For thousands of years, devices have been used to measure and keep track of time. The current sexagesimalsystem of time measurement dates to approximately 2000 BC from the Sumerians.
The 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 Zhou dynasty is believed to have used the outflow water clock around the same time, devices which were introduced from Mesopotamia as early as 2000 BC.
Other ancient timekeeping devices include the candle clock, used in ancient China, ancient Japan, England and Mesopotamia; 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 sundial, another early clock, relies on shadows to provide a good estimate of the hour on a sunny day. It is not so useful in cloudy weather or at night and requires recalibration as the seasons change (if the gnomon was not aligned with the Earth's axis).
The earliest known clock with a water-poweredescapement mechanism, which transferred rotational energy into intermittent motions, dates back to 3rd century BC in ancient Greece;Chinese engineers later invented clocks incorporating mercury-powered escapement mechanisms in the 10th century, followed by Iranian engineers inventing water clocks driven by gears and weights in the 11th century.
The first mechanical clocks, employing the verge escapement mechanism with a foliot or balance wheel timekeeper, were invented in Europe at around the start of the 14th century, and became the standard timekeeping device until the pendulum clock was invented in 1656. The invention of the mainspring in the early 15th century allowed portable clocks to be built, evolving into the first pocketwatches by the 17th century, but these were not very accurate until the balance spring was added to the balance wheel in the mid 17th century.
The pendulum clock remained the most accurate timekeeper until the 1930s, when quartz oscillators were invented, followed by atomic clocks after World War 2. Although initially limited to laboratories, the development of microelectronics in the 1960s made quartz clocks both compact and cheap to produce, and by the 1980s they became the world's dominant timekeeping technology in both clocks and wristwatches.
Atomic clocks are far more accurate than any previous timekeeping device, and are used to calibrate other clocks and to calculate the International Atomic Time; a standardized civil system, Coordinated Universal Time, is based on atomic time.
Timekeeping devices of early civilizations
See also: Water clock (clepsydra) and Water clock
Many ancient civilizations observed astronomical bodies, often the Sun and Moon, to determine times, dates, and seasons. 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.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. As those megalithic civilizations left no recorded history, little is known of their calendars or timekeeping methods. Methods of sexagesimal timekeeping, now common in both Western and Eastern societies, are first attested nearly 4,000 years ago in Mesopotamia and Egypt.Mesoamericans similarly modified their usual vigesimal counting system when dealing with calendars to produce a 360-day year.
See also: History of timekeeping devices in Egypt
The oldest known sundial is from Egypt; it dates back to around 1500 BC (19th Dynasty), and was discovered in the Valley of the Kings in 2013.Sundials have their origin in shadow clocks, which were the first devices used for measuring the parts of a day. Ancient Egyptian obelisks, constructed about 3500 BC, are also among the earliest shadow clocks.
Egyptian shadow clocks divided daytime into 12 parts with each part further divided into more precise parts. 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 wintersolstices. 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.
Although accurate, shadow clocks relied on the sun, and so were useless at night and in cloudy weather. The Egyptians therefore developed a number of alternative timekeeping instruments, including water clocks, 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. 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 pharaohAmenhotep I (1525–1504 BC), suggesting that they were first used in ancient Egypt. 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.
Ancient Greece and Rome
See also: Clock tower, Ancient Greek technology, and Roman timekeeping
Water clocks, or clepsydrae, were commonly used in Ancient Greece following their introduction by Plato, who also invented a water-based alarm clock. 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. 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. 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 BC.
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". 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". The clock in Apuleius's 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.
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. 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. 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.
Between 270 BC and AD 500, Hellenistic (Ctesibius, Hero of Alexandria, Archimedes) and Romanhorologists and astronomers began developing more elaborate mechanized water clocks. The added complexity was aimed at regulating the flow and at providing fancier displays of the passage of time. For example, some water clocks rang bells and gongs, while others opened doors and windows to show figurines of people, or moved pointers, and dials. Some even displayed astrological models of the universe.
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. 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. During the reign of Emperor Augustus, the Romans constructed the largest sundial ever built, the Solarium Augusti. Its gnomon was an obelisk from Heliopolis. Similarly, the obelisk from Campus Martius was used as the gnomon for Augustus's zodiacal sundial.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.
Ancient and medieval Persia
See also: History of science and technology in Persia
According to Callisthenes, the Persians were using water clocks in 328 BC to ensure a just and exact distribution of water from qanats to their shareholders for agricultural irrigation. The use of water clocks in Iran, especially in Zeebad, dates back to 500 BC. Later they were also used to determine the exact holy days of pre-Islamic religions, such as the Nowruz, Chelah, or Yaldā – the shortest, longest, and equal-length days and nights of the years. The water clocks used in Iran were one of the most practical ancient tools for timing the yearly calendar.
Water clocks, or Fenjaan, in Persia reached a level of accuracy comparable to today's standards of timekeeping. The fenjaan was the most accurate and commonly used timekeeping device for calculating the amount or the time that a farmer must take water from a qanat or well for irrigation of the farms, until it was replaced by more accurate current clock. Persian water clocks were a practical and useful tool for the qanat's shareholders to calculate the length of time they could divert water to their farm. The qanat was the only water source for agriculture and irrigation so a just and fair water distribution was very important. Therefore, a very fair and clever old person was elected to be the manager of the water clock, and at least two full-time managers were needed to control and observe the number of fenjaans and announce the exact time during the days and nights.
The fenjaan was a big pot full of water and a bowl with small hole in the center. When the bowl become full of water, it would sink into the pot, and the manager would empty the bowl and again put it on the top of the water in the pot. He would record the number of times the bowl sank by putting small stones into a jar.
The place where the clock was situated, and its managers, were collectively known as khaneh fenjaan. Usually this would be the top floor of a public-house, with west- and east-facing windows to show the time of sunset and sunrise. There was also another time-keeping tool named a staryab or astrolabe, but it was mostly used for superstitious beliefs and was not practical for use as a farmers' calendar. The Zeebad Gonabad water clock was in use until 1965 when it was substituted by modern clocks.
Ancient and medieval China
See also: History of science and technology in China and Su Song
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 550 AD, 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. 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.
Timekeeping innovations in medieval and pre-modern periods
The term 'clock' encompasses 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 Englishclokke, Old North Frenchcloque, or Middle Dutchclocke, all of which mean bell, and are derived from the Medieval Latinclocca, also meaning bell. 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. Mechanical clocks became widespread in the 14th century, when they were used in medieval monasteries to keep the regulated schedule of prayers. The clock continued to be improved, with the first pendulum clock being designed and built in the 17th century.
Main article: Candle clock
The earliest mention of candle clocks comes from a Chinese poem, written in AD 520 by You Jianfu. 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.
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 (2.54 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.
The most sophisticated candle clocks of their time were those of Al-Jazari in 1206. One of his candle clocks included a dial to display the time and, for the first time, employed a bayonet fitting, a fastening mechanism still used in modern times.Donald Routledge Hill described Al-Jazari's candle clocks as follows:
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. No other candle clocks of this sophistication are known.
A variation on this theme were oil-lamp clocks. These early timekeeping devices consisted of a graduated glass reservoir to hold oil — usually whale oil, which burned cleanly and evenly — supplying the fuel for a built-in lamp. As the level in the reservoir dropped, it provided a rough measure of the passage of time.
Main article: Incense clock
In addition to water, mechanical, and candle clocks, incense clocks were used in the Far East, and were fashioned in several different forms.Incense clocks were first used in China around the 6th century; in Japan, one still exists in the Shōsōin, although its characters are not Chinese, but Devanagari. 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. Although similar to the candle clock, incense clocks burned evenly and without a flame; therefore, they were more accurate and safer for indoor use.
Several types of incense clock have been found, the most common forms include the incense stick and incense seal. An incense stick clock was an incense stick with calibrations; 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. Sticks of incense with different scents were also used, so that the hours were marked by a change in fragrance. 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.
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. Incense seal clocks were used for similar occasions and events as the stick clock; while religious purposes were of primary importance, these clocks were also popular at social gatherings, and were used by Chinese scholars and intellectuals. The seal was a wooden or stone disk with one or more grooves etched in it into which incense was placed. These clocks were common in China, but were produced in fewer numbers in Japan. 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. 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.
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. 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.
Main article: Sundial
Sundials had been used for timekeeping since Ancient Egypt. Ancient dials were nodus-based with straight hour-lines that 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 sundial was further developed by Muslim astronomers. 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.
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. In 1524, the French astronomer Oronce Finé constructed an ivory sundial, which still exists; 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'sConstructio instrumenti ad horologia solaria (c. 1620) discusses how to construct sundials.
Main article: Hourglass
Since the hourglass was one of the few reliable methods of measuring time at sea, it is speculated that it was 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 unambiguous evidence of their use appears in the painting Allegory of Good Government, by Ambrogio Lorenzetti, from 1338. 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. Though also used in China, the hourglass's history there is unknown. The Portuguese navigator Ferdinand Magellan used 18 hourglasses on each ship during his circumnavigation of the globe in 1522.
Clocks with gears and escapements
The earliest instance of a liquid-driven escapement was described by the Greek engineer Philo of Byzantium (fl. 3rd century BC) in his technical treatise Pneumatics (chapter 31) where he likens the escapement mechanism of a washstandautomaton with those as employed in (water) clocks. Another early clock to use escapements was built during the 7th century in Chang'an, by Tantric monk and mathematician, Yi Xing, and government official Liang Lingzan. An astronomical instrument that served as a clock, it was discussed in a contemporary text as follows:
[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.
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. Furthermore, it featured the first known endless power-transmitting chain drive in horology. 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.
The clock towers built by Zhang Sixun and Su Song, in the 10th and 11th centuries, respectively, also incorporated a striking clock mechanism, the use of clock jacks to sound the hours. A striking clock outside of China was the Jayrun Water Clock, at the Umayyad Mosque in Damascus, Syria, which struck once every hour. It was constructed by Muhammad al-Sa'ati in the 12th century, and later described by his son Ridwan ibn al-Sa'ati, in his On the Construction of Clocks and their Use (1203), when repairing the clock. In 1235, an early monumental water-powered alarm clock that "announced the appointed hours of prayer and the time both by day and by night" was completed in the entrance hall of the Mustansiriya Madrasah in Baghdad.
The first geared clock was invented in the 11th century by the Arab engineerIbn Khalaf al-Muradi in Islamic Iberia; it was a water clock that employed a complex gear train mechanism, including both segmental and epicyclic gearing, capable of transmitting high torque. The clock was unrivalled in its use of sophisticated complex gearing, until the mechanical clocks of the mid-14th century. Al-Muradi's clock also employed the use of mercury in its hydraulic linkages, which could function mechanical automata. Al-Muradi's work was known to scholars working under Alfonso X of Castile, hence the mechanism may have played a role in the development of the European mechanical clocks. Other monumental water clocks constructed by medieval Muslim engineers also employed complex gear trains and arrays of automata. Like the earlier Greeks and Chinese, Arab engineers at the time also developed a liquid-driven escapement mechanism which they employed in some of their water clocks. Heavy floats were used as weights and a constant-head system was used as an escapement mechanism, which was present in the hydraulic controls they used to make heavy floats descend at a slow and steady rate.
A mercury clock, described in the Libros del saber de Astronomia, a Spanish work from 1277 consisting of translations and paraphrases of Arabic works, is sometimes quoted as evidence for Muslim knowledge of a mechanical clock. However, the device was actually a compartmented cylindrical water clock, which the Jewish author of the relevant section, Rabbi Isaac, constructed using principles described by a philosopher named "Iran", identified with Heron of Alexandria (fl. 1st century AD), on how heavy objects may be lifted.
Main article: Clock tower
See also: Turret clock and Striking clock
Clock towers in Western Europe in the Middle Ages were also sometimes striking clocks. The most famous original still standing is possibly St Mark's Clock on the top of St Mark's Clocktower in St Mark's Square in Venice, assembled in 1493 by the clockmaker Gian Carlo Rainieri from Reggio Emilia. In 1497, Simone Campanato moulded the great bell on which every definite time-lapse is beaten by two mechanical bronze statues (h. 2,60 m.) called Due Mori (Two Moors), handling a hammer. Possibly earlier (1490) is the Prague Astronomical Clock by clockmaster Jan Růže (also called Hanuš) – according to another source this device was assembled as early as 1410 by clockmaker Mikuláš of Kadaň and mathematician Jan Šindel. The allegorical parade of animated sculptures rings on the hour every day.
Main article: Astronomical clock
During the 11th century in the 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, such as the water-powered astronomical clock by Al-Jazari in 1206, and the astrolabic clock by Ibn al-Shatir in the early 14th century. The most sophisticated timekeeping astrolabes were the geared astrolabe mechanisms designed by Abū Rayhān Bīrūnī in the 11th century and by Muhammad ibn Abi Bakr in the 13th century. These devices functioned as timekeeping devices and also as calendars.
A sophisticated water-powered astronomical clock was built by Al-Jazari in 1206. This castle clock was a complex device that was about 11 feet (3.4 m) high, and had multiple functions alongside timekeeping. It included a display of the zodiac and the solar and lunar paths, and a pointer in the shape of the crescent moon which travelled across the top of a gateway, moved by a hidden cart and causing doors to open, each revealing a mannequin, every hour. It was possible to reset the length of day and night in order to account for the changing lengths of day and night throughout the year. This clock also featured a number of automata including falcons and musicians who automatically played music when moved by levers operated by a hidden camshaft attached to a water wheel.
Early mechanical clocks and watches
Main articles: Mechanical clock and Mechanical watch
See also: Alarm clock
The earliest medieval European clockmakers were Christian monks. Medieval religious institutions required clocks because they regulated daily prayer- and work-schedules strictly, using various types of time-telling and recording devices, such as water clocks, sundials and marked candles, probably in combination. When mechanical clocks came into use, they were often wound at least twice a day to ensure accuracy. Monasteries broadcast important times and durations with bells, rung either by hand or by a mechanical device, such as by a falling weight or by rotating beater.
Although the mortuary inscription of Pacificus, archdeacon of Verona, records that he constructed a night clock (horologium nocturnum) as early as 850, his clock has been identified as being an observation tube used to locate stars with an accompanying book of astronomical observations, rather than a mechanical or water clock, an interpretation supported by illustrations from medieval manuscripts.
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.
The appearance of clocks in writings of the 11th century implies that they were well known in Europe in that period. In the early 14th-century, the Florentine poet Dante Alighieri referred to a clock in his Paradiso; the first known literary reference to a clock that struck the hours.Giovanni da Dondi, Professor of Astronomy at Padua, presented the earliest detailed description of clockwork in his 1364 treatise Il Tractatus Astrarii. This has inspired several modern replicas, including some in London's Science Museum and the Smithsonian Institution. Other notable examples from this period were built in Milan (1335), Strasbourg (1354), Lund (1380), Rouen (1389), and Prague (1462).
Salisbury cathedral clock, dating from about 1386, is one of the oldest working clocks in the world, and may be the oldest. It still has most of its original parts, although its original verge and foliot timekeeping mechanism is lost, having been converted to a pendulum, which was replaced by a replica verge in 1956. It has no dial, as its purpose was to strike a bell at precise times. 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. Two large stones, hanging from pulleys, supply the power. 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.
Note also Peter Lightfoot's Wells Cathedral clock, constructed c. 1390. The dial represents a geocentric view of the universe, with the Sun and Moon revolving around a central fixed Earth. It is unique in having its original medieval face, showing a philosophical model of the pre-Copernican universe. 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. 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. Similar astronomical clocks, or horologes, survive at Exeter, Ottery St Mary
Previous (Clive Bell)
A clock (from the Latin word cloca, meaning "bell") is an instrument for measuring time. In its most common form, in use since at least the fourteenth century, it displays the time in hours, minutes, and often seconds, during a 12 or 24 hour period.
Clocks that are used for telling the time at very high accuracy are usually called chronometers. A common, portable timekeeping instrument for personal use is the pocket watch or wristwatch.
By definition, a "true" clock has an announcing or striking mechanism that sounds after each set interval of time. The sound could be the ringing of a bell, chimes, or gong. A silent clock without a striking mechanism is traditionally known as a timepiece, a term sometimes used by horologists and other specialists to describe devices such as ordinary wristwatches (Baillie et al., p. 307; Palmer, p. 19; Zea and Cheney, p. 172).
The clock is one of the oldest human inventions, requiring a physical process that will proceed at a known rate and a way to gauge how long that process has run. As the seasons and the phases of the moon can be used to measure the passage of longer periods of time, shorter processes had to be used to measure off hours and minutes.
Sundials and other techniques
The sundial, which measures the time of day by the direction of shadows cast by the sun, was widely used in ancient times. A well-designed 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 doesn't work at all during the night—encouraged the use of other techniques for measuring time.
Candles 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.
Vitruvius reported that the ancient Egyptians used a clepsydra, a time mechanism using flowing water. Herodotus had mentioned an ancient Egyptian time-keeping device that was based on mercury. By the ninth century C.E., a mechanical timekeeper had been developed that lacked only an escapement mechanism. Later years saw the rise of automated water clocks in Arabia, China, and Korea.
Early mechanical clocks
None of the first clocks survive from thirteenth century Europe, but various mentions in church records reveal some of the early history of the clock.
Medieval religious institutions required clocks to measure and indicate the passing of time because, for many centuries, daily prayer and work schedules had to be 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. Important times and durations were broadcast by bells, rung either by hand or by some mechanical device such as a falling weight or rotating beater.
The word horologia (from the Greek hora, hour, and legein, to tell) was used to describe all these devices, but the use of this word (still used in several romance languages) for all timekeepers conceals the true nature of the mechanisms. For example, there is a record that in 1176 Sens Cathedral installed a horologe but the mechanism used is unknown. 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.
These early clocks may not have used hands or dials, but “told” the time with audible signals.
A new mechanism
The word "clock" (from the Latin word for "bell"), which gradually supersedes "horologe," suggests that it was the sound of bells which also characterized the prototype mechanical clocks that appeared during the thirteenth century.
Between 1280 and 1320, there was an increase in the number of references to clocks and horologes in church records, and this probably indicates that a new type of clock mechanism had been devised. Existing clock mechanisms that used water power were being adapted to take their driving power from falling weights. This power was controlled by some form of oscillating mechanism, probably derived from existing bell-ringing or alarm devices. This controlled release of power—the escapement—marks the beginning of the true mechanical clock.
These mechanical clocks were intended for two main purposes: For signaling and notification (e.g. the timing of services and public events), and for modeling the solar system. The former purpose was administrative, the latter arose naturally given the scholarly interest in astronomy, science, astrology, and how these subjects integrated with the religious philosophy of the time. The astrolabe was used both by astronomers and astrologers, and it was natural to apply a clockwork drive to the rotating plate to produce a working model of the solar system.
Simple clocks intended mainly for notification were installed in towers, and did not always require dials or hands. They would have announced the canonical hours or intervals between set times of prayer. Canonical hours varied in length as the times of sunrise and sunset shifted. The more sophisticated astronomical clocks would have had moving dials or hands, and would have shown the time in various time systems, including Italian hours, canonical hours, and time as measured by astronomers at the time. Both styles of clock started acquiring extravagant features such as automata.
In 1283, a large clock was installed at Dunstable Priory; its location above the rood screen suggests that it was not a water clock. In 1292, Canterbury Cathedral installed a "great horloge." Over the next 30 years there are brief mentions of clocks at a number of ecclesiastical institutions in England, Italy, and France. In 1322, a new clock was installed in Norwich, an expensive replacement for an earlier clock installed in 1273. This had a large (2 meter) astronomical dial with automata and bells. The costs of the installation included the full-time employment of two technicians for two years.
Early astronomical clocks
The clocks constructed by Richard of Wallingford in St Albans by 1336, and by Giovanni de'Dondi in Padua from 1348 to 1364, no longer exist, but detailed descriptions of their design and construction survive, and modern reproductions have been made. They illustrate how quickly the theory of the mechanical clock had been translated into practical constructions, and also that one of the many impulses to their development had been the desire of astronomers to investigate celestial phenomena.
Wallingford's clock had a large astrolabe-type dial, showing the sun, the moon's age, phase, and node, a star map, and possibly the planets. In addition, it had a wheel of fortune and an indicator of the state of the tide at London Bridge. Bells rang every hour, the number of strokes indicating the time.
Dondi's clock was a seven-sided construction, 1 meter high, with dials showing the time of day, including minutes, the motions of all the known planets, an automatic calendar of fixed and movable feasts, and an eclipse prediction hand rotating once every 18 years.
It is not known how accurate or reliable these clocks would have been. They were probably adjusted manually every day to compensate for errors caused by wear and imprecise manufacture.
The Salisbury Cathedral clock, built toward the end of the fourteenth century, is considered to be the oldest surviving mechanical clock in the world.
Elements of the mechanical clock
These fourteenth century clocks show the four key elements common to all clocks in subsequent centuries, at least up to the digital age:
- the power, supplied by a falling weight, later by a coiled spring
- the escapement, a periodic repetitive action that allows the power to escape in small bursts rather than drain away all at once
- the going train, a set of interlocking gear wheels that controls the speed of rotation of the wheels connected between the power supply and the indicators
- indicators, such as dials, hands, and bells
Clock makers developed their art in various ways. Building smaller clocks was a technical challenge, as was improving accuracy and reliability. Clocks could be impressive showpieces to demonstrate skilled craftsmanship, or less expensive, mass-produced items for domestic use. The escapement in particular was an important factor affecting the clock's accuracy, so many different mechanisms were tried.
Spring-driven clocks were developed during the fifteenth century, and this gave the clockmakers many new problems to solve, such as how to compensate for the changing power supplied as the spring unwound.
The first record of a minute hand on a clock is 1475, in the Almanus Manuscript of Brother Paul.
During the fifteenth and sixteenth centuries, clockmaking flourished, particularly in the metalworking towns of Nuremberg and Augsburg, and in France, Blois. Some of the more basic table clocks have only one time-keeping hand, with the dial between the hour markers being divided into four equal parts making the clocks readable to the nearest 15 minutes. Other clocks were exhibitions of craftsmanship and skill, incorporating astronomical indicators and musical movements. The cross-beat escapement was developed in 1585 by Jost Burgi, who also developed the remontoire. Burgi's accurate clocks helped Tycho Brahe and Johannes Kepler to observe astronomical events with much greater precision than before.
The first record of a second hand on a clock is about 1560, on a clock now in the Fremersdorf collection. However, this clock could not have been accurate, and the second hand was probably for indicating that the clock was working.
The next development in accuracy occurred after 1657, with the invention of the pendulum clock. Galileo had the idea to use a swinging bob to propel the motion of a time telling device earlier in the seventeenth century. Christiaan Huygens, however, is usually credited as the inventor. He determined the mathematical formula that related pendulum length to time (99.38 centimeters or 39.13 inches for the one second movement) and had the first pendulum-driven clock made. In 1670, the English clockmaker William Clement created the anchor escapement, an improvement over Huygens' crown escapement. Within just one generation, minute hands and then second hands were added.
A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation. The position of a ship at sea could be determined with reasonable accuracy if a navigator could refer to a clock that lost or gained less than about 10 seconds per day. This clock could not contain a pendulum, which would be virtually useless on a rocking ship. Many European governments offered a large prize for anyone that could determine longitude accurately; for example, Great Britain offered 20,000 pounds, equivalent to millions of dollars today. The reward was eventually claimed in 1761 by John Harrison, who dedicated his life to improving the accuracy of his clocks. His H5 clock is reported to have lost less than 5 seconds over 10 days.
The excitement over the pendulum clock had attracted the attention of designers resulting in a proliferation of clock forms. Notably, the longcase clock (also known as the "grandfather clock") was created to house the pendulum and works. The English clockmaker William Clement is also credited with developing this form in 1670 or 1671. It was also at this time that clock cases began to be made of wood and clock faces to utilize enamel as well as hand-painted ceramics.
On November 17, 1797, Eli Terry received his first patent for a clock. Terry is known as the founder of the American clock-making industry.
Alexander Bain, Scottish clockmaker, patented the electric clock in 1840. The electric clock's mainspring is wound either with an electric motor or with an electro-magnet and armature. In 1841, he first patented the electromagnetic pendulum.
The development of electronics in the twentieth century led to clocks with no clockwork parts at all. Time in these cases is measured in several ways, such as by the vibration of a tuning fork, the behavior of quartz crystals, the decay of radioactive elements, or resonance of polycarbonates. Even mechanical clocks have since come to be largely powered by batteries, removing the need for winding.
Clocks can be classified by the type of time display, as well as by the method of timekeeping.
Time display methods
Analog clocks usually indicate time using angles. The most common clock face uses a fixed numbered dial or dials and moving hand or hands. It usually has a circular scale of 12 hours, which can also serve as a scale of 60 minutes, and often also as a scale of 60 seconds—though many other styles and designs have been used throughout the years, including dials divided into 6, 8, 10, and 24 hours. Of these alternative versions, the 24 hour analog dial is the main type in use today. The 10-hour clock was briefly popular during the French Revolution, when the metric system was applied to time measurement, and an Italian 6 hour clock was developed in the eighteenth century, presumably to save power (a clock or watch chiming 24 times uses more power).
Another type of analog clock is the sundial, which tracks the sun continuously, registering the time by the shadow position of its gnomon. Sundials use some or part of the 24 hour analog dial.
There also exist clocks which use a digital display despite having an analog mechanism—these are commonly referred to as flip clocks.
Main Article: Digital clock
Digital clocks display a numeric representation of time. Two numeric display formats are commonly used on digital clocks:
- the 24-hour notation with hours ranging 00–23;
- the 12-hour notation with a.m./p.m. indicator, with hours indicated as 12a.m., followed by 1 a.m.–11 a.m., followed by 12 p.m., followed by 1 p.m.–11 p.m. (a notation mostly used in the United States).
Most digital clocks use an LCD or LED display; many other display technologies are used as well (cathode ray tubes, nixie tubes, etc.). After a reset, battery change, or power failure, digital clocks without a backup battery or capacitor either start counting from 00:00, or stay at 00:00, often with blinking digits indicating that time needs to be set. Some newer clocks will actually reset themselves based on radio or Internet time servers that are tuned to national atomic clocks.
For convenience, distance, telephony, or blindness, auditory clocks present the time as sounds. The sound is either spoken natural language, (e.g. "The time is twelve thirty-five"), or as auditory codes (e.g. number of sequential bell rings on the hour represents the number of the hour, like the clock Big Ben).
Most types of clocks are built around some form of oscillator, an arrangement that goes through an endless sequence of periodic state changes, designed to provide a continuous and stable reference frequency. The periods of this oscillator are then counted and converted into the desired clock display.
- Mechanical clocks use a pendulum as their oscillator, which controls the rotation of a system of gears that drive the clock display.
- Crystal clocks use an electronic quartz crystal oscillator and a frequency divider or counter. Most battery-powered crystal clocks use a 215 Hertz (Hz) = 32.768 Kilohertz (kHz) oscillator.
- Atomic clocks use a microwave oscillator (maser) tuned by the energy transitions of elements such as caesium, rubidium, or hydrogen. These are the most precise clocks available. Atomic clocks based on caesium are used as the official definition of time today.
- Mains power clocks count the 50 or 60 hertz periods of their AC power.
- Radio clocks receive time signal broadcasts from a radio transmitter (which may be hundreds of kilometers away). The clock can decode the transmission and adjust its hands or display for perfect accuracy. The broadcast radio signals are generated by an atomic clock and typically have a data rate of 1 bit/s.
- Sundials observe the apparent rotation of the Sun around the Earth as their reference oscillation. They are observed with a solar tempometer.
Clocks are in homes and offices; smaller ones (watches) are carried; larger ones are in public places, e.g. a train station or church. A small clock is often shown in a corner of computer displays or mobile phones.
The purpose of a clock is not always to display the time. It may also be used to control a device according to time, e.g. an alarm clock, or a VCR, (see: counter). However, in this context, it is more appropriate to refer to it as a timer or trigger mechanism rather than strictly as a clock.
Computers depend on an accurate internal clock signal to allow synchronized processing. (A few research projects are developing CPUs based on asynchronous circuits.) Some computers also maintain time and date for all manner of operations whether these be for alarms, event initiation, or just to display the time of day. The internal computer clock is generally kept running by a small battery. Memory of this kind is often referred to as "non-volatile." Many computers will still function even if the internal clock battery is dead, but the computer clock will need to be reset each time the computer is restarted, since once power is lost, time is also lost.
An ideal clock is a scientific principle that measures the ratio of the duration of natural processes, and thus will give the time measure for use in physical theories. Therefore, to define an ideal clock in terms of any physical theory would be circular. An ideal clock is more appropriately defined in relationship to the set of all physical processes. An ideal clock should too measure time in consistent, for example decimalized time units.
This leads to the following definitions:
- A clock is a recurrent periodic process and a counter.
- A good clock is one which, when used to measure other recurrent processes, finds many of them to be periodic.
- An ideal clock is a clock (i.e., recurrent process) that makes the most other recurrent processes periodic.
The recurrent, periodic process (a metronome) is an oscillator and typically generates a "clock signal." Sometimes that signal alone is (confusingly) called "the clock," but sometimes "the clock" includes the counter, its indicator, and everything else supporting it.
This definition can be further improved by the consideration of successive levels of smaller and smaller error tolerances. While not all physical processes can be surveyed, the definition should be based on the set of physical processes which includes all individual physical processes which are proposed for consideration. Since atoms are so numerous and since, within current measurement tolerances they all beat in a manner such that if one is chosen as periodic then the others are all deemed to be periodic also, it follows that atomic clocks represent ideal clocks to within present measurement tolerances and in relation to all presently known physical processes. However, they are not so designated by fiat. Rather, they are designated as the current ideal clock because they are currently the best instantiation of the definition.
Navigation by ships depends on the ability to measure latitude and longitude. Latitude is fairly easy to determine through celestial navigation, but the measurement of longitude requires accurate measurement of time. This need was a major motivation for the development of accurate mechanical clocks. John Harrison created the first, highly accurate marine chronometer in the mid-eighteenth century. The Noon gun in Cape Town still fires an accurate signal to allow ships to check their chronometers.
Specific types of clocks
- Alarm clock
- Analog clock with digital display
- Astronomical clock
- Atomic clock
- Balloon clock
- Binary clock
- Bracket clock
- Carriage clock
- Cartel clock
- Chiming clock
- Clock network
- Clock of the Long Now
- Countdown clock
- Cuckoo clock
- Data clock for timescapes created with time-technology
- Digital clock
- Doll's head clock
- Electric clock
- Flip clock
- Floral clock
- Game clock
- Japanese clock
- Lantern clock
- Lighthouse Clock
- Longcase (or "grandfather") clock
- Mantel clock
- Master clock
- Paper clock
- Pedestal clock
- Pendulum clock
- Projection clock
- Quartz clock
- Railroad chronometers
- Reference clock
- Rolling ball clock
- Shelf clock
- Sidereal clock
- Skeleton clock
- Slave clock
- Striking clock
- Tall-case clock
- Tide clock
- Time ball
- Time clock
- Tower clock
- Torsion pendulum clock
- Water clock
- Wall clock
- World clock
- Baillie, G.H., O. Clutton, & C.A. Ilbert. 1956. Britten’s Old Clocks and Watches and Their Makers. 7th ed. Bonanza Books.
- Bolter, David J. 1984. Turing's Man: Western Culture in the Computer Age. Chapel Hill, NC: University of North Carolina Press. ISBN 0-8078-4108-0
- Bruton, Eric. 2003. The History of Clocks and Watches. New York: Little, Brown. ISBN 0316724262
- Edey, Winthrop. 1967. French Clocks. New York: Walker & Co. ISBN 0289370566
- Landes, David S. 1983. Revolution in Time: Clocks and the Making of the Modern World. Cambridge: Harvard University Press.
- Lloyd, Alan H. 1957. "Mechanical Timekeepers," in A History of Technology Vol. III. Edited by Charles Joseph Singer et al. Oxford: Clarendon Press.
- Macey, Samuel L. 1980. Clocks and the Cosmos: Time in Western Life and Thought. Hamden, CT: Archon Books. ISBN 0208017739
- North, John. 2007. God's Clockmaker: Richard of Wallingford and the Invention of Time. London: Hambledon and London. ISBN 1852855711
- Palmer, Brooks. 1979. The Book of American Clocks. London: Macmillan Co.
- Robinson, Tom. 2006. The Longcase Clock. Suffolk, UK: Antique Collector’s Club. ISBN 1851492321
- Smith, Alan. 1996. The International Dictionary of Clocks. London: Chancellor Press. ISBN 0671068091
- Tardy. 1981. French Clocks the World Over. Part I and II. Translated with the assistance of Alexander Ballantyne. Paris: Tardy.
- Yoder, Joella Gerstmeyer. 1988. Unrolling Time: Christiaan Huygens and the Mathematization of Nature. New York: Cambridge University Press. ISBN 052134140X
- Zea, Philip, and Robert Cheney. 1992. Clock Making in New England: 1725-1825. Sturbridge, MA: Old Sturbridge Village. ISBN 0913387037
All links retrieved March 6, 2017.
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