Modern clocks are harmonic oscillators in a box
A clock measures and shows time, one of humanity's oldest inventions for dividing the day into smaller pieces. Sundials and water clocks came first. The word itself means bell: horologists once reserved clock for machines that struck the hours aloud and called silent ones timepieces.
English got the word from Middle Dutch and Middle Low German klocke, since clocks reached England from the Low Countries, and it goes back to medieval Latin clocca, a bell. Every modern clock keeps time with something that oscillates at a steady rate: a swinging pendulum, a sprung balance wheel, a vibrating fork or crystal of quartz, or electrons in atoms, which are regular enough to define the second. Faces with hands are analogue, numerals digital, and speaking clocks announce the time for blind users and telephone callers.
Water clocks and sundials are probably the oldest timekeepers after tally sticks. Simple bowls that drained at a steady rate were in use by Babylonians and Egyptians some 3,500 years ago, and India and China had early versions. In the 1st century BC Andronicus of Cyrrhus built Athens' Tower of the Winds with a large water clock inside and sundials outside, a kind of early clock tower. Ancient societies used such devices mostly for astrology, not for scheduling work. Candle clocks, incense clocks and hourglasses measured intervals by burning or flowing at a constant pace.
Harun al-Rashid, the Abbasid caliph, sent Charlemagne an elephant named Abul-Abbas and an elaborate water clock around 797. In China the Tang monk Yi Xing and official Liang Lingzan fitted an escapement to a water-driven armillary sphere in about 723, the first clockwork escapement. Su Song's astronomical clock tower at Kaifeng, finished in 1088, used a driving wheel 11 feet across with 36 scoops filled from a constant-level tank, building on Zhang Sixun's mercury-driven design of 976. In the 12th century al-Jazari built elephant, scribe and castle clocks for the Artuqid court.
Europe's leap came with the verge escapement, which made mechanical clocks possible around 1300. Pope Sylvester II had brought clocks to northern and western Europe about 1000, spring-driven clocks followed in the 15th century, and clockmaking flourished through the 15th and 16th centuries.
Source: Clock
