metal, caesium) and which measure time to an accuracy of thousandths of a second per year.
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