The sundial, which measures the time of day by using the sun casting a shadow onto a cylindrical stone, 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.

Water clocks

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.[3] 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, includingIndia 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.[4]

Greek astronomer, Andronicus of Cyrrhus, supervised the construction of the Tower of the Winds in Athens in the 1st century B.C.[5]

The Greek and Roman civilizations are credited for initially advancing water clock design to include complex gearing,[6][dead link] which was connected to fanciful automata and also resulted in improved accuracy. These advances were passed on throughByzantium and Islamic times, eventually making their way back to Europe. Independently, the Chinese developed their own advanced water clocks（水鐘）in 725 A.D., passing their ideas on to Korea and Japan.

Some water clock designs were developed independently and some knowledge was transferred through the spread of trade. Pre-modern societies do not have the same precise timekeeping requirements that exist in modern industrial societies, where every hour of work or rest is monitored, and work may start or finish at any time regardless of external conditions. Instead, water clocks in ancient societies were used mainly for astrological reasons. These early water clocks were calibrated with a sundial. While never reaching the level of accuracy of a modern timepiece, the water clock was the most accurate and commonly used timekeeping device for millennia, until it was replaced by the more accurate pendulum clock in 17th century Europe.

Islamic civilization is credited with further advancing the accuracy of clocks with elaborate engineering. In 797 (or possibly 801), the Abbasid caliph of Baghdad, Harun al-Rashid, presented Charlemagne with anAsian Elephant named Abul-Abbas together with a "particularly elaborate example" of a water[7] clock.

Early mechanical clocks

None of the first clocks survived from 13th century Europe, but various mentions in church records reveal some of the early history of the clock.

The word horologia (from the Greek ὡρα, hour, and λέγειν, to tell) was used to describe all these devices,[9] but the use of this word (still used in several Romance languages) for all timekeepers conceals from us the true nature of the mechanisms. For example, there is a record that in 1176 Sens Cathedral installed a ‘horologe’[citation needed] but the mechanism used is unknown. According to Jocelin of Brakelond, in 1198 during a fire at the abbey of St Edmundsbury (now Bury St Edmunds), the monks 'ran to the clock' to fetch water, indicating that their water clock had a reservoir large enough to help extinguish the occasional fire.[10]

Specific types of clocks

By mechanism, By function, By style, Astronomical clock, Atomic clock, Candle clock, Congreve clock, Digital clock, Electric clock, Flip clock, Hourglass, Incense clock, Mechanical watch, Oil-lamp clock, Pendulum clock, Pipe organ clock, Projection clock, Quantum clock, Quartz clock, Radio clock, Rolling ball clock, Spring drive watch, Steam clock, Sundial, Torsion pendulum clock, Water clock, 10-hour clock, Alarm clock, Binary clock, Chiming clock, Chronometer watch, Cuckoo clock, Game clock, Japanese clock, Master clock, Musical clock, Railroad chronometer, Slave clock, Speaking clock, Stopwatch, Striking clock, Talking clock, Tide clock, Time ball, Time clock, World clock, American clock, Automaton clock, Balloon clock, Banjo clock, Bracket clock, Carriage clock, Cartel clock, Cat clock, Clock tower, Cuckoo clock, Doll's head clock, Floral clock, French Empire mantel clock, Granddaughter clock, Grandfather clock, Grandmother clock, Lantern clock, Lighthouse clock, Longcase clock (or tall-case clock), Mantel clock, Skeleton clock, Tower clock, Turret clock, Watch,