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 often 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, 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. The first clock with an escapement, which transferred rotational energy into discrete motions, appeared in China in the 8th century, and Muslim engineers invented weight-driven clocks in the 11th century. Mechanical clocks were introduced to Europe at the turn of the 14th century, and became the standard timekeeping device until the 20th 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.

NAVSTAR GPS was developed by the United States Department of Defense as a military program, intended to improve navigation. Each of the 12 satellites in the GPS system carries an onboard atomic clock synchronized to account for gravitational time dilation. In response to the shooting down of Korean Air Lines Flight 007, President Ronald Reagan issued a directive guaranteeing the free use of GPS for civilian purposes, to prevent further navigation accidents.

Many ancient civilizations observed astronomical bodies, often the Sun and Moon, to determine times, dates, and seasons.^[1][2] Methods of sexagesimal timekeeping, now common in Western society, first originated nearly 4,000 years ago in Mesopotamia and Egypt;^[1][3][4] a similar system was developed later in Mesoamerica.^[5] 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.^[2] Stone circles, such as England's Stonehenge, were built in various parts of the world, especially in Prehistoric Europe, to time and predict seasonal and annual events such as equinoxes or solstices.^[6][2] As those megalithic civilizations left no recorded history, little is known of their calendars or timekeeping methods.^[7]

The shadow clock was the first device able to measure the individual hours of a day.^[8] 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.^[2][9][10]

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.^[2][11] 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.^[12]

Although accurate, shadow clocks relied on the sun, and so were useless at night or in cloudy weather.^[11][13] 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.^[14] 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.^[11][15][16] 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 upside-down or rightside-up, grains of sand fell at a constant rate from one chamber to the other.^[13] 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.^[11][17]

Water clocks were commonly used in Ancient Greece following their introduction by Plato, who also invented a water-based alarm clock.^[19][20] 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.^[21] 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.^[20] The Greeks and Chaldeans regularly maintained timekeeping records as an essential part of their astronomical observations.

In Greek tradition, clepsydrae were used in court; later, the Romans adopted this practice, as well. There are several mentions 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".^[22] 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".^[23] 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.^[24]

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.^[25] 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.^[11] 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. Also, the water clock did not account for variation in the length of day and night during the year, so its accuracy varied depending on the season.^[26]

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.^[27] 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.^[28] During the reign of Emperor Augustus, the Romans constructed the largest sundial ever built, the Solarium Augusti. Its gnomon was an obelisk from Heliopolis.^[29] Similarly, the obelisk from Campus Martius was used as the gnomon for Augustus' zodiacal sundial.^[30] 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.^[31]

Joseph Needham speculated that the introduction of the outflow clepsydra 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. By the beginning of the Han Dynasty, in 202 BC, the outflow clepsydra was gradually replaced by the inflow clepsydra, which featured an indicator rod on a float. To compensate for the falling pressure head in the reservoir, which slowed timekeeping as the vessel filled, Zhang Heng added an extra tank between the reservoir and the inflow vessel. Around AD 550, Yin Gui was the first in China to write of the overflow or constant-level tank added to the series, which was later described in detail by the inventor Shen Kuo. Around 610, this design was trumped by two Sui Dynasty inventors, Geng Xun and Yuwen Kai, who were the first to create the balance clepsydra, with standard positions for the steelyard balance.^[32] Joseph Needham states that:

... [the balance clepsydra] permitted the seasonal adjustment of the pressure head in the compensating tank by having standard positions for the counterweight graduated on the beam, and hence it could control the rate of flow for different lengths of day and night. With this arrangement no overflow tank was required, and the two attendants were warned when the clepsydra needed refilling.^[32]

Some of the most elaborate water clocks were designed by Muslim engineers. In particular, the water clocks by Al-Jazari in 1206 are credited for going "well beyond anything" that had preceded them. In his treatise, he describes one of his water clocks, the elephant clock. The clock recorded the passage of temporal hours, which meant that the rate of flow had to be changed daily to match the uneven length of days throughout the year. To accomplish this, the clock had two tanks: the top tank was connected to the time indicating mechanisms and the bottom was connected to the flow control regulator. At daybreak the tap was opened and water flowed from the top tank to the bottom tank via a float regulator that maintained a constant pressure in the receiving tank.^[33]

Many developments in horology, not limited to water clocks, occurred in China between 200 and 1300.^[11] For example, the first mechanical clock—the first to use escapements—was built in Chang'an, by Tantric monk and mathematician, Yi Xing, and government official Liang Lingzan.^[34][35] An astronomical instrument that served as a clock, it was discussed in a contemporary text as follows:^[36]

[It] was made in the image of the round heavens and on it were shown the lunar mansions in their order, the equator and the degrees of the heavenly circumference. Water, flowing into scoops, turned a wheel automatically, rotating it one complete revolution in one day and night. Besides this, there were two rings fitted around the celestial sphere outside, having the sun and moon threaded on them, and these were made to move in circling orbit ... And they made a wooden casing the surface of which represented the horizon, since the instrument was half sunk in it. It permitted the exact determinations of the time of dawns and dusks, full and new moons, tarrying and hurrying. Moreover, there were two wooden jacks standing on the horizon surface, having one a bell and the other a drum in front of it, the bell being struck automatically to indicate the hours, and the drum being beaten automatically to indicate the quarters. All these motions were brought about by machinery within the casing, each depending on wheels and shafts, hooks, pins and interlocking rods, stopping devices and locks checking mutually.^[36]

Since Yi Xing's clock was a water clock, it was affected by temperature variations. That problem was solved in 976 by Zhang Sixun by replacing the water with mercury, which remains liquid down to −39 °C (−38 °F). Zhang implemented the changes into his clock tower, which was about 10 metres (33 ft) tall, with escapements to keep the clock turning and bells to signal every quarter-hour. Another noteworthy clock, the elaborate Cosmic Engine, was built by Su Song, in 1088. It was about the size of Zhang's tower, but had an automatically rotating armillary sphere—also called a celestial globe—from which the positions of the stars could be observed. It also featured five panels with mannequins ringing gongs or bells, and tablets showing the time of day, or other special times.^[11] Originally built in the capital of Kaifeng, it was dismantled by the Jin army and sent to the capital of Yanjing (now Beijing), where they were unable to put it back together. As a result, Su Song's son Su Xie was ordered to build a replica.^[37]

The first mechanical clocks to be driven by weights and gears were invented by medieval Muslim engineers.^[38][39] The first geared mechanical clock was invented by the 11th-century Arab engineer Ibn Khalaf al-Muradi in Islamic Spain; the first weight-driven mechanical clocks, employing a mercury escapement mechanism and a clock face similar to an astrolabe dial, were also invented by Muslim engineers in the 11th century. A similar weight-driven mechanical clock later appeared in a Spanish language work compiled from earlier Arabic sources for Alfonso X in 1277.^[40] The knowledge of weight-driven mechanical clocks produced by Muslim engineers in Spain was transmitted to other parts of Europe through Latin translations of Arabic and Spanish texts on Muslim mechanical technology.^[40][41]

It is not known specifically where and when candle clocks were first used; however, their earliest mention comes from a Chinese poem, written in 520 by You Jiangu. According to the poem, the graduated candle was a means of determining time at night. Similar candles were used in Japan until the early 10th century.^[42]

The candle clock most commonly mentioned and written of is attributed to King Alfred the Great. It consisted of six candles made from 72 pennyweights of wax, each 12 inches (30 cm) high, and of uniform thickness, marked every inch (2.5 cm). As these candles burned for about four hours, each mark represented 20 minutes. Once lit, the candles were placed in wooden framed glass boxes, to prevent the flame from extinguishing.^[43]

The most sophisticated candle clocks of their time were those of Al-Jazari in 1206. Donald Routledge Hill described one of al-Jazari's candle clocks as follows:^[44]

The candle, whose rate of burning was known, bore against the underside of the cap, and its wick passed through the hole. Wax collected in the indentation and could be removed periodically so that it did not interfere with steady burning. The bottom of the candle rested in a shallow dish that had a ring on its side connected through pulleys to a counterweight. As the candle burned away, the weight pushed it upward at a constant speed. The automata were operated from the dish at the bottom of the candle.

In addition to water, mechanical, and candle clocks, incense clocks were used in the Far East, and were fashioned in several different forms.^[45]

Incense clocks were first used in China around the 6th century; in Japan, one still exists in the Shōsōin,^[46] although its characters are not Chinese, but Devanagari.^[47] Due to their frequent use of Devanagari characters, suggestive of their use in Buddhist ceremonies, Edward H. Schafer speculated that incense clocks were invented in India.^[47] Although similar to the candle clock, incense clocks burned evenly and without a flame; therefore, they were more accurate and safer for indoor use.^[48] Several types of incense clock have been found, the most common being the incense stick and incense seal ones.^[49][50] An incense stick clock was an incense stick with calibrations;^[50] most were elaborate, sometimes having threads, with weights attached, at even intervals. The weights would drop onto a platter or gong below, signifying that a certain amount of time had elapsed. Some incense clocks were held in elegant trays; open-bottomed trays were also used, to allow the weights to be used together with the decorative tray.^[51][52] Sticks of incense with different scents were also used, so that the hours were marked by a change in fragrance.^[53] The incense sticks could be straight or spiraled; the spiraled ones were longer, and were therefore intended for long periods of use, and often hung from the roofs of homes and temples.^[54] In Japan, a geisha was paid for the number of senkodokei (incense sticks) that had been consumed while she was present, a practice which continued until 1924.^[55] Incense seal clocks were used for similar occasions and events as the stick clock; while religious purposes were of primary importance,^[49] these clocks were also popular at social gatherings, and were used by Chinese scholars and intellectuals.^[56] The seal was a wooden or stone disk with one or more grooves etched in it^[49] into which incense was placed.^[57] These clocks were common in China,^[56] but were produced in fewer numbers in Japan.^[58] To signal the passage of a specific amount of time, small pieces of fragrant woods, resins, or different scented incenses could be placed on the incense powder trails. Different powdered incense clocks used different formulations of incense, depending on how the clock was laid out.^[59] The length of the trail of incense, directly related to the size of the seal, was the primary factor in determining how long the clock would last; all burned for long periods of time, ranging between 12 hours and a month.^[60][61][62] While early incense seals were made of wood or stone, the Chinese gradually introduced disks made of metal, most likely beginning during the Song dynasty. This allowed craftsmen to more easily create both large and small seals, as well as design and decorate them more aesthetically. Another advantage was the ability to vary the paths of the grooves, to allow for the changing length of the days in the year. As smaller seals became more readily available, the clocks grew in popularity among the Chinese, and were often given as gifts.^[63] Incense seal clocks are often sought by modern-day clock collectors; however, few remain that have not already been purchased or been placed on display at museums or temples.^[58]

During the 11th-century Song Dynasty, the Chinese astronomer, horologist and mechanical engineer Su Song created a water-driven astronomical clock for his clock-tower of Kaifeng City. It incorporated an escapement mechanism as well as the earliest known endless power-transmitting chain drive, which drove the armillary sphere.

Contemporary Muslim astronomers also constructed a variety of highly accurate astronomical clocks for use in their mosques and observatories,^[41] such as the timekeeping astrolabe by Abd al-Rahman al-Sufi (Azophi) in the 10th century,^[64] the water-powered astronomical clock by Al-Jazari in 1206,^[65][66] and the astrolabic clock by Ibn al-Shatir in the early 14th century.^[67]

Sundials were further developed by Muslim astronomers. As the ancient dials were nodus-based with straight hour-lines, they indicated unequal hours—also called temporary hours—that varied with the seasons. Every day was divided into 12 equal segments regardless of the time of year; thus, hours were shorter in winter and longer in summer. The idea of using hours of equal length throughout the year was the innovation of Abu'l-Hasan Ibn al-Shatir in 1371, based on earlier developments in trigonometry by Muhammad ibn Jābir al-Harrānī al-Battānī (Albategni). Ibn al-Shatir was aware that "using a gnomon that is parallel to the Earth's axis will produce sundials whose hour lines indicate equal hours on any day of the year". His sundial is the oldest polar-axis sundial still in existence. The concept appeared in Western sundials starting in 1446.^[68][69]

Following the acceptance of heliocentrism and equal hours, as well as advances in trigonometry, sundials appeared in their present form during the Renaissance, when they were built in large numbers.^[70] In 1524, the French astronomer Oronce Finé constructed an ivory sundial, which still exists;^[71] later, in 1570, the Italian astronomer Giovanni Padovani published a treatise including instructions for the manufacture and laying out of mural (vertical) and horizontal sundials. Similarly, Giuseppe Biancani's Constructio instrumenti ad horologia solaria (c. 1620) discusses how to construct sundials.^[72]

Ferdinand Magellan used 18 hourglasses on each ship during his circumnavigation of the globe in 1522.^[73] Since the hourglass was one of the few reliable methods of measuring time at sea, it is speculated that it had been used on board ships as far back as the 11th century, when it would have complemented the magnetic compass as an aid to navigation. However, the earliest evidence of their use appears in the painting Allegory of Good Government, by Ambrogio Lorenzetti, from 1338.^[74] From the 15th century onwards, hourglasses were used in a wide range of applications at sea, in churches, in industry, and in cooking; they were the first dependable, reusable, reasonably accurate, and easily constructed time-measurement devices. The hourglass also took on symbolic meanings, such as that of death, temperance, opportunity, and Father Time, usually represented as a bearded, old man.^[75] Though also used in China, the hourglass's history there is unknown.^[76]

Clocks encompass a wide spectrum of devices, ranging from wristwatches to the Clock of the Long Now. The English word clock is said to derive from the Middle English clokke, Old North French cloque, or Middle Dutch clocke, all of which mean bell, and are derived from the Medieval Latin clocca, also meaning bell.^[77][78][79] Indeed, bells were used to mark the passage of time; they marked the passage of the hours at sea and in abbeys.

Throughout history, clocks have had a variety of power sources, including gravity, springs, and electricity.^[80][81] The invention of mechanical clockwork itself is usually credited to the Chinese official Liang Lingzan and monk Yi Xing.^[35][34][82] However, mechanical clocks were not widely used in the West until the 14th century. Clocks were used in medieval monasteries to keep the regulated schedule of prayers. The clock continued to be improved, and the first pendulum clock was designed and built in the 17th century by Christiaan Huygens, a Dutch scientist.

The first mechanical alarm clock was invented by the Ottoman engineer Taqi al-Din. He described the alarm clock in his book, The Brightest Stars for the Construction of Mechanical Clocks (Al-Kawākib al-durriyya fī wadh' al-bankāmat al-dawriyya), published in 1559. His alarm clock was capable of sounding at a specified time, achieved by placing a peg on the dial wheel. At the requested time, the peg activated a ringing device.^[83] In the same treatise, he described a mechanical astronomical clock called the "observational clock", which was the first to measure time in minutes. He made use of his mathematical knowledge to design three dials which showed the hours, degrees and minutes.^[83] He later improved the design of his observational clock to measure time in seconds in an astronomical treatise written at his Istanbul observatory of al-Din (1577–1580). He described his observational clock as "a mechanical clock with three dials which show the hours, the minutes, and the seconds". This was an important innovation in 16th-century practical astronomy, as previous clocks were not accurate enough to be used for astronomical purposes.^[84] He further improved the observational clock to use only one dial face, describing it as "a mechanical clock with a dial showing the hours, minutes and seconds and we divided every minute into five seconds".^[85]

The earliest medieval European clockmakers were Christian monks.^[86] Medieval religious institutions required clocks because daily prayer and work schedules were strictly regulated. This was done by various types of time-telling and recording devices, such as water clocks, sundials and marked candles, probably used in combination.^[87][81] When mechanical clocks were used, they were often wound at least twice a day to ensure accuracy.^[88] Important times and durations were broadcast by bells, rung either by hand or by a mechanical device, such as a falling weight or rotating beater.

The religious necessities and technical skill of the medieval monks were crucial factors in the development of clocks, as the historian Thomas Woods writes:

The monks also counted skillful clock-makers among them. The first recorded clock was built by the future Pope Sylvester II for the German town of Magdeburg, around the year 996. Much more sophisticated clocks were built by later monks. Peter Lightfoot, a 14th-century monk of Glastonbury, built one of the oldest clocks still in existence, which now sits in excellent condition in London's Science Museum.^[89]

The appearance of clocks in writings of the 11th century implies that they were well-known in Europe in that period.^[91] In the early 14th century, the Florentine poet Dante Alighieri referred to a clock in his Paradiso;^[92] considered to be the first literary reference to a clock that struck the hours.^[91] The earliest detailed description of clockwork was presented by Giovanni da Dondi, Professor of Astronomy at Padua, in his 1364 treatise Il Tractatus Astrarii.^[82] This has inspired several modern replicas, including some in London's Science Museum and the Smithsonian Institution.^[82] Other notable examples from this period were built in Milan (1335), Strasbourg (1354), Lund (1380), Rouen (1389), and Prague (1462).^[82]

Salisbury cathedral clock, dating from about 1386, is the oldest working clock in the world, still with most of its original parts.^[93] It has no dial, as its purpose was to strike a bell at precise times.^[93] The wheels and gears are mounted in an open, box-like iron frame, measuring about 1.2 metres (3.9 ft) square. The framework is held together with metal dowels and pegs, and the escapement is the verge and foliot type, standard for clocks of this age. The power is supplied by two large stones, hanging from pulleys. As the weights fall, ropes unwind from the wooden barrels. One barrel drives the main wheel, which is regulated by the escapement, and the other drives the striking mechanism and the air brake.^[93]

Peter Lightfoot's Wells Cathedral clock, constructed c. 1390, is also of note.^[94][95] The dial represents a geocentric view of the universe, with the Sun and Moon revolving around a centrally fixed Earth. It is unique in having its original medieval face, showing a philosophical model of the pre-Copernican universe.^[96] Above the clock is a set of figures, which hit the bells, and a set of jousting knights who revolve around a track every 15 minutes.^[97][96] The clock was converted to pendulum and anchor escapement in the 17th century, and was installed in London's Science Museum in 1884, where it continues to operate.^[97] Similar astronomical clocks, or horologes, can be seen at Exeter, Ottery St Mary, and Wimborne Minster.

One clock that has not survived to the present-day is that of the Abbey of St Albans, built by the 14th-century abbot Richard of Wallingford.^[98] It may have been destroyed during Henry VIII's Dissolution of the Monasteries, but the abbot's notes on its design have allowed a full-scale reconstruction. As well as keeping time, the astronomical clock could accurately predict lunar eclipses, and may have shown the Sun, Moon (age, phase, and node), stars and planets, as well as a wheel of fortune, and an indicator of the state of the tide at London Bridge.^[99] According to Thomas Woods, "a clock that equaled it in technological sophistication did not appear for at least two centuries".^[89][100] Giovanni de Dondi was another early mechanical clockmaker, whose clock did not survive, but has been replicated based on the designs. De Dondi's clock was a seven-faced construction with 107 moving parts, showing the positions of the Sun, Moon, and five planets, as well as religious feast days.^[99] Around this period, mechanical clocks were introduced into abbeys and monasteries to mark important events and times, gradually replacing water clocks which had served the same purpose.^[101][102]

During the Middle Ages, clocks were primarily used for religious purposes; the first employed for secular timekeeping emerged around the 15th century. In Dublin, the official measurement of time became a local custom, and by 1466 a public clock stood on top of the Tholsel (the city court and council chamber).^[103] It was probably the first of its kind in Ireland, and would only have had an hour hand.^[103] The increasing lavishness of castles led to the introduction of turret clocks.^[104] A 1435 example survives from Leeds castle; its face is decorated with the images of the Crucifixion of Jesus, Mary and St George.^[104]

Innovations to the mechanical clock continued, with miniaturization leading to domestic clocks in the 15th century, and personal watches in the 16th.^[82] In the 1580s, the Italian polymath Galileo Galilei investigated the regular swing of the pendulum, and discovered that it could be used to regulate a clock.^[105][81] Although Galileo studied the pendulum as early as 1582, he never actually constructed a clock based on that design.^[81] The first pendulum clock was designed and built by Dutch scientist Christiaan Huygens, in 1656.^[81] Early versions erred by less than one minute per day, and later ones only by 10 seconds, very accurate for their time.^[81]

The Jesuits were another major contributor to the development of pendulum clocks in the 17th and 18th centuries, having had an "unusually keen appreciation of the importance of precision".^[106][107] In measuring an accurate one-second pendulum, for example, the Italian astronomer Father Giovanni Battista Riccioli persuaded nine fellow Jesuits "to count nearly 87,000 oscillations in a single day".^[107] They served a crucial role in spreading and testing the scientific ideas of the period, and collaborated with contemporary scientists, such as Huygens.^[106]

The modern longcase clock, also known as the grandfather clock, has its origins in the invention of the anchor escapement mechanism in about 1670.^[108] Before then, pendulum clocks had used the older verge escapement mechanism, which required very wide pendulum swings of about 100°. To avoid the need for a very large case, most clocks using the verge escapement had a short pendulum. The anchor mechanism, however, reduced the pendulum's necessary swing to between 4° to 6°, allowing clockmakers to use longer pendulums with consequently slower beats. These required less power to move, caused less friction and wear, and were more accurate than their shorter predecessors. Most longcase clocks use a pendulum about a metre (39 inches) long to the center of the bob, with each swing taking one second. This requirement for height, along with the need for a long drop space for the weights that power the clock, gave rise to the tall, narrow case.^[109]

In 1675, 18 years after inventing the pendulum clock, Huygens devised the spiral balance spring for the balance wheel of pocket watches, an improvement on the straight spring invented by English natural philosopher Robert Hooke.^[105] This resulted in a great advance in accuracy of pocket watches, from perhaps several hours per day to 10 minutes per day, similar to the effect of the pendulum upon mechanical clocks.^[110][11]

The first professional clockmakers came from the guilds of locksmiths and jewellers. Clockmaking developed from a specialized craft into a mass production industry over many years.^[111] Paris and Blois were the early centers of clockmaking in France. French clockmakers such as Julien Le Roy, clockmaker of Versailles, were leaders in case design and ornamental clocks.^[111] Le Roy belonged to the fifth generation of a family of clockmakers, and was described by his contemporaries as "the most skillful clockmaker in France, possibly in Europe". He invented a special repeating mechanism which improved the precision of clocks and watches, a face that could be opened to view the inside clockwork, and made or supervised over 3,500 watches. The competition and scientific rivalry resulting from his discoveries further encouraged researchers to seek new methods of measuring time more accurately.^[112]

Between 1794 and 1795, in the aftermath of the French Revolution, the French government briefly mandated decimal clocks, with a day divided into 10 hours of 100 minutes each.^[113] The astronomer and mathematician Pierre-Simon Laplace, among other individuals, modified the dial of his pocket watch to decimal time.^[113] A clock in the Palais des Tuileries kept decimal time as late as 1801, but the cost of replacing all the nation's clocks prevented decimal clocks from becoming widespread.^[114] Because decimalized clocks only helped astronomers rather than ordinary citizens, it was one of the most unpopular changes associated with the metric system, and it was abandoned.^[114]

In Germany, Nuremberg and Augsburg were the early clockmaking centers, and the Black Forest came to specialize in wooden cuckoo clocks.^[115] The English became the predominant clockmakers of the 17th and 18th centuries. Switzerland established itself as a clockmaking center following the influx of Huguenot craftsmen, and in the 19th century, the Swiss industry "gained worldwide supremacy in high-quality machine-made watches". The leading firm of the day was Patek Philippe, founded by Antoni Patek of Warsaw and Adrien Philippe of Berne.^[111]

In 1904, Alberto Santos-Dumont, an early aviator, asked his friend, a French watchmaker called Louis Cartier, to design a watch that could be useful during his flights.^[116] The wristwatch had already been invented by Patek Philippe, in 1868, but only as a "lady’s bracelet watch", intended as jewelry. As pocket watches were unsuitable, Louis Cartier created the Santos wristwatch, the first man's wristwatch and designed for practical use.^[117]

Wristwatches gained in popularity during World War I, when officers found them to be more convenient than pocket watches in battle. Also, because the pocket watch was mainly a middle class item, the enlisted men usually owned wristwatches, which they brought with them. Artillery and infantry officers depended on their watches as battles became more complicated and coordinated attacks became necessary. Wristwatches were found to be needed in the air as much as on the ground: military pilots found them more convenient than pocket watches for the same reasons as Santos-Dumont had. Eventually, army contractors manufactured watches en masse, for both infantry and pilots. In World War II, the A-11 was a popular watch among American airmen, with its simple black face and clear white numbers for easy readability.^[118]

Marine chronometers are clocks used at sea as time standards, to determine longitude by celestial navigation.^[119] They were first developed by Yorkshire carpenter John Harrison, who won the British government's Longitude Prize in 1759. Marine chronometers keep the time of a fixed location—usually Greenwich Mean Time—allowing seafarers to determine longitude by comparing the local high noon to the clock.^{[119][120][121]}

A chronometer is a portable timekeeper that meets certain precision standards. Initially, the term was used to refer to the marine chronometer, a timepiece used to determine longitude by means of celestial navigation.^[119] More recently, the term has also been applied to the chronometer watch, a wristwatch that meets certain precision standards set by the Swiss agency COSC.^[122] Over 1,000,000 "Officially Certified Chronometer" certificates, mostly for mechanical wrist-chronometers—wristwatches—with sprung balance oscillators, are delivered each year, after passing the COSC's most severe tests, and being singly identified by an officially recorded individual serial number. According to COSC, a chronometer is a high-precision watch, capable of displaying the seconds and housing a movement that has been tested over several days, in different positions, and at different temperatures, by an official, neutral body. To meet this requirement, each movement is individually tested for several consecutive days, in five positions, and at three temperatures. Any watch with the designation chronometer has a certified movement.^[123]

The piezoelectric properties of crystalline quartz were discovered by Jacques and Pierre Curie in 1880.^[124][81] The first quartz crystal oscillator was built by Walter G. Cady in 1921, and in 1927 the first quartz clock was built by Warren Marrison and J. W. Horton at Bell Telephone Laboratories in Canada.^[125][126] The following decades saw the development of quartz clocks as precision time measurement devices in laboratory settings—the bulky and delicate counting electronics, built with vacuum tubes, limited their practical use elsewhere. In 1932, a quartz clock able to measure small weekly variations in the rotation rate of the Earth was developed.^[126] The National Bureau of Standards (now NIST) based the time standard of the United States on quartz clocks from late 1929 until the 1960s, when it changed to atomic clocks.^[127] In 1969, Seiko produced the world's first quartz wristwatch, the Astron.^[128] Their inherent accuracy and low cost of production has resulted in the subsequent proliferation of quartz clocks and watches.^[81]

Atomic clocks are the most accurate timekeeping devices. Accurate to a within few seconds over many thousands of years, they are used to calibrate other clocks and timekeeping instruments.^[129] The first atomic clock, invented in 1949, is on display at the Smithsonian Institution.^[127] It was based on the absorption line in the ammonia molecule,^[130][131] but most are now based on the spin property of the cesium atom. The International System of Units standardised its unit of time, the second, on the properties of cesium in 1967.^[131] SI defines the second as 9,192,631,770 cycles of the radiation which corresponds to the transition between two electron spin energy levels of the ground state of the ¹³³Cs atom.^[132] The cesium atomic clock, maintained by the National Institute of Standards and Technology, is accurate to 30 billionths of a second per year.^[131] Atomic clocks have employed other elements, such as hydrogen and rubidium vapor, offering greater stability—in the case of hydrogen clocks—and smaller size, lower power consumption, and thus lower cost (in the case of rubidium clocks).^[131]

The Global Positioning System (GPS), in coordination with the network time protocol, is a radio-navigation system used to synchronize timekeeping systems across the globe.^[133] GPS was developed by the US Department of Defense to provide constant, all-weather navigation capabilities for military ground, sea, and air forces.^[134] In 1983, following the shooting down of Korean Air Lines Flight 007 after it accidentally entered Soviet airspace, President Ronald Reagan issued a directive allowing the free commercial use of GPS, to prevent further navigational errors.^[135] GPS time is not corrected to match the rotation of the Earth, so it does not account for leap seconds or other corrections which are periodically applied to systems such as Universal Coordinated Time (UTC). GPS time was set to match UTC in 1980, but has since diverged because of the absence of corrections. GPS time therefore remains at a constant offset of 19 seconds from International Atomic Time (TAI). The on-board satellite clocks are periodically corrected to compensate for relativistic effects, and to keep them synchronized with ground clocks. The GPS navigation message includes the difference between GPS time and UTC, which is 14 seconds, as of 2007. Receivers subtract this offset from GPS time to calculate UTC and specific timezone values.^[136] In the United States, the Navstar GPS system is maintained by 24 satellites circling the Earth every 12 hours, travelling in 6 orbits; Russia operates a system known as GLONASS (Global Navigation Satellite System). In 2007, the European Union approved funding for 30 satellites scheduled to be operational by 2013. China has two orbiting satellites out of 35 planned for its Beidou navigation system.^[133]

• A walk through time
• Measuring time in ancient Egypt
• A history of the watch