The history of timekeeping devices dates back to when ancient civilizations first observed astronomical bodies as they moved across the sky. Devices and methods for keeping time have gradually improved through a series of new inventions, starting with measuring time by continuous processes, such as the flow of liquid in water clocks, to mechanical clocks, and eventually repetitive, oscillatory processes, such as the swing of pendulums. Oscillating timekeepers are used in modern timepieces. Sundials and water clocks were first used in ancient Egypt BC and later by the Babylonians, the Greeks and the Chinese. Incense clocks were being used in China by the 6th century. In the medieval period, Islamic water clocks were unrivalled in their sophistication until the mid-14th century. The hourglass, invented in Europe, was one of the few reliable methods of measuring time at sea.
In medieval Europe, purely mechanical clocks were developed after the invention of the bell-striking alarm, used to signal the correct time to ring monastic bells. The weight-driven mechanical clock controlled by the action of a verge and foliot was a synthesis of earlier ideas from European and Islamic science. Mechanical clocks were a major breakthrough, one notably designed and built by Henry de Vick in , which established basic clock design for the next 300 years. Minor developments were added, such as the invention of the mainspring in the early 15th century, which allowed small clocks to be built for the first time.
The next major improvement in clock building, from the 17th century, was the discovery that clocks could be controlled by harmonic oscillators. Leonardo da Vinci had produced the earliest known drawings of a pendulum in 14931494, and in 1582 Galileo Galilei had investigated the regular swing of the pendulum, discovering that frequency was only dependent on length, not weight. The pendulum clock, designed and built by Dutch polymath Christiaan Huygens in 1656, was so much more accurate than other kinds of mechanical timekeepers that few verge and foliot mechanisms have survived. Other innovations in timekeeping during this period include inventions for striking clocks, the repeating clock and the deadbeat escapement.
Error factors in early pendulum clocks included temperature variation, a problem tackled during the 18th century by the English clockmakers John Harrison and George Graham. Following the Scilly naval disaster of 1707, after which governments offered a prize to anyone who could discover a way to determine longitude, Harrison built a succession of accurate timepieces, introducing the term chronometer. The electric clock, invented in 1840, was used to control the most accurate pendulum clocks until the 1940s, when quartz timers became the basis for the precise measurement of time and frequency. The wristwatch, which had been recognised as a valuable military tool during the Boer War, became popular after World War I, in variations including non-magnetic, battery-driven, and solar powered, with quartz, transistors and plastic parts all introduced. Since the early 2010s, smartphones and smartwatches have become the most common timekeeping devices. The most accurate timekeeping devices in practical use today are atomic clocks, which can be accurate to a few billionths of a second per year and are used to calibrate other clocks and timekeeping instruments.
Continuous timekeeping devices
Ancient civilizations observed astronomical bodies, often the Sun and Moon, to determine time. According to the historian Eric Bruton, Stonehenge is likely to have been the Stone Age equivalent of an astronomical observatory, used for seasonal and annual events such as equinoxes or solstices. As megalithic civilizations left no recorded history, little is known of their timekeeping methods. The Warren Field calendar monument in Scotland is currently considered to be the oldest lunisolar calendar yet found.
Mesoamericans modified their usual vigesimal (base-20) counting system when dealing with calendars to produce a 360-day year. Aboriginal Australians understood the movement of objects in the sky well, and used their knowledge to construct calendars and aid navigation; most Aboriginal cultures had seasons that were well-defined and determined by natural changes throughout the year, including celestial events. Lunar phases were used to mark shorter periods of time; the Yaraldi of South Australia being one of the few people recorded as having a way to measure time during the day, which was divided into seven parts using the position of the Sun.
All timekeepers before the 13th century relied upon methods that used something that moved continuously. No early method of keeping time changed at a steady rate. Devices and methods for keeping time have improved continuously through a long series of new inventions and ideas.
Shadow clocks and sundials
The first devices used for measuring the position of the Sun were shadow clocks, which later developed into the sundial. The oldest known sundial dates back to BC (during the 19th Dynasty), and was discovered in the Valley of the Kings in 2013. Obelisks could indicate whether it was morning or afternoon, as well as the summer and winter solstices. A kind of shadow clock was developed BC that was similar in shape to a bent T-square. It measured the passage of time by the shadow cast by its crossbar, and was oriented eastward in the mornings, and turned around at noon, so it could cast its shadow in the opposite direction.
A sundial is referred to in the Bible, in 2 Kings 20:911, when Hezekiah, king of Judea during the 8th century BC, is recorded as being healed by the prophet Isaiah and asks for a sign that he would recover:
A clay tablet from the late Babylonian period describes the lengths of shadows at different times of the year. The Babylonian writer Berossos () is credited by the Greeks with the invention of a hemispherical sundial hollowed out of stone; the path of the shadow was divided into 12 parts to mark the time. Greek sundials evolved to become highly sophisticated—Ptolemy's Analemma, written in the 2nd century AD, used an early form of trigonometry to derive the position of the Sun from data such as the hour of day and the geographical latitude.
The Romans inherited the sundial from the Greeks. The first sundial in Rome arrived in 264 BC, looted from Catania in Sicily. This sundial offered the innovation of the hours of the "horologium" throughout the day where before the Romans simply split the day into early morning and forenoon (mane and ante merididiem). Still, there were unexpected astronomical challenges; this clock gave the incorrect time for a century. This mistake was noticed only in 164 BC, when the Roman censor came to check and adjusted for the appropriate latitude.
According to the German historian of astronomy Ernst Zinner, sundials were developed during the 13th century with scales that showed equal hours. The first based on polar time appeared in Germany ; an alternative theory proposes that a Damascus sundial measuring in polar time can be dated to 1372. European treatises on sundial design appeared .
An Egyptian method of determining the time during the night, used from at least 600 BC, was a type of plumb-line called a merkhet. A north–south meridian was created using two merkhets aligned with Polaris, the north pole star. The time was determined by observing particular stars as they crossed the meridian.
The Jantar Mantar in Jaipur built in 1727 by Jai Singh II includes the Vrihat Samrat Yantra, 88 feet (27 m) tall sundial. It can tell local time to an accuracy of about two seconds.
Water clocks
The oldest description of a clepsydra, or water clock, is from the tomb inscription of an early 18th Dynasty ( BC) Egyptian court official named Amenemhet, who is identified as its inventor. It is assumed that the object described on the inscription is a bowl with markings to indicate the time. The oldest surviving water clock was found in the tomb of pharaoh Amenhotep III (1379 BC). There are no recognised examples in existence of outflowing water clocks from ancient Mesopotamia, but written references have survived.
The introduction of the water clock to China, perhaps from Mesopotamia, occurred as far back as the 2nd millennium BC, during the Shang dynasty, and at the latest by the 1st millennium BC. Around 550 AD, Yin Kui (殷蘷) was the first in China to write of the overflow or constant-level tank in his book "Lou ke fa (漏刻法)". Around 610, two Sui dynasty inventors, Geng Xun (耿詢) and Yuwen Kai (宇文愷), created the first balance clepsydra, with standard positions for the steelyard balance. In 721 the mathematician Yi Xing and government official Liang Lingzan regulated the power of the water driving an astronomical clock, dividing the power into unit impulses so that motion of the planets and stars could be duplicated. In 976, the Song dynasty astronomer Zhang Sixun addressed the problem of the water in clepsydrae freezing in cold weather when he replaced the water with liquid mercury. A water-powered astronomical clock tower was built by the polymath Su Song in 1088, which featured the first known endless power-transmitting chain drive.
The Greek philosophers Anaxagoras and Empedocles both referred to water clocks that were used to enforce time limits or measure the passing of time. The Athenian philosopher Plato is supposed to have invented an alarm clock that used lead balls cascading noisily onto a copper platter to wake his students.
A problem with most clepsydrae was the variation in the flow of water due to the change in fluid pressure, which was addressed from 100 BC when the clock's water container was given a conical shape. They became more sophisticated when innovations such as gongs and moving mechanisms were included. There is strong evidence that the 1st century BC Tower of the Winds in Athens once had a water clock, and a wind vane, as well as the nine vertical sundials still visible on the outside. In Greek tradition, clepsydrae were used in court, a practise later adopted by the Ancient Romans.
Ibn Khalaf al-Muradi in medieval Al-Andalus described a water clock that employed both segmental and epicyclic gearing. Islamic water clocks, which used complex gear trains and included arrays of automata, were unrivalled in their sophistication until the mid-14th century. Liquid-driven mechanisms (using heavy floats and a constant-head system) were developed that enabled water clocks to work at a slower rate. Some have argued that the first known geared clock was rather invented by the great mathematician, physicist, and engineer Archimedes during the 3rd century BC. Archimedes created his astronomical clock, which was also a cuckoo clock with birds singing and moving every hour. It is the first carillon clock as it plays music simultaneously with a person blinking his eyes, surprised by the singing birds. The Archimedes clock works with a system of four weights, counterweights, and strings regulated by a system of floats in a water container with siphons that regulate the automatic continuation of the clock. The principles of this type of clock are described by the mathematician and physicist Hero, who says that some of them work with a chain that turns a gear in the mechanism.
The 12th-century Jayrun Water Clock at the Umayyad Mosque in Damascus was constructed by Muhammad al-Sa'ati, and was later described by his son Ridwan ibn al-Sa'ati in his On the Construction of Clocks and their Use (1203). A sophisticated water-powered astronomical clock was described by Al-Jazari in his treatise on machines, written in 1206. This castle clock was about high. In 1235, a water-powered clock that "announced the appointed hours of prayer and the time both by day and by night" stood in the entrance hall of the Mustansiriya Madrasah in Baghdad.
Chinese incense clocks