History of timekeeping devices

The history of timekeeping devices dates back to when ancient civilizations observed the Sun and the Moon as they moved across the sky. The current sexagesimal system of time measurement dates to approximately 2000 BCE from the Sumerians.

A marine sandglass. It is related to the hourglass, nowadays often used symbolically to represent the concept of time.

The ancient Egyptians divided the day into two 12-hour periods, and used obelisks to track the sun. They developed water clocks, later employed by the Chinese (after they were introduced from Mesopotamia), the Persians, and the Greeks. Other ancient timekeeping devices include the candle clock, the timestick, and the hourglass.

The escapement mechanism is known to have been used in ancient Greece. The Chinese used a mercury-powered escapement mechanism in their 10th century clocks, and medieval Islamic inventions included clocks driven by gears and weights. Mechanical clocks that used the verge escapement mechanism with a foliot timekeeper were invented in Europe around the start of the 14th century. Portable clocks were first built after the invention of the mainspring in the early 15th century; the first pocketwatches appeared during the 17th century, their accuracy improving after the balance spring was added to the balance wheel.

During the Dutch Golden Age, the polymath Christiaan Huygens invented an accurate and practical pendulum clock and the hairspring, which led to the development of the watch. His inventions increased the accuracy of timekeeping dramatically and became widely used. Quartz oscillators were invented in the 1930s, and atomic clocks emerged after World War II. Technological advances during the 1960s made quartz clocks compact and cheap, leading to their dominance during the 1980s. Atomic clocks are more accurate than any other timekeeping device. They are used to calibrate other devices. A standardized system for measuring time, Coordinated Universal Time, is based on atomic time.

Timekeeping by early civilizations

The sun rising over Stonehenge in southern England on the June solstice

Ancient civilizations observed astronomical bodies, often the Sun and Moon, to determine time.[1] Stonehenge is likely to have been an astronomical observatory, used to seasonal and annual events such as equinoxes or solstices.[2] As megalithic civilizations left no recorded history, little is known of their timekeeping methods.[3] Mesoamericans modified their usual vigesimal counting system when dealing with calendars to produce a 360-day year.[4]

The Aboriginal Australians understood the motion of objects in the sky well, and used their knowledge to construct calendars and aid navigation; most Aboriginal cultures had seasons that were well-defined and determined by natural changes throughout the year, including celestial events. The phases of the moon were used to mark shorter periods of time; the Yaraldi of South Australia were one of the few people recorded as having a way to measure time during the day, which was divided into seven parts using the position of the Sun.[5]

Ancient Egypt and Mesopotamia

A limestone Egyptian water clock, 285-246 BC (Oriental Institute, Chicago)

All timekeepers prior to the development of the escapement relied upon methods that used something that moved continuously. No early method of keeping time changed at a steady rate.[6] The oldest known timekeeping devices were made in Ancient Egypt, since when the devices and methods for keeping time have improved continuously through a long series of new inventions and ideas.[7]

The first devices used for measuring the position of the Sun were shadow clocks, which later developed into the sundial.[8][note 1] Ancient Egyptian obelisks, constructed c.3500 BC, are also among the earliest shadow clocks.[9] The oldest of all known sundials dates back to c.1500 BC (during the 19th Dynasty), and was discovered in the Valley of the Kings in 2013.[10] 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. Each part was further divided into more precise parts.[10][11]

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.[12] A third shadow clock, developed c.500 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.[13]

Although accurate, shadow clocks were useless at night and in cloudy weather.[14] The Egyptians therefore developed other timekeeping instruments, including the water clock, and a system for tracking star movements. The oldest description of a water clock is from the tomb inscription of the early 18th Dynasty (c.1500 BC) court official Amenemhet, now lost, identifying him as its inventor.[15] It is assumed that the object described on the inscription is a classical Egyptian water clock, i.e. 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.[16] The oldest known water clock was found in the tomb of pharaoh Amenhotep III (c.14171379 BC).[17]

Babylonian clay tablet BM 29371, which describes a water clock (British Museum)

Another Egyptian method of determining the time during the night was using a type of plumb-line called a merkhet. In use since at least 600 BC, two merkhets were aligned with Polaris, the north pole star, to create a north–south meridian. The time was determined by observing particular stars as they crossed the meridian.[18]

Water clocks and sundials were known from classical antiquity;[19] a clay tablet from the late Babylonian period describes the lengths of shadows at different times of the year.[20] There are no recognised examples in existence of outflowing water clocks from ancient Mesopotamia of outflowing water clocks, but written references have survived.[20] The text in a tablet in the British Museum describes a water clock used by astronomers (who also worked as celestial diviners), that measured time using weights of water. It "explicitly describes a ratio of the longest to the shortest night as 3:2 in terms of weight".[21]

Ancient Greece and Rome

The Babylonian writer Berossos (fl.3rd century BC) is credited by the Greeks with the invention of a hemispherical sundial hollowed out of stone, which was designed so that the path of the shadow was divided into 12 parts as a way to mark the time.[22] Greek sundials evolved to become highly sophisticated; Ptolemy's Analemma, written in the 2nd century AD, used an early form of trigonometry to derive the position of the sun from data such as the hour of day and the geographical latitude. The Romans borrowed the idea of the sundial from the Greeks.[23][24][note 2]

The Greek philosophers Anaxagoras and Empedocles both referred to a simple form of water clock, of which two kinds were known—vessels that were used to enforce time limits, and others that marked the passing of time.[27][28] The Athenian philosopher Plato is supposed to have invented a form of alarm clock to wake his students,[29] that might have consisted of lead balls in a floating vessel. The lead balls cascaded noisily onto a copper platter after the floating vessel reached the top of its container of water.[30][note 3]

The Tower of the Winds in Athens (1st century BC)

The Greek astronomer Andronicus of Cyrrhus designed the Tower of the Winds in Athens in the 1st century BC; there is evidence that it once had eight sundials, a water clock, and a wind vane.[31] 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".[32]

Although still not as accurate as sundials, Greek water clocks became more accurate around 325 BC, and they were adapted to have a face with an hour hand, making the reading of the clock more precise and convenient. One of the more common problems in most types of clepsydrae was caused by water pressure: when the container holding the water was full, the increased pressure caused the water to flow more rapidly. This problem was addressed by Greek and Roman horologists beginning in 100 BC, and improvements continued to be made in the following centuries. To counteract the increased water flow, the clock's water containers—usually bowls or jugs—were given a conical shape; positioned with the wide end up, a greater amount of water had to flow out in order to drop the same distance as when the water was lower in the cone. Along with this improvement, clocks were constructed more elegantly in this period, with hours marked by gongs, doors opening to miniature figurines, bells, or moving mechanisms.[14] One problems associated with water clocks that meant they did not work well was the effect of temperature on liquid water. Water flows more slowly when cold, or freezes, and the rate of evaporation from the surface is temperature-dependent.[33]

Between 270 BC and AD 500, the Hellenistic mathematicians Ctesibius, Hero of Alexandria, and Archimedes, and Roman horologists 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.[citation needed] The Greek engineer Philo of Byzantium (fl. 3rd century BC) described how liquid was used to slow down the speed of a water clock in his technical treatise Pneumatics (chapter 31) where he likens the mechanism of a washstand automaton with those as employed in (water) clocks.[34]

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 is said to have invented a universal sundial that was accurate anywhere on Earth, though little is known about it.[35] The obelisk from Campus Martius was used as the gnomon for Augustus's zodiacal sundial.[36] The Roman military commander and naturalist 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.[37]

Chinese water clocks

The original diagram of Su Song's book showing the inner workings of his clock tower

British historian of Chinese science 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.[38] Needham stated 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.[38]

In 721 the Tantric monk and mathematician Yi Xing and government official Liang Lingzan regulated the power of the water driving an astronomical clock, dividing the power into unit impulses so that motion of the planets and stars could be duplicated.[39] The liquid in water clocks was liable to freezing, and had to be kept warm with torches, a problem that was solved in 976 by the Chinese astronomer and engineer Zhang Sixun. His invention—a considerable improvement on Yi Xing's clock—used mercury instead of water. Mercury is a liquid at room temperature, and freezes at −38.9 °C (−38.0 °F), lower than any air temperature normally found on Earth.[40][41] A water-powered astronomical clock tower was built by the polymath Su Song in 1088,[14] which featured the first known endless power-transmitting chain drive in horology.[42]

Chinese incense clocks

Incense clocks were first used in China around the 6th century; in Japan, one still exists in the Shōsōin,[43] although its characters are not Chinese, but Devanagari.[44] Due to their frequent use of Devanagari characters, suggestive of their use in Buddhist ceremonies, American sinologist Edward H. Schafer speculated that incense clocks were invented in India.[44] As they burn evenly and without a flame; they are accurate and safe for indoor use.[45]

A Chinese incense clock. Time is measured by burning powdered incense along a pre-measured path.

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; some dropped weights at even intervals.[46] Incense with different scents was used, so that the hours were marked by a change in odour as the sticks burnt away.[47] Incense sticks could be straight or spiralled; the spiralled ones were intended for long periods of use, and often hung from the roofs of homes and temples.[48] 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.[49]

Incense seal clocks were used for similar occasions and events as the stick clock; while religious purposes were of primary importance,[46] these clocks were also popular at social gatherings, and were used by Chinese scholars and intellectuals.[50] The seal was a wooden or stone disk with one or more grooves etched in it[46] into which incense was placed.[51] These clocks were common in China,[50] but were produced in fewer numbers in Japan.[52] To mark different hours, differently scented incenses (made from different recipes) could be used.[53] 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; to burn 12 hours an incense path of around 20 feet (6.1 m) has been estimated.[54]

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.[55] 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.[52]

One of the earliest mentions of a candle clock is in a Chinese poem, written in AD 520 by You Jianfu, who wrote of the graduated candle being a means of determining time at night. Similar candles were used in Japan until the early 10th century.[56]

Ancient and medieval Persia

A Persian water clock, or fenjaan

The use of water clocks by the Persians dates to 500 BC, the time of the Achaemenid Empire. According to the Greek historian Callisthenes, farmers used a water clock (called a fenjaan) in 328 BC to ensure a just and exact distribution of water from qanats for irrigation. A bowl with a small hole floated in a large pot of water. As soon as the bowl had sunk, the manager (called the khaneh fenjaan) emptied it and put it on the top of the water again. Stones were used to record the number of times the bowl sank. More than one manager—usually a wise elder—was needed to be in charge of continuously keeping the time using the fenjaan.[citation needed]

The place where the clock was situated, also called a khaneh fenjaan, would usually be the top floor of a building, with west- and east-facing windows to allow the times of sunset and sunrise to be viewed. The fenjaan was also used determine the days of pre-Islamic religions, such as the Nowruz, Chelah, or Yaldā—the shortest, longest, and equal-length days and nights of the years. Water clocks were at that time one of the most practical ancient tools for timing the calendar.[citation needed]

Other early references to timekeeping

A sundial is referred to in the Bible, when Hezekiah, king of Judea during the 8th century BC, was healed by the prophet Isaiah. After the king asked for a sign he would recover, the Old Testament reads:[57]

And Isaiah said, This sign shalt thou have of the Lord, that the Lord will do the thing that he hath spoken: shall the shadow go forward ten degrees, or go back ten degrees? And Hezekiah answered, It is a light thing for the shadow to go down ten degrees: nay, but let the shadow return backward ten degrees. And Isaiah the prophet cried unto the Lord: and he brought the shadow ten degrees backward, by which it had gone down in the dial of Ahaz.

Early timekeeping innovations

Candle clocks

(left) Al-Jazari's 'scribe' candle clock, illustrated in his Book of Knowledge of Ingenious Mechanical Devices; (right) An 18th century candle clock

In the 10th century, the invention of the candle clock was attributed by the Anglo-Saxons to Alfred the Great, king of Wessex. The story of how the clock was created was narrated by Asser, the king's biographer, who lived at Alfred's court and became his close associate.[58] Alfred used six candles, each made from 12 pennyweights of wax, and made to be 12 inches (30 cm) high and of a uniform thickness. The candles were marked at intervals of an inch. Once lit, they protected from the wind by being placed in a lantern made of wood and transparent horn. It would have taken 20 minutes to burn down to the next mark; the candles, burning one after the other, lasted for 24 hours.[59]

The 12th century Muslim inventor Al-Jazari described four different designs for a candle clock in his book The Book of Knowledge of Ingenious Mechanical Devices (IKitab fi Ma'rifat al-Hiyal al-Handasiyya).[60][61] His so-called 'scribe' candle clock was invented to mark the passing of 14 hours of equal length: a precisely engineering mechanism caused a candle of specific dimensions to be slowly pushed upwards, which caused an indicator to move along a scale. Every hour a small ball emerged from the beak of a bird.[60]


According to the German historian of astronomy Ernst Zinner, during the 13th century sundials were developed with scales that showed equal hours, whilst the first based on polar time appeared in Germany c.1400; an alternative theory proposes that a Damascus sundial measuring in polar time can be dated to 1372.[62] The modern sundial first appeared following the Copernican Revolution and the adoption of equal hours.[63]

European treatises on sundial design appeared c.1500.[64] In 1524, the French astronomer Oronce Finé who wrote a treatise, and constructed an example of a sundial made of ivory, now in the Museo Poldi Pezzoli, Milan. The instrument, intended for the court of Francis I of France, was in the shape of a shipl: when oriented correctly, a plumb line cast a shadow on the dial. The hours and two zodiacal scales are engraved on the hull, while the signs of the constellations appear along the mast.[65]


A detail from Ambrogio Lorenzetti's Allegory of Good Government (c.1338), Palazzo Pubblico, Siena

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 the Italian artist Ambrogio Lorenzetti, from 1338.[66] 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.[67] The Portuguese navigator Ferdinand Magellan used 18 hourglasses on each ship during his circumnavigation of the globe in 1522.[68] Though also used in China, the hourglass's history there is unknown,[69] but does not seem to have been used in China before the mid 16th century,[70] and the hourglass implies the use of glassblowing, which appears to be an entirely European and Western art.[71]

Gears in clocks and astrolabes

The first innovations to improve on the accuracy of the hourglass and the water clock occurred in the 10th century, when attempts were made to use weights or friction to slow the rate of flow of the sand or water. [72] The first geared clock was invented in the 11th century by the Arab engineer Ibn Khalaf al-Muradi in Islamic Iberia; it was a water clock that employed both segmental and epicyclic gearing, capable of transmitting high torque.[73] Islamic water clocks, which used complex gear trains and included arrays of automata, were unrivalled in their sophistication until the mid-14th century.[73][74] They developed a liquid-driven mechanism (using heavy floats and a constant-head system) to cause water clocks to descend at a slower rate.[74]

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.[75] 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.[76]

(left) al-Bīrūnī's 11th century description of a geared astrolabe; (right) the astrolabe made in c.1221 by the astronomer al‐Farisi (History of Science Museum, Oxford)

Contemporary Muslim astronomers constructed a variety of highly accurate astronomical clocks for use in their mosques and observatories,[77] such as the astrolabic clock by Ibn al-Shatir in the early 14th century.[78] Sophisticated timekeeping astrolabes with geared mechanisms were made in Persia, built by the polymath Abū Rayhān Bīrūnī in the 11th century and the astronomer Muhammad ibn Abi Bakr al‐Farisi in c.1221.[79][80]

The brass and silver astrolabe made in Isfahan by Muhammad ibn Abi Bakr al‐Farisi is the earliest machine with its gears still intact. It is both an astrolabe and a calendar. The design originates from a text by Abū Rayhān Bīrūnī, but the gearing has been simplified. Openings on the back of the astrolabe depict the lunar phases and gives the Moon's age; within a zodiacal scale are two concentric rings that show the relative positions of the Sun and the Moon.[81]

A sophisticated water-powered astronomical clock was described by Al-Jazari in his treatise on machines, written in 1206.[82] 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.[83] 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.[84]

Mechanical innovations in medieval Europe

The English word clock first appeared in Middle English as clok, cloke, or clokke. The origin of the word is not known for certain; it may be a borrowing from French or Dutch, and can perhaps be traced to the post-classical Latin clocca ('bell'). 7th century Irish and 9th century Germanic sources recorded clock as meaning ‘bell’.[85]

Judaism, Christianity and Islam all had times set aside for prayer, although Christians alone were expected to attend prayers at specific hours of the day and night—what the historian Jo Ellen Barnett describes as "a rigid adherence to repetitive prayers said many times a day".[86] The bell-striking alarms warned the monk on duty to toll the monastic bell. His alarm was a timer that used a form of escapement to ring a small bell. This mechanism was the forerunner of the escapement device found in the mechanical clock.[87][88]

Large mechanical clocks were invented which were mounted in towers to ring the bell directly. The earliest known are the tower clock of Norwich Cathedral (constructed c.1321 1325), the clock at St Albans Abbey (completed c.1360), and an astronomical clock designed and built by Giovanni Dondi dell'Orologio that was completed in 1364.[note 4] None of these early clocks have survived.[89] During the 14th century, striking clocks appeared with increasing frequency in public spaces, first in Italy, slightly later in France and England—between 1371 and 1380, public clocks were introduced in over 70 European cites.[90] The first professional clockmakers[when?] came from the guilds of locksmiths and jewellers.[91] The weight-driven mechanism is probably a Western European invention, as a picture of a 13th-century clock shows a weight pulling an axle around, its motion slowed by a system of holes that slowly released water.[92]

Salisbury Cathedral's restored medieval clock at the Science Museum, London

At around the same time as the invention of the escapement mechanism, the Florentine poet Dante Alighieri used clock imagery to depict the souls of the blessed in Paradiso, the third part of the Divine Comedy. It may be the first known literary description of a mechanical clock.[93] Giovanni da Dondi, Professor of Astronomy at Padua, presented the earliest detailed description of clockwork in his 1364 treatise Il Tractatus Astrarii.[94] This has inspired several modern replicas, including some in London's Science Museum and the Smithsonian Institution.[94] Other notable examples from this period were built in Milan (1335), Strasbourg (1354), Rouen (1389), Lund (c.1425) and Prague (1462).[94] Early clock dials showed hours; a clock with a minutes dial is mentioned in a 1475 manuscript.[95] By 1577 the Danish astronomer Tycho Brahe had obtained the first of four clocks that measured in seconds.[96]

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.[97][note 5] Wells Cathedral clock, built in 1392, is unique in that it still has its original medieval face, showing a model of the pre-Copernican, geocentric universe. Above the clock are figures which hit the bells, and a set of jousting knights who revolve around a track every 15 minutes.[citation needed][note 6] Similar astronomical clocks, or horologes, survive at Exeter, Ottery St Mary, and Wimborne Minster.[citation needed] Clock towers in Western Europe in the Middle Ages struck the time. 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.

The Ottoman engineer Taqi al-Din described a weight-driven clock with a verge-and-foliot escapement, a striking train of gears, an alarm, and a representation of the moon's phases in his book The Brightest Stars for the Construction of Mechanical Clocks (Al-Kawākib al-durriyya fī wadh' al-bankāmat al-dawriyya), written around 1556.[99]

Era of precision timekeeping

Pendulum clocks

(left and center) The first pendulum clock, invented by Christiaan Huygens in 1656. His invention increased the accuracy of clocks more than sixty-fold; (right) Netscher's portrait of Huygens (1671).

The first pendulum clock was designed and built by Dutch polymath Christiaan Huygens in 1656.[100] Early versions erred by less than one minute per day, and later ones only by 10 seconds, very accurate for their time.[100] Dials that showed minutes and seconds became common after the increase in accuracy made possible by the pendulum clock. The 16th-century astronomer Tycho Brahe used clocks with minutes and seconds to observe stellar positions.[95]

The first pendulum clocks used a verge escapement, which required wide swings of about 100°, and so by necessity had short, light pendulums.[101] The swing was reduced to around 6° after the invention of the anchor mechanism enabled the use of longer, heavier pendulums with slower beats that had less variation, as they more closely resembled simple harmonic motion, required less power, and caused less friction and wear.[102] The first known anchor escapement clock was built by the English clockmaker William Clement in 1671 for KIng's Collage, Cambridge,[103] now in the Science Museum, London.[104] The anchor escapement originated with Hooke, although it has been argued that it was invented by Clement, [105] or the English clockmaker Joseph Knibb.[104]

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] 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.[108]

Detail from the face of an equation clock made by Ferdinand Berthoud, c.1752 (Metropolitan Museum of Art)

Huygens first used a clock to calculate the equation of time (the difference between the apparent solar time and the time given by a clock), publishing his results in 1665. The relationship enabled astronomers to use the stars to measure sidereal time, which provided an accurate method for setting clocks. The equation of time was engraved on sundials so that clocks could be set using the sun. In 1720, Joseph Williamson claimed to have invented an clock that showed solar time, fitted with a cam and differential gearing, so that the clock indicated true solar time.[109][110][111]

Other innovations in timekeeping during this period include the invention of the rack and snail striking mechanism for striking clocks by the English mechanician Edward Barlow, the invention by either Barlow or Daniel Quare, a London clock-maker, in 1676 of the repeating clock, that chimes the number of hours (or even minutes),[112] and the deadbeat escapement, and the deadbeat escapement, invented around 1675 by the astronomer Richard Towneley,[113] George Graham invented the mercury pendulum in 1726,[114] and Graham's pupil Thomas Mudge created the first lever escapement in 1754.[115]

Paris and Blois were the early centres of clockmaking in France, and French clockmakers such as Julien Le Roy, clockmaker of Versailles, were leaders in case design and ornamental clocks.[91] 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.[when?] The competition and scientific rivalry resulting from his discoveries further encouraged researchers to seek new methods of measuring time more accurately.[116]

Between 1794 and 1795, in the aftermath of the French Revolution, the French government mandated the use of decimal time, with a day divided into 10 hours of 100 minutes each. A clock in the Palais des Tuileries kept decimal time as late as 1801.[117]

John Harrison and the marine chronometer

John Harrison's H5 chronometer

Marine chronometers are clocks used at sea as time standards, to determine longitude by celestial navigation. 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. The marine chronometer kept the time of a fixed location—usually Greenwich Mean Time—which allowed seafarers to determine longitude by comparing the local time at noon with the time on the chronometer.[118][119]

After the Scilly naval disaster of 1707 where four ships ran aground due to navigational mistakes, the British government offered a large prize of £20,000, equivalent to millions of pounds today, for anyone who could determine longitude accurately. The reward was eventually claimed in 1761 by Yorkshire carpenter John Harrison, who dedicated his life to improving the accuracy of his clocks.[citation needed]

At the age of 23, Harrison had used his carpentry skills to construct a wooden eight-day clock, now in the British Museum.[120] In 1735 he built his first chronometer, which he steadily improved on over the next thirty years before submitting it for examination. The clock had many innovations, including the use of bearings to reduce friction, weighted balances to compensate for the ship's pitch and roll in the sea and the use of two different metals to reduce the problem of expansion from heat.[citation needed]

The chronometer was trialled in 1761 by Harrison's son and by the end of 10 weeks the clock was in error by less than 5 seconds.[121][better source needed]

Electric clocks

One of Alexander Bain's early electromagnetic clocks, from the 1840s

In 1815, the prolific English inventor Francis Ronalds produced the forerunner of the electric clock, the electrostatic clock. It was powered with dry piles, a high voltage battery with extremely long life but the disadvantage of its electrical properties varying according to the air temperature and humidity. He experimented with ways of regulating the electricity and his improved devices proved to be more reliable.[122]

In 1840 the Scottish clock and instrument maker Alexander Bain, first used electricity to sustain the motion of a pendulum clock, and so can be credited with the invention of the electric clock.[123] On January 11, 1841, Bain and the chronometer maker John Barwise took out a patent describing a clock with an electromagnetic pendulum. The English scientist Charles Wheatstone, whom Bain met in London to discuss his ideas for an electric clock, produced his own version of the clock in November 1840, but Bain won a legal battle to establish himself as the inventor.[124][125]

The piezoelectric properties of crystalline quartz were discovered by the French physicist brothers Jacques and Pierre Curie in 1880.[126] The first quartz crystal oscillator was built by the American engineer Walter G. Cady in 1921, and in 1927 the first quartz clock was built at Bell Telephone Laboratories.[127] 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.[128] Their inherent physical and chemical stability and accuracy has resulted in the subsequent proliferation of quartz devices, and since the 1940s they have formed the basis for precision measurements of time and frequency worldwide.[129]

Development of the watch

(above) An illustration of a Huygens balance spring attached to a balance wheel; (below) An early balance spring watch by Thomas Tompion

The concept of the wristwatch goes back to the production of the very earliest watches in the 16th century. Elizabeth I of England what was received a described as an arm watch from Robert Dudley, 1st Earl of Leicester in 1571.[citation needed] The invention of the mainspring in the early 15th century allowed portable clocks to be built, which could be made to accurate enough for minute hands to be included in their construction, but the first pocketwatches were less accurate, as their size precluded them from having sufficiently well-made moving parts.[130]

Dials that showed minutes and seconds became common after the increase in accuracy made possible by the balance spring (or hairspring).[95] Invented separately In 1675 by Huygens and Hooke, it enabled the oscillations of the balance wheel to have a fixed frequency.[131] The invention resulted in a great advance in the accuracy of the mechanical watch, from around half an hour to within a few minutes per day.[132] Some dispute remains as to whether the balance spring was first invented by Huygens or by Hooke; both scientists claimed to have come up with the idea of the balance spring first. Huygens' design for the balance spring is the type used in virtually all watches up to the present day.[132]

Thomas Tompion was one of the first clockmakers to recognise the potential of the balance spring and use it successfully in his pocket watches;[133] the improved accuracy enabled watches to perform as well as they are generally used today, as a second hand to be added to the face, a development that occurred during the 1690s.[134] The concentric minute hand was an earlier invention, but a mechanism was devised by Quare that enabled the hands to be actuated together.[135] Nicolas Fatio de Duillier, a Swiss natural philosopher, is credited with the design of the first jewel bearings in watches in 1704.[136]

Other notable 18th century English horologists include John Arnold and Thomas Earnshaw, who devoted their careers towards constructing high quality chronometers and so-called 'deck watches', smaller versions of the chronometer that could be kept in a pocket.[137]

Military use of the watch

Watches were worn during the Franco-Prussian War (18701871), and by the time of the Boer War (18991902), they has been recognised as a valuable tool.[138] Early models were essentially standard pocket watches fitted to a leather strap, but by the early 20th century, manufacturers began producing purpose-built wristwatches. In 1904, Alberto Santos-Dumont, an early aviator, asked his friend the French watchmaker Louis Cartier to design a watch that could be useful during his flights.[139]

During World War I, wristwatches were used by artillery officers.[140] The so-called trench watch, or 'wristlets' were practical, as they freed up one hand that would normally be used to operate a pocket watch, and became standard equipment.[141][142] The demands of trench warfare meant that soldiers needed to protect the glass of their watches, and a guard in the form of a hinged cage was sometimes used.[142] The guard was designed to allow the numerals to be read easily, but it obscured the hands—a problem that was solved after the introduction of shatter-resistant Plexiglass in the 1930s. [142] Prior to the advent of its military use, the wristwatch was typically only worn by women, but during World War I they became symbols of masculinity and bravado.[142]

Modern watches

Quartz watches with their case backs removed. The arrows point to the crystal resonators, which keep the time.

In 1969, Seiko produced the world's first quartz wristwatch, the Astron.[143]

Atomic clocks

Atomic clocks are the most accurate timekeeping devices in practical use today. Accurate to within a few seconds over many thousands of years, they are used to calibrate other clocks and timekeeping instruments.[144] The National Bureau of Standards (now NIST) changed the way it based the time standard of the United States from quartz to atomic clocks in the 1960s.[145]

Louis Essen (right) and Jack Parry standing next to the world's first caesium-133 atomic clock

The idea of using atomic transitions to measure time was first suggested by the British scientist Lord Kelvin in 1879,[146][non-primary source needed] although it was only in the 1930s with the development of magnetic resonance that there was a practical method for measuring time in this way.[147] A prototype ammonia maser device was built in 1948 at the U.S. National Bureau of Standards (NBS, now National Institute of Standards and Technology (NIST)). Although it was less accurate than existing quartz clocks, it served to demonstrate the concept.[148]

The first accurate atomic clock, a caesium standard based on a certain transition of the caesium-133 atom, was built by the English physicist Louis Essen in 1955 at the National Physical Laboratory in London.[149] The clock was calibrated by the use of the astronomical time scale ephemeris time (ET).[150]

The International System of Units (SI) standardized its unit of time, the second, on the properties of cesium in 1967.[148] The 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.[151] The cesium atomic clock maintained by NIST is accurate to 30 billionths of a second per year.[148] 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).[148]

See also


  1. The inventor of the quartz clock, Warren Marrison, noted that the sundial is not a timekeeping device, as it could only "at best keep local solar time".[7]
  2. A verse by Plautus (c.254  184 BC) shows that sundials were familiar to the Romans:[25][26]

    The gods confound the man who first found out
      How to distinguish hours! Confound him too,
    Who in this place set up a sundial,
      To cut and hack my days so wretchedly
    Into small portions—When I was a boy,
      My belly was my sun-dial: one more sure,
    Truer, and more exact than any of them.
      This dial told me when 'twas proper time
    To go to dinner, when I had aught to eat—
      But now-a-days, why, even when I have,
    I can’t fall to, unless the sun gives leave.
      The town’s so full of these confounded dials,
    The greatest part of its inhabitants
      Shrunk up with hunger, creep along the streets.

  3. Plato's invention may alternatively have consisted of two jars connected to enable water to force air through a whistle.[29]
  4. Giovanni de Dondi's work has been replicated based on the designs. His 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.[citation needed]
  5. The original verge and foliot timekeeping mechanism for the Salisbury Cathedral clock 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.[97] The wheels and gears are mounted in a 1.2 metres (3.9 ft) iron frame, held together with metal dowels and pegs. Two large stones supply the power, and cause ropes to unwind from wooden barrels. The barrels drive the main wheel (regulated by the escapement), and the striking mechanism and air brake.[97]
  6. 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.[98]


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Further reading