In general usage today a "clock" refers to any device for measuring and displaying the time
Watches and other timepieces that can be carried on one's person are often distinguished from clocks.
Clock at the Royal Observatory, Greenwich
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
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 using the sun, was widely used in ancient times
A well-constructed 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.
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
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.
Greek astronomer, Andronicus of Cyrrhus, supervised the construction of the Tower of the Winds in Athens in the 1st century B.C.