Clocks

A clock is an instrument for measuring and indicating the time. The word "clock" is derived ultimately (via Dutch, Northern French, and Medieval Latin) from the Celtic words clagan and clocca meaning "bell". more...

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For horologists and other specialists the term "clock" continues to mean exclusively a device with a striking mechanism for announcing intervals of time acoustically, by ringing a bell, a set of chimes, or a gong. A silent instrument lacking such a mechanism has traditionally been known as a timepiece. In general usage today, however, a "clock" refers to any device for measuring and displaying the time which, unlike a watch, is not worn on the person.

History

Further information: History of timekeeping devices

The clock is one of the oldest human inventions, meeting the need to consistently measure intervals of time shorter than the natural units, the day, the lunar month, and the year. Such measurement requires devices. Devices operating on several different physical processes have been used over the millennia, culminating in the clocks of today.

Sundials and other devices

The sundial, which measures the time of day by the direction of shadows cast by the sun, was widely used in ancient times. A well-designed sundial can measure local solar time with reasonable accuracy, and sundials continued to be used to monitor the performance of clocks until the modern era. However, its practical limitations - it requires the sun to shine and does not work at all during the night - encouraged the use of other techniques for measuring time.

Candle clocks and sticks of incense that burn down at, approximately, predictable speeds have also been used to estimate the passing of time. In an hourglass, fine sand pours through a tiny hole at a constant rate and indicates a predetermined passage of an arbitrary period of time.

Water clocks

Water clocks, along with the sundials, are possibly the oldest time-measuring instruments, with the only exceptions being the gnomon and day-counting tally stick.

Given their great antiquity, where and when they first existed are not known and perhaps unknowable. The simplest form of water clocks, the bowl-shaped outflow type, are known to have existed in Babylon and in Egypt around the 16th century B.C.E. Other regions of the world, including India and China, also have early evidence of water clocks but the earliest dates are less certain. Some authors, however, write about water clocks appearing as early as 4000 B.C.E..

Read more at Wikipedia.org