History of timekeeping devices
The history of timekeeping devices dates back to when ancient civilizations first observed astronomical bodies as they moved across the sky. Devices and methods for keeping time have since then improved through a long series of new inventions and ideas. Sundials and water clocks originated from ancient Egypt, and were later used by the Babylonians, the Greeks and the Chinese; medieval Islamic water clocks were unrivalled in their sophistication until the mid-14th century. Incense clocks, which may have been invented in India, were being used in China by the 6th century. The hourglass, one of the few reliable methods of measuring time at sea, was a European invention and does not seem to have been used in China before the mid-16th century.
In medieval Europe, purely mechanical clocks were developed after the invention of the bell-striking alarm, used to warn a man to toll the monastic bell. The weight-driven mechanical clock, controlled by the action of a verge and foliot, was a synthesis of earlier ideas derived from European and Islamic science, and one of the most important inventions in the history of the timekeeping. The most famous mechanical clock was designed and built by Henry de Vick in c.1360—for the next 300 years, all the improvements in timekeeping were essentially developments based on it. The invention of the mainspring in the early 15th century allowed small clocks to be built for the first time.
From the 17th century, the discovery that clocks could be controlled by harmonic oscillators led to the most productive era in the history of timekeeping. Leonardo da Vinci had produced the earliest known drawings of a pendulum in 1493–1494, and in 1582 Galileo Galilei had investigated the regular swing of the pendulum, discovering that frequency was only dependent on length. The pendulum clock, designed and built by Dutch polymath Christiaan Huygens in 1656, was so much more accurate than other kinds of mechanical timekeepers that few clocks have survived with their verge and foliot mechanisms intact. Other innovations in timekeeping during this period include inventions for striking clocks, the repeating clock and the deadbeat escapement. Errors in early pendulum clocks were eclipsed by those caused by temperature variation, a problem tackled during the 18th century by the English clockmakers John Harrison and George Graham; only the invention of invar in 1895 eliminated the need for such innovations.
From the 18th century, a succession of innovations and inventions led to timekeeping devices becoming increasingly accurate. Following the Scilly naval disaster of 1707, after which governments offered a prize to anyone who could discover a way to determine longitude, Harrison built a succession of accurate timepieces. The electric clock, invented in 1840, was used to control the most accurate pendulum clocks until the 1940s, when quartz timers became the basis for the precise measurement of time and frequency. The wristwatch, which had been recognised as a valuable military tool during the Boer War, became a symbol of masculinity and bravado after World War I. During the 20th century the non-magnetic wristwatch, battery-driven watches, the quartz wristwatch, and transistors and plastic parts were all invented. The most accurate timekeeping devices in practical use today are atomic clocks, which can be accurate to within a few billionths of a second per year. They are used to calibrate other clocks and timekeeping instruments.
Ancient civilizations observed astronomical bodies, often the Sun and Moon, to determine time.  According to the historian Eric Bruton, Stonehenge is likely to have been the Stone Age equivalent of an astronomical observatory, used to seasonal and annual events such as equinoxes or solstices.  As megalithic civilizations left no recorded history, little is known of their timekeeping methods. 
Mesoamericans modified their usual vigesimal (base-20) counting system when dealing with calendars to produce a 360-day year.  The Aboriginal Australians understood the movement of objects in the sky well, and used their knowledge to construct calendars and aid navigation; most Aboriginal cultures had seasons that were well-defined and determined by natural changes throughout the year, including celestial events. Lunar phases were used to mark shorter periods of time; the Yaraldi of South Australia being one of the few people recorded as having a way to measure time during the day, which was divided into seven parts using the position of the Sun. 
All timekeepers before the 13th century relied upon methods that used something that moved continuously. No early method of keeping time changed at a steady rate.  Devices and methods for keeping time have improved continuously through a long series of new inventions and ideas. 
The first devices used for measuring the position of the Sun were shadow clocks, which later developed into the sundial.  [note 1] Egyptian obelisks, constructed c. 3500 BC, are among the earliest shadow clocks.  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.   Obelisks could indicate whether it was morning or afternoon, as well as the summer and winter solstices.  A kind of shadow clock was developed c. 500 BC that was similar in shape to a bent T-square. It measured the passage of time by the shadow cast by its crossbar, and was oriented eastward in the mornings, and turned around at noon, so it could cast its shadow in the opposite direction. 
A sundial is referred to in the Bible, in 2 Kings 20:9–11, when Hezekiah, king of Judea during the 8th century BC, is recorded as being healed by the prophet Isaiah and asks for a sign that he would recover: 
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.
A clay tablet from the late Babylonian period describes the lengths of shadows at different times of the year.  The Babylonian writer Berossos ( fl. 3rd century BC) is credited by the Greeks with the invention of a hemispherical sundial hollowed out of stone; the path of the shadow was divided into 12 parts to mark the time.  Greek sundials evolved to become highly sophisticated— Ptolemy's Analemma, written in the 2nd century AD, used an early form of trigonometry to derive the position of the sun from data such as the hour of day and the geographical latitude.  [note 2] The Romans borrowed the idea of the sundial from the Greeks.  The military commander Pliny the Elder recorded that the first sundial in Rome arrived in 264 BC, looted from Catania in Sicily; according to him, it gave the incorrect time for a century, until the markings and angle appropriate for Rome's latitude were used. 
According to the German historian of astronomy Ernst Zinner, sundials were developed during the 13th century with scales that showed equal hours. The first based on polar time appeared in Germany c. 1400; an alternative theory proposes that a Damascus sundial measuring in polar time can be dated to 1372.  European treatises on sundial design appeared c. 1500. 
An Egyptian method of determining the time during the night, used from at least 600 BC, was a type of plumb-line called a merkhet. A north–south meridian was created using two merkhets aligned with Polaris, the north pole star. The time was determined by observing particular stars as they crossed the meridian. 
The oldest description of a clepsydra, or water clock, is from the tomb inscription of an early 18th Dynasty ( c. 1500 BC) Egyptian court official named Amenemhet, who is identified as its inventor.  It is assumed that the object described on the inscription is a bowl with markings to indicate the time.  The oldest surviving water clock was found in the tomb of pharaoh Amenhotep III ( c. 1417–1379 BC).  There are no recognised examples in existence of outflowing water clocks from ancient Mesopotamia of outflowing water clocks, but written references have survived. 
The introduction of the water clock to China, perhaps from Mesopotamia, occurred as far back as the 2nd millennium BC, during the Shang Dynasty, and at the latest by the 1st millennium BC. Around 550 AD, Yin Gui was the first in China to write of the overflow or constant-level tank. Around 610, two Sui Dynasty inventors, Geng Xun and Yuwen Kai, created the first balance clepsydra, with standard positions for the steelyard balance.  In 721 the mathematician Yi Xing and government official Liang Lingzan regulated the power of the water driving an astronomical clock, dividing the power into unit impulses so that motion of the planets and stars could be duplicated.  In 976, the Song dynasty astronomer Zhang Sixun addressed the problem of the water in clepsydrae freezing in cold weather when he replaced the water with liquid mercury.  A water-powered astronomical clock tower was built by the polymath Su Song in 1088,  which featured the first known endless power-transmitting chain drive. 
The Greek philosophers Anaxagoras and Empedocles both referred to water clocks that were used to enforce time limits or measure the passing of time.   The Athenian philosopher Plato is supposed to have invented an alarm clock that used lead balls cascading noisily onto a copper platter to wake his students. 
A problem with most clepsydrae was the variation in the flow of water due to the change in fluid pressure, which was addressed from 100 BC when the clock's water container was given a conical shape. They became more sophisticated when innovations such as gongs and moving mechanisms were included.  There is evidence that the 1st century BC Tower of the Winds in Athens once had eight sundials, a water clock, and a wind vane.  In Greek tradition, clepsydrae were used in court; later, a practise later adopted by the Ancient Romans. 
The first geared clock, invented in the 11th century by the Arab engineer Ibn Khalaf al-Muradi in Islamic Iberia, was a water clock that employed both segmental and epicyclic gearing. Islamic water clocks, which used complex gear trains and included arrays of automata, were unrivalled in their sophistication until the mid-14th century.   Liquid-driven mechanisms (using heavy floats and a constant-head system) were developed that enabled water clocks to work at a slower rate. 
The 12th-century Jayrun Water Clock at the Umayyad Mosque in Damascus was constructed by Muhammad al-Sa'ati, and was later described by his son Ridwan ibn al-Sa'ati in his On the Construction of Clocks and their Use (1203).  A sophisticated water-powered astronomical clock was described by Al-Jazari in his treatise on machines, written in 1206.  This castle clock was about 11 metres (36 ft) high, and included a display of the zodiac and the solar and lunar paths, and doors that opened on the hour, to reveal a mannequin.  In 1235, a water-powered clock that "announced the appointed hours of prayer and the time both by day and by night" stood in the entrance hall of the Mustansiriya Madrasah in Baghdad. 
Incense clocks were first used in China around the 6th century,  mainly for religious purposes, but also for social gatherings or by scholars.   Due to their frequent use of Devanagari characters, American sinologist Edward H. Schafer has speculated that incense clocks were invented in India.  As incense burns evenly and without a flame, the clocks were safe for indoor use.  To mark different hours, differently scented incenses (made from different recipes) were used. 
The incense sticks used 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.  Some clocks were designed to drop weights at even intervals, 
Incense seal clocks had a disk etched with one or more grooves, into which incense was placed.  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 metres (66 ft) has been estimated.  The gradual introduction of metal disks, most likely beginning during the Song dynasty, allowed craftsmen to more easily create seals of different sizes, design and decorate them more aesthetically, and vary the paths of the grooves, to allow for the changing length of the days in the year. As smaller seals became available, incense seal clocks grew in popularity and were often given as gifts. 
Sophisticated timekeeping astrolabes with geared mechanisms were made in Persia. Examples include those built by the polymath Abū Rayhān Bīrūnī in the 11th century and the astronomer Muhammad ibn Abi Bakr al‐Farisi in c.1221.   A brass and silver astrolabe (which also acts as a calendar) made in Isfahan by al‐Farisi is the earliest surviving machine with its gears still intact. 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. 
Muslim astronomers constructed a variety of highly accurate astronomical clocks for use in their mosques and observatories,  such as the astrolabic clock by Ibn al-Shatir in the early 14th century. 
One of the earliest references to a candle clock is in a Chinese poem, written in 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. 
The invention of the candle clock was attributed by the Anglo-Saxons to Alfred the Great, king of Wessex, who used six candles marked at intervals of one inch (25 mm), each made from 12 pennyweights of wax, and made to be 12 centimetres (4.7 in) high and of a uniform thickness. 
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).   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. 
The hourglass was one of the few reliable methods of measuring time at sea, and it has been speculated that it was used on board ships as far back as the 11th century, when it would have complemented the compass as an aid to navigation. The earliest unambiguous evidence of the use an hourglass appears in the painting Allegory of Good Government, by the Italian artist Ambrogio Lorenzetti, from 1338. 
The Portuguese navigator Ferdinand Magellan used 18 hourglasses on each ship during his circumnavigation of the globe in 1522.  Though used in China, the hourglass's history there is unknown,  but does not seem to have been used before the mid-16th century,  as the hourglass implies the use of glassblowing, then an entirely European and Western art. 
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 took on symbolic meanings, such as that of death, temperance, opportunity, and Father Time, usually represented as a bearded, old man. 
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'. 
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".  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.  
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 slow their rate of flow using friction or the force of gravity.  The earliest depiction of a clock powered by a hanging weight is from the Bible of St. Louis, an illuminated manuscript that shows a clock being slowed by water acting on a wheel. The illustration seems to show that weight-driven clocks were invented in western Europe.  A treatise written by Robert the Englishman in 1271 shows that medieval craftsmen were attempting to design a purely mechanical clock (i.e. only driven by gravity) during this period.  Such clocks were a synthesis of earlier ideas derived from European and Islamic science, such as gearing systems, weight drives, and striking mechanisms. 
In 1250, the artist Villard de Honnecourt illustrated a device that was the step towards the development of the escapement.  Another forerunner of the escapement was the horologia nocturna, which used an early kind of verge mechanism to operate a knocker that continuously struck a bell.  The weight-driven clock was probably a Western European invention, as a picture of a clock shows a weight pulling an axle around, its motion slowed by a system of holes that slowly released water.  In 1271, the English astronomer Robertus Anglicus wrote of his contemporaries that they were in the process of developing a form of mechanical clock.  [note 3]
The invention of the verge and foliot escapement in c.1275  was one of the most important inventions in both the history of the clock  and the history of technology.  It was the first type of regulator in horology.  A verge, or vertical shaft, is forced to rotate by a weight-driven crown wheel, but is stopped from rotating freely by a foliot. The foliot, which cannot vibrate freely, swings back and forth, which allows a wheel to rotate one tooth at a time.   Although the verge and foliot was an advancement on previous timekeepers, it was impossible to avoid fluctuations in the beat caused by changes in the applied forces—the earliest mechanical clocks were regularly reset using a sundial.  
At around the same time as the invention of the escapement, the Florentine poet Dante Alighieri used clock imagery to depict the souls of the blessed in Paradiso, the third part of the Divine Comedy, written in the early part of the 14th century. It may be the first known literary description of a mechanical clock.  There are references to house clocks from 1314 onwards; by 1325 the development of the mechanical clock can be assumed to have occurred. 
Large mechanical clocks were built that were mounted in towers so as to ring the bell directly. The tower clock of Norwich Cathedral (constructed c.1321 –1325) is the earliest such large clock known. The clock has not survived.  The first clock known to strike regularly on the hour, a clock with a verge and foliot mechanism, is recorded in Milan in 1336.  By 1341, clocks driven by weights were familiar enough to be able to be adapted for grain mills,  and by 1344 the clock in London's Old St Paul's Cathedral had been replaced by one with an escapement.  The foliot was first illustrated by Dondi in 1364,  and mentioned by the court historian Jean Froissart in 1369. 
The most famous example of a timekeeping device during the medieval period was a clock designed and built by the clockmaker Henry de Vick in c.1360,   which was said to have varied by up to two hours a day. For the next 300 years, all the improvements in timekeeping were essentially developments based on the principles of de Vick's clock.  Between 1348 and 1364, Giovanni Dondi dell'Orologio, the son of Jacopo Dondi, built a complex astrarium in Florence.  [note 4]
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.  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.  [note 5] The Wells Cathedral clock, built in 1392, is unique in that it still has its original medieval face. Above the clock are figures which hit the bells, and a set of jousting knights who revolve around a track every 15 minutes.  [note 6]
The invention of the mainspring in the early 15th century—a device first used in locks and for flintlocks in guns— allowed small clocks to be built for the first time.  The need for an escapement mechanism that steadily controlled the release of the stored energy, led to the development of two devices, the stackfreed (which although invented in the 15th century can be documented no earlier than c.1535) and the fusee, which first originated from medieval weapons such as the crossbow.  There is a fusee in the earliest surviving spring-driven clock, a chamber clock made for Philip the Good in c. 1430.  Leonardo da Vinci, who produced the earliest known drawings of a pendulum in 1493–1494,  illustrated a fusee in c. 1500, a quarter of a century after the coiled spring first appeared. 
Clock towers in Western Europe in the Middle Ages struck the time. Early clock dials showed hours; a clock with a minutes dial is mentioned in a 1475 manuscript.  During the 16th century, timekeepers became more refined and sophisticated, so that by 1577 the Danish astronomer Tycho Brahe was able to obtain the first of four clocks that measured in seconds,  and in Nuremberg, the clockmaker Peter Henlein was paid for making what is thought to have been the earliest example of a watch, made in 1524.  By 1500, the use of the foliot in clocks had begun to decline.  The oldest surviving spring-driven clock is a device made by Jacob Zech in 1525.   The first person to suggest travelling with a clock to determine longitude, in 1530, was the Dutch instrument maker Gemma Frisius. The clock would be set to the local time of a starting point whose longitude was known, and the longitude of any other place could be determined by comparing its local time with the clock time.  
The Ottoman engineer Taqi ad-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. 
The Italian polymath Galileo Galilei is thought to have first realised that the pendulum could be used as an accurate timekeeper after watching the motion of suspended lamps at Pisa Cathedral.  In 1582, he investigated the regular swing of the pendulum, and discovered that this was only dependent on its length. Galileo never constructed a clock based on his discovery, but prior to his death he dictated instructions for building a pendulum clock to his son, Vincenzo. 
The first accurate timekeepers depended on the phenomenon known as harmonic motion, in which the restoring force acting on an object moved away from its equilibrium position—such as a pendulum or an extended spring—acts to return the object to that position, and causes it to oscillate.  Harmonic oscillators can be used as accurate timekeepers as the period of oscillation does not depend on the amplitude of the motion—and so it always takes the same time to complete one oscillation.  The period of a harmonic oscillator is completely dependent on the physical characteristics of the oscillating system and not the starting conditions or the amplitude. 
The period when clocks were controlled by harmonic oscillators was the most productive era in timekeeping.  [note 7] The first invention of this type was the pendulum clock, which was designed and built by Dutch polymath Christiaan Huygens in 1656. Early versions erred by less than one minute per day, and later ones only by 10 seconds, very accurate for their time. Dials that showed minutes and seconds became common after the increase in accuracy made possible by the pendulum clock. Brahe used clocks with minutes and seconds to observe stellar positions.  The pendulum clock outperformed all other kinds of mechanical timekeepers to such an extent that these were usually refitted with a pendulum—a task that could be done without difficulty —so that few verge escapement devices have survived in their original form. 
The first pendulum clocks used a verge escapement, which required wide swings of about 100° and so had short, light pendulums.  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.  The first known anchor escapement clock was built by the English clockmaker William Clement in 1671 for KIng's College, Cambridge,  now in the Science Museum, London.  The anchor escapement originated with Hooke, although it has been argued that it was invented by Clement,  or the English clockmaker Joseph Knibb. 
The Jesuits made major contributions to the development of pendulum clocks in the 17th and 18th centuries, having had an "unusually keen appreciation of the importance of precision".  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".  They served a crucial role in spreading and testing the scientific ideas of the period, and collaborated with Huygens and his contemporaries. 
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.   
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 minutes,  and the deadbeat escapement, invented around 1675 by the astronomer Richard Towneley. 
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.  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 during his career of almost five decades, which ended with his death in 1759. The competition and scientific rivalry resulting from his discoveries further encouraged researchers to seek new methods of measuring time more accurately. 
Any inherent errors in early pendulum clocks were smaller than other errors caused by factors such as temperature variation.  In 1729 the Yorkshire carpenter and self-taught clockmaker John Harrison invented the gridiron pendulum, which used at least three metals of different lengths and expansion properties, connected so as to maintain the overall length of the pendulum when it is heated or cooled by its surroundings.  In 1781 the clockmaker George Graham compensated for temperature variation in an iron pendulum by using a bob made from a glass jar of mercury—a liquid metal at room temperature that expands faster than glass. More accurate versions of this innovation contained the mercury in thinner iron jars to make them more responsive. This type of temperature compensating pendulum was improved still further when the mercury was contained within the rod itself, which allowed the two metals to be thermally coupled more tightly.  In 1895, the invention of invar, an alloy made form iron and nickel that expands very little, largely eliminated the need for earlier inventions designed to compensate for the variation in temperature. 
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. 
After the Scilly naval disaster of 1707 where four ships ran aground due to navigational mistakes, the British government offered a prize of £20,000, equivalent to millions of pounds today, for anyone who could determine the longitude to within 50 kilometres (31 mi) at a latitude just north of the equator.  The position of a ship at sea could be determined to within 100 kilometres (62 mi) if a navigator could refer to a clock that lost or gained less than about six seconds per day.  Proposals were examined by a newly created Board of Longitude.  Among the many people who attempted to claim the prize was the Yorkshire clockmaker Jeremy Thacker, who first used the term chronometer in a pamphlet published in 1714.  Huygens built the first sea clock, designed to remain horizontal aboard a moving ship, but that stopped working if the ship moved suddenly. 
In 1715, at the age of 22, Harrison had used his carpentry skills to construct a wooden eight-day clock.  His clocks had innovations that included the use of wooden parts to remove the need for additional lubrication (and cleaning), rollers to reduce friction, a new kind of escapement, and the use of two different metals to reduce the problem of expansion caused by temperature variation.  He traveled to London to seek assistance from the Board of Longitude in making a sea clock. He was sent to visit Graham, who assisted Harrison by arranging to finance his work to build a clock. After 30 years, his device, now named "H1" was built and in 1736 it was tested at sea. Harrison then went on to design and make two other sea clocks, "H2" (completed in around 1739) and "H3", both of which were ready by 1755.  
Harrison made two watches, "H4" and "H5". Eric Bruton, in his book The History of Clocks and Watches, has described H4 as "probably the most remarkable timekeeper ever made".  After the completion of its sea trials during the winter of 1761–1762 it was found that it was three times more accurate than was needed for Harrison to be awarded the Longitude prize.  
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. 
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.  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.  
In 1857, the French physicist Jules Lissajous showed how an electric current can be used to vibrate a tuning fork indefinitely, and was probably the first to use the invention as a method for accurately measuring frequency.  The piezoelectric properties of crystalline quartz were discovered by the French physicist brothers Jacques and Pierre Curie in 1880. 
The most accurate pendulum clocks were controlled electrically.  The Shortt–Synchronome clock, an electrical driven pendulum clock designed in 1921, was the first clock to be a more accurate timekeeper than the Earth itself. 
A succession of innovations and discoveries led to the invention of the modern quartz timer. The vacuum tube oscillator was invented in 1912.  An electrical oscillator was first used to sustain the motion of a tuning fork by the British physicist William Eccles in 1919;  his achievement removed much of the damping associated with mechanical devices and maximised the stability of the vibration's frequency.  The first quartz crystal oscillator was built by the American engineer Walter G. Cady in 1921, and in October 1927 the first quartz clock was described by Joseph Horton and Warren Marrison at Bell Telephone Laboratories.  [note 8] 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.  Their inherent physical and chemical stability and accuracy has resulted in the subsequent proliferation, and since the 1940s they have formed the basis for precision measurements of time and frequency worldwide. 
The first wristwatches were made in the 16th century. Elizabeth I of England had made an inventory in 1572 of the watches she acquired, all of which were considered to be part of her jewellery collection.  The first pocketwatches were inaccurate, as their size precluded them from having sufficiently well-made moving parts.  Unornamented watches began to appear in c. 1625. 
Dials that showed minutes and seconds became common after the increase in accuracy made possible by the balance spring (or hairspring).  Invented separately in 1675 by Huygens and Hooke, it enabled the oscillations of the balance wheel to have a fixed frequency.  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.  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. 
Thomas Tompion was one of the first clockmakers to recognise the potential of the balance spring and use it successfully in his pocket watches;  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.  The concentric minute hand was an earlier invention, but a mechanism was devised by Quare that enabled the hands to be actuated together.  Nicolas Fatio de Duillier, a Swiss natural philosopher, is credited with the design of the first jewel bearings in watches in 1704. 
Other notable 18th century English horologists include John Arnold and Thomas Earnshaw, who devoted their careers to constructing high quality chronometers and so-called 'deck watches', smaller versions of the chronometer that could be kept in a pocket. 
Watches were worn during the Franco-Prussian War (1870–1871), and by the time of the Boer War (1899–1902), watches had been recognised as a valuable tool.  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. 
During World War I, wristwatches were used by artillery officers.  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.   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.  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.  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. 
Fob watches were starting to be replaced at the turn of the 20th century.  The Swiss, who were neutral throughout World War I, produced wristwatches for both sides of the conflict. The introduction of the tank influenced the design of the Cartier Tank watch,  and the design of watches during the 1920s was influenced by the Art Deco style.  The automatic watch, first introduced with limited success in the 18th century, was reintroduced in the 1920s by the English watchmaker John Harwood.  After he went bankrupt in 1929, restrictions on automatic watches were lifted and companies such as Rolex were able to produce them.  In 1930, Tissot produced the first ever non-magnetic wristwatch. 
The first battery-driven watches were developed in the 1950s.  High quality watches were produced by firms such as Patek Philippe, an example made in 1933, an example being a Patek Philippe ref. 1518, possibly the most complicated wristwatch ever made in stainless steel, which fetched a world record price in 2016 when it was sold at auction for $11,136,642.   
The manual winding Speedmaster Professional or "Moonwatch" was worn during the first United States spacewalk as part of NASA's Gemini 4 mission and was the first watch worn by an astronaut walking on the Moon during the Apollo 11 mission.  In 1969, Seiko produced the world's first quartz wristwatch, the Astron. 
During the 1960s, the introduction of watches made using transistors and plastic parts enabled companies to reduce their work force. By the 1970s, many of those firms that maintained more complicated metalworking techniques had gone bankrupt. 
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.  The U.S. National Bureau of Standards (NBS, now National Institute of Standards and Technology (NIST)) changed the way it based the time standard of the United States from quartz to atomic clocks in the 1960s. 
The idea of using atomic transitions to measure time was first suggested by the British scientist Lord Kelvin in 1879,  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.  A prototype ammonia maser device was built in 1948 at NIST. Although less accurate than existing quartz clocks that existed at that time, it served to demonstrate the concept. 
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.  It was calibrated by the use of the astronomical time scale ephemeris time (ET). 
In 1967 the International System of Units (SI) standardized its unit of time, the second, on the properties of cesium.  The SI defined 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.  The cesium atomic clock maintained by NIST is accurate to 30 billionths of a second per year.  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). 
- 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". 
- A verse by
c. 254 – 184 BC) shows that sundials were familiar to the Romans:
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.
- Nor is it possible for any clock to follow the judgment of astronomy with complete accuracy. Yet clockmakers are trying to make a wheel which will make one complete revolution for every one of the equinoctial circle, but they cannot quite perfect their work. ( Latin: Nec est hoc possibile, quod aliquod horologium sequatur omnino iudicium astronomie secundum veritatem. Conantur tamen artifices horologiorum facere circulum unum qui omnino moveatur secundum motum circuli equinoctialis, sed non possunt omnino complere opus eorum, quod, si possent facere, esset horologium verax valde et valeret plus quam astrolabium quantum ad horas capiendas vel aliud instrumentum astronomie, si quis hoc sciret facere secundum modum antedictum.) 
- 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. His clock has inspired several modern replicas, including some in London's Science Museum and the Smithsonian Institution.  
- 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.  The wheels and gears are mounted in a 1.2 metres (3 ft 11 in) 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. 
- 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. 
- Harmonically-driven clocks depend on some form of deformation from an equilibrium position; the resulting oscillations have a maximum amplitude when they receive energy at a frequency close to their natural undamped frequency. The main examples of such harmonic oscillators used to keep time are: the electrical resonance circuit; the gravity pendulum; the quartz crystal oscillator and the tuning fork; the balance spring; the torsion spring; and the vertical pendulum. 
- Quartz resonators can vibrate with very a small amplitude that can be precisely controlled, properties that allow them to have a remarkable degree of frequency stability. 
- Bruton 2000, p. 11.
- Bruton 2000, pp. 235–237.
- Richards 1999, p. 130.
- Aveni 1980, pp. 158–159.
- Norris 2016, p. 27.
- Barnett 1999, p. 64.
- Marrison 1948, p. 510.
- Major 1998, p. 9.
- "Sundial". Encyclopædia Britannica. Retrieved April 4, 2008.
- "One of world's oldest sun dial dug up in Kings' Valley, Upper Egypt". ScienceDaily. March 14, 2013. Retrieved May 10, 2021.
- "Sundials". Royal Museums Greenwich. 2021. Retrieved May 27, 2021.
- Bruton 2000, p. 14.
- Barnett 1999, p. 18.
- Dolan 1975, pp. 31–32.
- Brown, Fermor & Walker 1999, p. 130.
- Dolan 1975, p. 34.
- Hart, Graham (1999). "Ptolemy on Sundials". Starry Messenger. Retrieved May 27, 2021.
- Dolan 1975, pp. 37–38.
- Thornton 1767, pp. 368–369.
- Dolan 1975, p. 35.
- Barnett 1999, p. 21.
- & Dolan 1975, p. 43.
- & Dolan 1975, p. 60.
- Magdolen 2001, p. 84.
- von Lieven 2016, p. 207.
- von Lieven 2016, p. 218.
- Cotterell & Kamminga 1990, p. 59.
- Needham 1965, pp. 479–480.
- Schafer 1967, p. 128.
- Needham 1965, pp. 469–471.
- "Earliest Clocks". A Walk Through Time. National Institute of Standards and Technology Physics Laboratory. Archived from the original on March 15, 2008. Retrieved April 2, 2008.
- Needham 1965, p. 411.
- van Dusen 2014, p. 257.
- Allen 1996, p. 157.
- Hellemans & Bunch 2004, p. 65.
- Noble & de Solla Price 1968, pp. 345–347.
- Humphrey 1998, pp. 518–519.
- Hill 2016, p. 17.
- Hill 1997, p. 242.
- Hill 1997, p. 234.
- Hill 1997, p. 203.
- al-Jazari 1974, p. 241.
- Hill 2016, p. 43.
- Pagani 2001, p. 209.
- Fraser 1990, pp. 55–56.
- Bedini 1994, pp. 103–104.
- Schafer 1963, pp. 160–161.
- Chang, Edward; Lu, Yung-Hsiang (December 1996). "Visualizing Video Streams using Sand Glass Metaphor". Stanford University. Retrieved June 20, 2008.
- Bedini 1963, p. 37.
- Rossotti 2002, p. 157.
- Fraser 1990, pp. 52, 55–56.
- Fraser 1990, p. 56.
- Bedini 1994, pp. 104–106.
- al-Hassan & Hill 1986, p. 24.
- Hill, Donald R.; al-Hassan, Ahmad Y. "Engineering in Arabic-Islamic Civilisation". History of Science and Technology in Islam. Retrieved May 28, 2021.
- "Inventory no. 48213 – Former Display Label". History of Science Museum, Oxford. Retrieved May 28, 2021.
- Ajram 1992, Appendix B.
- King 1983, pp. 545–546.
- Flamer, Keith (2006). "History of Time". International Watch Magazine. Archived from the original on July 16, 2011. Retrieved April 8, 2008.
- Asser 1983, p. 108.
- Hill 1997, p. 238.
- al-Jazari 1974, pp. 83–92.
- Frugoni 1988, p. 83.
- Bergreen 2003, p. 53.
- Blaut 2000, p. 186.
- Needham 1965, figure 995.
- Needham 1965, p. 570.
- Macey 1994, p. 209.
- "Clock". OED. 2021. Retrieved May 29, 2021.
- Barnett 1999, pp. 33–34, 37.
- Landes 1985, p. 67.
- Truitt 2015, pp. 145–146.
- Marrison 1948, pp. 813–814.
- White 1964, pp. 120–121.
- White 1964, p. 122.
- Hill 1997, pp. 223, 242–243.
- Baillie, Clutton & Ilbert 1969, p. 4.
- Landes 1985, pp. 67–68.
- White 1964, p. 120.
- Barnett 1999, p. 67.
- Thorndike, de Sacro Bosco & Robertus Anglicus 1949, pp. 180, 230.
- Bruton 2000, p. 49.
- Marrison 1948, p. 514.
- Hill 1997, p. 243.
- Barnett 1999, pp. 64, 79.
- Bruton 2000, p. 248.
- Barnett 1999, pp. 87–88.
- Moevs 1999, pp. 59–60.
- Baillie, Clutton & Ilbert 1969, pp. 5–6.
- Landes 1985, p. 53.
- Barnett 1999, p. 75.
- White 1964, p. 134.
- Baillie, Clutton & Ilbert 1969, p. 5.
- Bruton 2000, p. 244.
- Bruton 2000, p. 35.
- Barnett 1999, p. 64–65.
- Marrison 1948, p. 515.
- Baillie, Clutton & Ilbert 1969, p. 7.
- Davies 1996, p. 434.
- Bradbury & Collette 2009, pp. 353, 356.
- "Oldest Working Clock, Frequently Asked Questions, Salisbury Cathedral". Retrieved April 4, 2008.
- Colchester 1987, pp. 116–120.
- "Wells Cathedral clock, c.1392". Science Museum (London). Retrieved May 7, 2020.
- White 1964, pp. 126–128.
- Baillie, Clutton & Ilbert 1969, p. 66.
- Baillie, Clutton & Ilbert 1969, p. 19.
- Lankford 1997, p. 529.
- Thoren 1990, p. 123.
- Baillie, Clutton & Ilbert 1969, pp. 20–22.
- Baillie, Clutton & Ilbert 1969, p. 15.
- "History". Jacob Zech Original. 2021. Retrieved June 18, 2021.
- Pogo, A (1935). "Gemma Frisius, His Method of Determining Differences of Longitude by Transporting Timepieces (1530), and His Treatise on Triangulation (1533)". Isis. 22 (2): 469–506. doi: 10.1086/346920. S2CID 143585356.
- Meskens 1992, p. 259.
- al-Hassan & Hill 1986, p. 59.
- Cotterell & Kamminga 1990, p. 20.
- Baillie, Clutton & Ilbert 1969, pp. 67–68.
- Frautschi et al. 2008, p. 297.
- Frautschi et al. 2008, p. 309.
- Hüwel 2018, section 2–17.
- Marrison 1948, pp. 515–516.
- Bruton 2000, p. 72.
- Marrison 1948, p. 518.
- Headrick 2002, p. 44.
- Headrick 2002, pp. 44–45.
- Barnett 1999, p. 90.
- Bruton 2000, p. 70.
- Headrick 2002, p. 41.
- Woods 2005, pp. 100–101, 103.
- Woods 2005, p. 103.
- Woods 2005, p. 100.
- Buick 2013, p. 159.
- Richards 1999, pp. 24–25.
- Macey 1994, p. 125.
- Landes 1985, p. 220.
- Macey 1994, p. 126.
- Davies 1996, p. 435.
- "Julien Le Roy". Getty Center. Retrieved April 5, 2008.
- Marrison 1948, pp. 518–519.
- Baker 2011, p. 79–80.
- Matthys 2004, pp. 7–8.
- Baker 2011, p. 82.
- Alder 2002, p. 150.
- Bruton 2000, pp. 86–87.
- Bruton 2000, p. 89.
- Bruton 2000, p. 87.
- Bruton 2000, p. 90.
- "Harrison's eight-day wooden clock movement, 1715". Science Museum Group Collection. Retrieved June 4, 2021.
- Landes 1985, pp. 147–148.
- Bruton 2000, pp. 90–93.
- Barnett 1999, p. 111.
- Bruton 2000, p. 93.
- Bruton 2000, p. 94.
- Barnett 1999, p. 112.
- Ronalds 2015, p. 224.
- Marrison 1948, p. 522.
- Marrison 1948, p. 583.
- Thomson 1972, pp. 65–66.
- Marrison 1948, p. 524.
- "Pierre Curie". American Institute of Physics. Retrieved April 8, 2008.
- Marrison 1948, p. 523.
- Sidgwick & Muirden 1980, p. 478.
- Marrison 1948, p. 526.
- Marrison 1948, p. 527.
- Marrison 1948, p. 538.
- Marrison 1948, p. 533.
- Marrison 1948, p. 564.
- Marrison 1948, pp. 531–532.
- Bruton 2000, pp. 56–57.
- Landes 1985, p. 114.
- Baillie, Clutton & Ilbert 1969, p. 39.
- Landes 1985, pp. 124–125.
- Landes 1985, p. 128.
- Landes 1985, p. 219.
- Landes 1985, p. 129.
- Baillie, Clutton & Ilbert 1969, p. 280.
- "Nicolas Fatio de Duillier (1664–1753)". Famous Watchmakers. Fondation de la Haute Horlogerie. 2019. Retrieved May 22, 2021.
- Landes 1985, pp. 172, 185.
- Glasmeier 2000, p. 141.
- Hoffman 2004, p. 3.
- Bruton 2000, p. 183.
- Barnett 1999, p. 141.
- Pennington, Cole (September 24, 2019). "How World War I Changed Watches Forever". Bloomberg News. Retrieved June 3, 2021.
- Miller 2009, p. 9.
- Miller 2009, p. 26.
- Miller 2009, p. 30.
- Miller 2009, p. 39.
- Miller 2009, p. 51.
- "Non-magnetism". Tissot. Retrieved August 15, 2021.
- Miller 2009, p. 137.
- Miller 2009, p. 13.
- Touchot, Arthur (November 12, 2016). "Stainless Steel Patek Philippe Ref. 1518 Sells For Over $11,000,000 At Phillips Geneva". Hodinkee. Retrieved August 15, 2021.
- Clymer, Benjamin. "The Patek Philippe 1518 In Steel". Hodinkee. Retrieved August 15, 2021.
- Nelson 1993, pp. 33–38.
- "Electronic Quartz Wristwatch, 1969". IEEE History Center. Retrieved July 11, 2015.
- "Alarm Clocks from the Black Forest". Deutsches Uhrenmuseum. Retrieved August 17, 2021.
- Dick 2002, p. 484.
- Sullivan, D.B. (2001). "Time and frequency measurement at NIST: The first 100 years" (PDF). Time and Frequency Division, National Institute of Standards and Technology. p. 5. Archived from the original (PDF) on September 27, 2011.
- "Atomic ticker clocks up 50 years". BBC News. June 2, 2005. Retrieved August 1, 2021.
- Lombardi, Heavner & Jefferts 2007, p. 74.
- "The "Atomic Age" of Time Standards". National Institute of Standards and Technology. Archived from the original on April 12, 2008. Retrieved May 2, 2008.
- Essen & Parry 1955, p. 280.
- Markowitz et al. 1958, pp. 105–107.
- "What is a Cesium Atomic Clock?". National Research Council Canada. January 9, 2020. Retrieved May 15, 2021.
- Ajram, K. (1992). Miracle of Islamic Science. Cedar Rapids, Iowa: Knowledge House Publishers. ISBN 978-0-911119-43-5.
- Alder, Ken (2002). The Measure of All Things: The Seven-Year Odyssey and Hidden Error that Transformed the World. London: Little, Brown. ISBN 978-03168-5-989-9.
- Allen, Danielle (1996). "A Schedule of Boundaries: An Exploration, Launched from the Water-Clock, of Athenian Time". Greece & Rome. Cambridge University Press. 43 (2): 157–168. doi: 10.1093/gr/43.2.157. JSTOR 643092 – via JSTOR.
- Asser (1983) [before 909]. Alfred the Great: Asser's Life of King Alfred and other contemporary sources. Translated by Keynes, Simon; Lapidge, Michael. London; New York: Penguin Books. ISBN 978-01404-4-409-4.
- Aveni, Anthony (1980). Skywatchers of Ancient Mexico. Austin, Texas: University of Texas Press. ISBN 978-02927-0-502-9.
- Baillie, G.H.; Clutton, C.; Ilbert, C.A. (1969) . Britten's Old Clocks and Watches and their Makers (7th ed.). London: Eyre & Spottiswoode; E. & F.N. Spon Ltd. ISBN 9-780-41327-3-901.
- Baker, Gregory L. (2011). Seven Tales of the Pendulum. Oxford: Oxford University Press. ISBN 978-01995-8-951-7.
- Barnett, Jo Ellen (1999). Time's Pendulum: From Sundials to Atomic Clocks, the Fascinating History of Timekeeping and How Our Discoveries Changed the World (1st ed.). San Diego: Harcourt Trade Publishers. ISBN 978-01560-0-649-1.
- Bedini, Silvio A. (1963). "The Scent of Time. A Study of the Use of Fire and Incense for Time Measurement in Oriental Countries". Transactions of the American Philosophical Society. Philadelphia: American Philosophical Society. 53 (5): 1–51. doi: 10.2307/1005923. hdl: 2027/mdp.39076006361401. ISSN 0065-9746. JSTOR 1005923.
- Bedini, Silvio (1994). The Trail of Time: Shih-chien Ti Tsu-chi: Time Measurement with Incense in East Asia. Cambridge: Cambridge University Press. ISBN 978-0-521-37482-8.
- Bergreen, Laurence (2003). Over the Edge of the World: Magellan's Terrifying Circumnavigation of the Globe. New York: Morrow. ISBN 978-0-06-621173-2.
- Blaut, James Morris (2000). Eight Eurocentric Historians. Guildford Press. ISBN 978-1-57230-591-5.
- Bradbury, Nancy Mason; Collette, Carolyn P. (2009). "Changing Times: The Mechanical Clock In Late Medieval Literature". The Chaucer Review. Penn State University Press. 43 (4): 351–375. doi: 10.1353/cr.0.0027. ISSN 0009-2002.
- Brown, David; Fermor, John; Walker, Christopher (1999). "The Water Clock in Mesopotamia". Archiv für Orientforschung. 46/47: 130–148. JSTOR 41668444 – via JSTOR.
- Bruton, Eric (2000). The History of Clocks and Watches. London: Little, Brown. ISBN 978-05173-7-744-4.
- Buick, Tony (2013). Orrery: A Story of Mechanical Solar Systems, Clocks, and English Nobility. New York: Springer. ISBN 978-14614-7-042-7.
- Colchester, L.S. (1987). Wells Cathedral. London: Unwin Hyman. ISBN 978-00444-0-012-7.
- Cotterell, Brian; Kamminga, Johan (1990). Mechanics of Pre-Industrial Technology: An Introduction to the Mechanics of Ancient and Traditional Material Culture. Cambridge: Cambridge University Press. ISBN 978-05213-4-194-3.
- Davies, Norman (1996). Europe: A History. Oxford: Oxford University Press. ISBN 978-01982-0-171-7.
- Dick, Stephen (2002). Sky and Ocean Joined: The U.S. Naval Observatory, 1830–2000. Cambridge University Press. ISBN 978-0-521-81599-4.
- Dolan, Winthrop W. (1975). A Choice of Sundials. Brattleboro, Vermont: The Stephen Greene Press. ISBN 9780828902106. OCLC 471181086.
- van Dusen, David (2014). The Space of Time: a sensualist interpretation of time in Augustine, Confessions X to XII. Leiden; Boston (Massachusetts): Brill. ISBN 978-90042-6-686-5.
- Essen, L.; Parry, J. V. L. (1955). "An Atomic Standard of Frequency and Time Interval: A Cæsium Resonator". Nature. 176 (4476): 280. Bibcode: 1955Natur.176..280E. doi: 10.1038/176280a0. S2CID 4191481.
- Fraser, Julius (1990). Of Time, Passion, and Knowledge: Reflections on the Strategy of Existence. Princeton, New Jersey: Princeton University Press. ISBN 978-0-691-02437-0.
- Frautschi, Steven C.; Olenick, Richard P.; Apostol, Tom M.; Goodstein, David L.· (2008). The Mechanical Universe: Mechanics and Heat (Advanced ed.). Cambridge: Cambridge University Press. ISBN 978-11396-4-290-3.
- Frugoni, Chiara (1988). Pietro et Ambrogio Lorenzetti. New York: Scala Books. ISBN 978-09357-4-880-2.
- Glasmeier, Amy K (2000). Manufacturing Time: Global Competition in the Watch Industry, 1795-2000. New York: The Guilford Press. ISBN 978-15723-0-589-2.
- al-Hassan, Ahmad Y.; Hill, Donald R. (1986). Islamic Technology: an illustrated history. Cambridge: Cambridge University Press. ISBN 978-0-521-42239-0.
- Headrick, Mark V. (April 2002). "Origin and Evolution of the Anchor Clock Escapement" (PDF). IEEE Control Systems Magazine. New York.
- Hellemans, Alexander; Bunch, Bryan H. (2004). The History of Science and Technology: A Browser's Guide to the Great Discoveries, Inventions, and the People Who Made Them, From the Dawn of Time to Today. Boston: Houghton Mifflin. ISBN 978-06182-2-123-3.
- Hill, Donald R. (2016) . King, David A. (ed.). Studies in Medieval Islamic Technology From Philo to Al-Jazari – from Alexandria to Diyar Bakr. London; New York: Routledge. ISBN 978-08607-8-606-1.
- Hill, Donald Routledge (1997). A History of Engineering in Classical and Medieval Times. Routledge. ISBN 978-0-415-15291-4.
- Hoffman, Paul (2004). Wings of Madness: Alberto Santos-Dumont and the Invention of Flight. Hyperion Press. ISBN 978-0-7868-8571-8.
- Humphrey, John William (1998). Greek and Roman Technology: A Sourcebook. Routledge. ISBN 978-04150-6-136-0.
- Hüwel, Lutz (2018). Of Clocks and Time. San Rafael, California: Morgan & Claypool Publishers. ISBN 978-16817-4-096-6.
- al-Jazari, Ismail (1974). The Book of Knowledge of Ingenious Mechanical Devices (Kitab fi Ma'rifat al-Hiyal al-Handasiyya) by ibn al-Razzaz al-Jazari. Translated by Hill, Donald R. (1st (reprinted) ed.). Dordrecht: D. Reidel Publishing Company. ISBN 978-90277-0-329-3.
- King, David A. (1983). "The Astronomy of the Mamluks". Isis. 74 (4): 531–555. doi: 10.1086/353360. ISSN 0021-1753. JSTOR 232211. S2CID 144315162 – via JSTOR.
- Landes, David S. (1985). Revolution in Time: Clocks and the Making of the Modern World. Cambridge, Massachusetts: Harvard University Press. ISBN 9780674768024. OCLC 29148451.
- Lankford, John (1997). "Time and Timekeeping Instruments". History of Astronomy: an Encyclopedia. Hoboken: Taylor & Francis. ISBN 978-0-8153-0322-0.
- von Lieven, Alexandra (2016). "The Movement of Time. News from the "Clockmaker" Amenemhet". In Landgráfová, Renata; Mynářová, Jana (eds.). Rich and Great: Studies in Honour of Anthony J. Spalinger on the Occasion of his 70th Feast of Thoth. Prague: Charles University in Prague. pp. 207–231. ISBN 978-80730-8-668-8.
- Lombardi, Michael A.; Heavner, Thomas P.; Jefferts, Steven R. (2007). "NIST Primary Frequency Standards and the Realization of the SI Second" (PDF). Measure. NCSL International. 2 (4): 74–89. ISSN 1674-8042.
- Macey, Samuel L. (1994). Encyclopedia of Time. New York: Garland Publishing. ISBN 978-0-8153-0615-3.
- Magdolen, Dušan (2001). "An astronomical inscription on the Berlin merkhet" (PDF). Asian and African Studies. 10 (1): 80–87.
- Major, Fouad G. (1998). The Quantum Beat: The Physical Principles of Atomic Clocks. New York, NY: Springer. ISBN 978-0-387-98301-1. OCLC 37315254. Retrieved June 22, 2008.
- Markowitz, W.; Hall, R.G.; Essen, L.; Parry, J.V.L. (1958). "Frequency of Cesium in Terms of Ephemeris Time". Physical Review Letters. 1 (3): 105–107. Bibcode: 1958PhRvL...1..105M. doi: 10.1103/PhysRevLett.1.105. ISSN 1079-7114.
- Marrison, Warren A. (1948). "The Evolution of the Quartz Crystal Clock". Bell System Technical Journal. New York: AT&T. 27 (3): 510–588. doi: 10.1002/j.1538-7305.1948.tb01343.x. OCLC 10999639. S2CID 88503681.
- Matthys, Robert J. (2004). Accurate Clock Pendulums. Oxford: Oxford University Press. ISBN 978-01915-1-368-8.
- Meskens, Ad (1992). "Michiel Coignet's Nautical Instruction". The Mariner's Mirror. 78 (3): 257–276. doi: 10.1080/00253359.1992.10656406.
- Miller, Judith (2009). Watches: the ultimate accessory. London; New York: Miller's. ISBN 978-18453-3-476-5.
- Moevs, Christian (1999). "Miraculous Syllogisms: Clocks, Faith and Reason in Paradiso 10 and 24". Dante Studies. The Johns Hopkins University Press. 117 (117): 59–84. ISSN 0070-2862. JSTOR 40166538 – via JSTOR.
- Needham, Joseph (1965). Physics and Physical Technology, Part 2: Mechanical Engineering. Science and Civilization in China. 4. Cambridge: Cambridge University Press. ISBN 978-05216-5-270-4.
- Nelson, A. A. (1993). "The Moon Watch: a history of the Omega Speedmaster Professional". Bulletin of the National Association of Watch and Clock Collectors. 35 (282): 33–38.
- Noble, Joseph V.; de Solla Price, Derek J. (1968). "The Water Clock in the Tower of the Winds". American Journal of Archaeology. 72 (4): 345–355. doi: 10.2307/503828. ISSN 0002-9114. JSTOR 503828 – via JSTOR.
- Norris, R. (2016). "Dawes Review 5: Australian Aboriginal Astronomy and Navigation". Publications of the Astronomical Society of Australia. Cambridge University Press. 33 (33, E039): 1–39. arXiv: 1607.02215. doi: 10.1017/pasa.2016.25. ISSN 1323-3580. S2CID 119304459.
- Pagani, Catherine (2001). Eastern Magnificence and European Ingenuity: Clocks of Late Imperial China. Ann Arbor, Michigan: University of Michigan Press. ISBN 978-04721-1-208-1.
- Richards, Edward Graham (1999). Mapping Time: The Calendar and its History. New York: Oxford University Press. ISBN 978-01928-6-205-1.
- Ronalds, Beverley F. (2015). "Remembering the first battery-operated clock". Antiquarian Horology and the Proceedings of the Antiquarian Horological Society. 36 (2): 244–248. ISSN 0003-5785. S2CID 198943520.
- Rossotti, Hazel (2002). Fire: Servant, Scourge, and Enigma. Dover Publications. ISBN 978-0-486-42261-9.
- Schafer, Edward (1963). The Golden Peaches of Samarkand: A Study of T'ang Exotics. University of California Press. ISBN 978-0-520-05462-2.
- Schafer, Edward H. (1967). Great Ages of Man: Ancient China. New York: Time-Life Books. ISBN 978-0-900658-10-5.
- Sidgwick, Benson John; Muirden, James (1980). Amateur Astronomer's Handbook (4th ed.). Hillside, New Jersey: Enslow Publishers. ISBN 9780894900495. OCLC 610565755.
- Thomson, A.G. (1972). "The First Electric Clock: Alexander Bain's gold contact system" (PDF). Gold Bulletin (5): 65–66. doi: 10.1007/BF03215167. ISSN 0017-1557. S2CID 134442458.
- Thoren, Victor E. (1990). The Lord of Uraniborg: a biography of Tycho Brahe. Cambridge; New York: Cambridge University Press. ISBN 978-05213-5-158-4.
- Thorndike, Lynn; de Sacro Bosco, Johannes; Robertus Anglicus (1949). The Sphere of Sacrobosco and its Commentators. Chicago: University of Chicago Press. OCLC 897640056.
- Thornton, Bonnell (1767). The Comedies of Plautus, Translated Into Familiar Blank Verse. London: T. Becket & P. A. de Hondt. OCLC 1125642326.
- Truitt, Elly Rachel (2015). Medieval Robots: Mechanism, Magic, Nature, and Art. Philadelphia: University of Pennsylvania Press. ISBN 978-08122-2-357-6.
- White, Lynn Townsend (1964). Medieval Technology and Social Change. New York: Oxford University Press. ISBN 978-01950-0-266-9.
- Woods, Thomas (2005). How the Catholic Church Built Western Civilization. Washington, D.C.: Regnery Publications. ISBN 978-14815-6-390-1.
- Relativity Science Calculator – Philosophic Question: are clocks and time separable?
- Ancient Discoveries Islamic Science Part 4 clip from History Repeating of Islamic time-keeping inventions (YouTube).