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For thousands of years, devices have been used to measure and keep track of time. The current sexagesimal system of time measurement dates to approximately 2000 BC, in Sumer. The Ancient Egyptians divided the day into two 12-hour periods, and used large obelisks to track the movement of the Sun. They also developed water clocks, which were probably first used in the Precinct of Amun-Re, and later outside Egypt as well; they were employed frequently by the Ancient Greeks, who called them clepsydrae. The Shang Dynasty is believed to have used the outflow water clock around the same time; the clocks were introduced from Mesopotamia, possibly as early as 2000 BC. Other ancient timekeeping devices include the candle clock, used in China, Japan, England and Iraq; the timestick, widely used in India and Tibet, as well as some parts of Europe; and the hourglass, which functioned similarly to a water clock.

The earliest clocks relied on shadows cast by the sun, so they were not useful in cloudy weather or at night, and required recalibration as the seasons changed if the gnomon was not aligned with the Earth's axis. The first clock with an escapement mechanism, which transferred rotational energy into intermittent motions,[1] dates back to 3rd century BC Ancient Greece,[2] and Arabic engineers invented water clocks driven by gears and weights in the 11th century.[3]

Mechanical clocks employing the verge escapement mechanism were invented in Europe at the turn of the 14th century, and became the standard timekeeping device until the spring-powered clock and pocket watch in the 16th century, followed by the pendulum clock in the 18th century. During the 20th century, quartz oscillators were invented, followed by atomic clocks. Although first used in laboratories, quartz oscillators were both easy to produce and accurate, leading to their use in wristwatches. Atomic clocks are far more accurate than any previous timekeeping device, and are used to calibrate other clocks and to calculate the proper time on Earth; a standardized civil system, Coordinated Universal Time, is based on atomic time.
Early timekeeping devices
See also: History of calendars
 
The sun rising over Stonehenge on the June solstice

Many ancient civilizations observed astronomical bodies, often the Sun and Moon, to determine times, dates, and seasons.[4][5] Methods of sexagesimal timekeeping, now common in Western society, first originated nearly 4,000 years ago in Mesopotamia and Egypt;[4][6][7] a similar system was developed later in Mesoamerica.[8] The first calendars may have been created during the last glacial period, by hunter-gatherers who employed tools such as sticks and bones to track the phases of the moon or the seasons.[5] Stone circles, such as England's Stonehenge, were built in various parts of the world, especially in Prehistoric Europe, and are thought to have been used to time and predict seasonal and annual events such as equinoxes or solstices.[5][9] As those megalithic civilizations left no recorded history, little is known of their calendars or timekeeping methods.[10]

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3500 BC – 500 BC
See also: History of timekeeping devices in Egypt

The shadow clock was the first device able to measure the individual hours of a day.[11] The oldest known shadow clock is from Egypt, and was made from green schist. Ancient Egyptian obelisks, constructed about 3500 BC, are also among the earliest shadow clocks.[5][12][13]
 
The Luxor Obelisk in Place de la Concorde, Paris, France

Egyptian shadow clocks divided daytime into 10 parts, with an additional four "twilight hours"—two in the morning, and two in the evening. One type of shadow clock consisted of a long stem with five variable marks and an elevated crossbar which cast a shadow over those marks. It was positioned eastward in the morning, and was turned west at noon. Obelisks functioned in much the same manner: the shadow cast on the markers around it allowed the Egyptians to calculate the time. The obelisk also indicated whether it was morning or afternoon, as well as the summer and winter solstices.[5][14] A third shadow clock, developed c. 1500 BC, was similar in shape to a bent T-square. It measured the passage of time by the shadow cast by its crossbar on a non-linear rule. The T was oriented eastward in the mornings, and turned around at noon, so that it could cast its shadow in the opposite direction.[15]

Although accurate, shadow clocks relied on the sun, and so were useless at night and in cloudy weather.[14][16] The Egyptians therefore developed a number of alternative timekeeping instruments, including water clocks, hourglasses, and a system for tracking star movements. The oldest description of a water clock is from the tomb inscription of the 16th-century BC Egyptian court official Amenemhet, identifying him as its inventor.[17] There were several types of water clocks, some more elaborate than others. One type consisted of a bowl with small holes in its bottom, which was floated on water and allowed to fill at a near-constant rate; markings on the side of the bowl indicated elapsed time, as the surface of the water reached them. The oldest-known waterclock was found in the tomb of pharaoh Amenhotep I (1525–1504 BC), suggesting that they were first used in ancient Egypt.[14][18][19] The ancient Egyptians are also believed to be the inventors of the hourglass, which consisted of two vertically aligned glass chambers connected by a small opening. When the hourglass was turned over, grains of sand fell at a constant rate from one chamber to the other.[16] Another Egyptian method of determining the time during the night was using plumb-lines called merkhets. In use since at least 600 BC, two of these instruments were aligned with Polaris, the north pole star, to create a north–south meridian. The time was accurately measured by observing certain stars as they crossed the line created with the merkhets.[14][20]

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500 BC – 1 BC
 
Ctesibius's clepsydra from the 3rd century BC. Clepsydra, literally water thief, was the Greek word for water clock.[21]

Water clocks, or clepsydrae, were commonly used in Ancient Greece following their introduction by Plato, who also invented a water-based alarm clock.[22][23] One account of Plato's alarm clock describes it as depending on the nightly overflow of a vessel containing lead balls, which floated in a columnar vat. The vat held a steadily increasing amount of water, supplied by a cistern. By morning, the vessel would have floated high enough to tip over, causing the lead balls to cascade onto a copper platter. The resultant clangor would then awaken Plato's students at the Academy.[24] Another possibility is that it comprised two jars, connected by a siphon. Water emptied until it reached the siphon, which transported the water to the other jar. There, the rising water would force air through a whistle, sounding an alarm.[23] The Greeks and Chaldeans regularly maintained timekeeping records as an essential part of their astronomical observations.

Greek astronomer, Andronicus of Cyrrhus, supervised the construction of the Tower of the Winds in Athens in the 1st century B.C.[25]

In Greek tradition, clepsydrae were used in court; later, the Romans adopted this practice, as well. There are several mentions of this in historical records and literature of the era; for example, in Theaetetus, Plato says that "Those men, on the other hand, always speak in haste, for the flowing water urges them on".[26] Another mention occurs in Lucius Apuleius' The Golden Ass: "The Clerk of the Court began bawling again, this time summoning the chief witness for the prosecution to appear. Up stepped an old man, whom I did not know. He was invited to speak for as long as there was water in the clock; this was a hollow globe into which water was poured through a funnel in the neck, and from which it gradually escaped through fine perforations at the base".[27] The clock in Apuleius' account was one of several types of water clock used. Another consisted of a bowl with a hole in its centre, which was floated on water. Time was kept by observing how long the bowl took to fill with water.[28]

Although clepsydrae were more useful than sundials—they could be used indoors, during the night, and also when the sky was cloudy—they were not as accurate; the Greeks, therefore, sought a way to improve their water clocks.[29] Although still not as accurate as sundials, Greek water clocks became more accurate around 325 BC, and they were adapted to have a face with an hour hand, making the reading of the clock more precise and convenient. One of the more common problems in most types of clepsydrae was caused by water pressure: when the container holding the water was full, the increased pressure caused the water to flow more rapidly. This problem was addressed by Greek and Roman horologists beginning in 100 BC, and improvements continued to be made in the following centuries. To counteract the increased water flow, the clock's water containers—usually bowls or jugs—were given a conical shape; positioned with the wide end up, a greater amount of water had to flow out in order to drop the same distance as when the water was lower in the cone. Along with this improvement, clocks were constructed more elegantly in this period, with hours marked by gongs, doors opening to miniature figurines, bells, or moving mechanisms.[14] There were some remaining problems, however, which were never solved, such as the effect of temperature. Water flows more slowly when cold, or may even freeze. [30]

Although the Greeks and Romans did much to advance water clock technology, they still continued to use shadow clocks. The mathematician and astronomer Theodosius of Bithynia, for example, is said to have invented a universal sundial that was accurate anywhere on Earth, though little is known about it.[31] Others wrote of the sundial in the mathematics and literature of the period. Marcus Vitruvius Pollio, the Roman author of De Architectura, wrote on the mathematics of gnomons, or sundial blades.[32] During the reign of Emperor Augustus, the Romans constructed the largest sundial ever built, the Solarium Augusti. Its gnomon was an obelisk from Heliopolis.[33] Similarly, the obelisk from Campus Martius was used as the gnomon for Augustus' zodiacal sundial.[34] Pliny the Elder records that the first sundial in Rome arrived in 264 BC, looted from Catania, Sicily; according to him, it gave the incorrect time until the markings and angle appropriate for Rome's latitude were used—a century later.[35]