Clock

History
Further information: History of timekeeping devices
Replica of an ancient Chinese incense clock

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.

[edit] 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, it has 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. In India, the King of Jaipur the Pink City, Jai Singh II constructed many instruments and sundials in the observatories in cities Jaipur, Varanasi, Ujjain, Mathura between 1724-1730 A.D. He had a good interest in astronomy and town planning.

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.

[edit] Water clocks

Main article: Water clock

A scale model of Su Song's Astronomical Clock Tower, built in 11th century Kaifeng, China. It was driven by a large waterwheel, chain drive, and escapement mechanism.

Water clocks, also known as clepsydrae (sg: clepsydra), along with the sundials, are possibly the oldest time-measuring instruments, with the only exceptions being the vertical gnomon and the day-counting tally stick.[2] Given their great antiquity, where and when they first existed are not known and perhaps unknowable. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BC. 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 BC in these regions of the world.[3]