History of timekeeping devices since 3000 BC

The history of timekeeping instruments encompasses the development of technical devices for measuring time from prehistory to the present day. It can be traced back to the Sumerians and Egyptians , who around 3000 BC had sundials based on simple shadow sticks. ^{[ 1 ]} The shadow stick has also been known in China since 2400 BC. The Greeks later called it a " gnomon ."

Around 2000 BC, the Babylonians used the sexagesimal system with base number 60, which later developed into the duodecimal system for measuring the hours. ^{[ 2 ]} The ancient Egyptians divided the day into two twelve-hour periods and used large obelisks on which the movement of the sun could be tracked. Water clocks were among the first timepieces not based on observations of the celestial bodies. One of the oldest was found in the tomb of the Egyptian pharaoh Amenhotep I , around 1500 BC. Around 325 BC, the water clock came to the Greeks, who called it a clepsydra ('water thief'). ^{[ 3 ]} Other ancient timepieces include candle clocks , used in China , Japan , England , and Iraq . In India and Tibet, the so-called timesticks (incense stick clocks) were widespread, as were hourglasses in some parts of Europe.

The oldest clocks used the sun's shadow – thus failing in cloudy weather or at night – and displayed the time only very imprecisely. More accurate sundials required taking the seasons into account, which was difficult with the gnomon and later led to the alignment of the shadow pointer with the celestial axis . The first clock with an escapement mechanism , which converted rotational energy into oscillations , ^{[ 4 ]} was developed by a Greek in the 3rd century BC. ^{[ 5 ]} In the 11th century, Arab engineers invented clocks whose gears and weights were powered by water. ^{[ 6 ]}

Mechanical clocks with averge escapement originated in Europe around 1300 and became the standard timekeeping device until spring-driven and pocket watches followed in the 16th century, and the pendulum clock around 1650. In the 20th century, quartz clocks were invented, followed by atomic clocks . Although the first quartz oscillators were developed for laboratories because of their accuracy, they were soon easy to produce and incorporated into wristwatches. Atomic clocks are by far the most accurate timekeeping devices to date. In order to calibrate other clocks and define a standard time for the Earth, the atomic-based system of " Coordinated Universal Time " was finally introduced in 1968. ^{[ 7 ]}

Many ancient civilizations observed celestial bodies, particularly the sun and moon, to determine times, dates, and the seasons. ^{[ 8 ] [ 9 ]} Methods of sexagesimal timekeeping, commonly used in Western society, first emerged in Mesopotamia and Egypt nearly 4,000 years ago; a similar system was later developed in Central America. ^{[ 8 ] [ 10 ] [ 11 ] [ 12 ]} The first calendars may have been created by hunter-gatherers during the last Ice Age . They employed sticks and bones that corresponded to the phase lengths of the moon or the seasons. ^{[ 9 ]} Stone circles, such as Stonehenge in England, were built primarily in prehistoric Europe and various parts of the world. They are thought to have been used to predict seasonal and annual events such as the equinox or solstices. ^{[ 9 ] [ 13 ]} Since these megalithic cultures left no records, little is known about their calendars or timekeeping methods. ^{[ 14 ]}

Sundials have their origins in shadow clocks , which were the first devices used to measure the parts of a day. ^{[ 15 ]} The oldest shadow clock comes from Egypt and was made of green slate. Egyptian obelisks, erected around 3500 BC, are also among the first shadow clocks. ^{[ 9 ] [ 9 ] [ 16 ]}

Egyptian shadow clocks were divided into ten parts during the day, with an additional four "twilight hours" – two in the morning and two in the evening. One type of shadow clock consisted of a long stem with five variable markers and a raised staff that cast a shadow on these markers. It was positioned facing east in the morning and west at noon. Obelisks worked in the same way; the shadows cast on the markers allowed the Egyptians to calculate time. Furthermore, the obelisk made it possible to determine the summer and winter solstices. The Egyptians also discovered the meridian , noting that when an obelisk's shadow is at its shortest, it always falls in the same direction regardless of the time of year. ^{[ 9 ] [ 17 ]} Around 1500 BC, a shadow clock was developed, shaped like a bent T-bar. The T-bar was oriented east in the morning and turned around at noon so that its shadow was cast in the opposite direction. ^{[ 18 ]} The Egyptians developed a number of alternative timekeeping devices, including water clocks and a system for tracking the movements of the stars . The oldest description of a water clock dates from the 16th century BC and was found in the tomb of the Egyptian court official Amenemhet . ^{[ 19 ]} There were several types of water clocks, ranging from simple to elaborate. One type of water clock was the so-called inlet clock, which consisted of a bowl with small holes in the bottom. The bowl floated on water, and the holes allowed the bowl to fill at a near-constant rate. Marks were made on the inside of the bowl to indicate elapsed time as the water level rose. In outlet clocks, a falling water level indicated the 'running out' of time. The oldest water clock was found in the tomb of Pharaoh Amenhotep I , suggesting that they were first used in ancient Egypt. ^{[ 17 ] [ 20 ] [ 21 ]} Furthermore, the Chaldeans proved that water clocks were multifunctional as early as the 1st millennium BC – they developed a closed measuring system in the form of a water-filled cube that combined time, weight, and length measurements. ^{[ 22 ]} Another Egyptian method for determining time at night was with plumb lines (merkhet). This method has been in use since at least 600 BC. Two merkhets, aligned with Kochab , the then North Star, were used to form a north-south line (or meridian). This allowed the exact hour of the night to be measured by observing certain stars when the meridian was crossed. ^{[ 17 ] [ 23 ]}

Around 425 BC, the water clock came to the Greeks, who called it a clepsydra ("water thief"). ^{[ 3 ]} After its introduction , Plato invented a water-based alarm clock. ^{[ 24 ] [ 25 ]} Plato's water alarm clock depended on the nightly overflow of a vessel filled with lead balls, which was suspended on a column and constantly supplied with water from a cistern. This caused the vessel to rise up the column until it struck the end of the column in the morning and tipped over, so that the lead balls rattled onto a copper plate. This sound of the lead balls woke Plato's students at the Academy. ^{[ 26 ]} Another version of the water alarm clock is based on two jugs connected next to a siphon . One jug was filled with water until it overflowed and the water ran through the siphon into the other, empty jug. The rising water forced the air out of the empty vessel, producing a loud whistle. ^{[ 25 ]} The Greeks and Chaldeans regularly maintained timekeeping records as an essential part of their astronomical observations. The Greek astronomer Andronicus of Cyrrhus built the "Tower of the Winds" in Athens in 50 BC, with a water clock inside the tower and several sundials on the outer walls. ^{[ 27 ]}

In Greek tradition, water clocks (clepsydra) were used during Socrates' lifetime to limit speaking time in court; the Romans later adopted this practice. ^{[ 28 ]} There are several references to this in the historical records and literature of the period. For example, in the Theaetetus , Plato says that "the men on the other side always speak in a hurry because the running water prompts them to do so." ^{[ 29 ]} Another mention is made in Lucius Apuleius's "The Golden Ass": "The secretary of the court called the summoned chief witness for the prosecution. After this, an old man whom I did not know stepped forward. He was invited to speak as long as there was water in the hollow sphere. The water was poured into the neck of the hollow sphere through a funnel and flowed out again through the fine perforation in the bottom of the sphere." ^{[ 30 ]} The clock in Apuleius's account was only one of several types of water clocks used.

Clepsydrums were more useful than sundials because they could be used indoors, at night, and when the sky was cloudy. Since they were not as accurate as sundials, the Greeks looked for a way to improve their water clocks. ^{[ 31 ]} Around 325 BC, the Greek water clock was adapted; it was given a face on which exactly one hour could be read. This made reading the water clock more precise and comfortable. One of the most common problems in most types of clepsydrums was caused by water pressure. When the container was full, the water flowed out more quickly due to the higher pressure. This meant that the water flowed at a different speed depending on its level. This problem was addressed by Greek and Roman clockmakers from 100 BC onwards. To counteract the increased water pressure, the water clocks were given a conical shape. The thin outlet allowed the water to drip off evenly, regardless of the current water surface in the clock. In the following centuries, further improvements were made to the water clock. The clocks became more elegant in design and were equipped with gongs to announce the full hours. Other water clocks were fitted with miniature figures, or moving mechanisms opened a door or rang a bell every hour. ^{[ 17 ]} There were still unsolved problems, such as the effect of temperature. Cold water is denser than warm water, which causes a different flow rate. Furthermore, the water clock could not be used in frosty conditions. ^{[ 32 ]}

Although the Greeks and Romans were far ahead in water clock technology, shadow clocks continued to be used. The mathematician and astronomer Theodosius of Bithynia is said to have invented a universal sundial that displayed the correct time everywhere on Earth. ^{[ 33 ]} Other contemporaries wrote about the sundial in mathematics and literature of the time. The Roman architect and chronicler Marcus Vitruvius Pollio described the mathematics of the gnomon (shadow pointer) in his standard work De Architectura and described 13 different types of sundials. ^{[ 34 ]} The Romans excelled in timekeeping less through innovation than through conquest and written records. This is surprising, as their precise language and jurisprudence suggest that exact time measurement and division would have been essential, especially given the size of the Roman Empire. Due to the significantly longer shifts than in the south of the empire, the Roman legionaries stationed in Britain complained to their military leaders. In 55 BC,Julius Caesar, during a personal visit to Britain, noticed that British summer nights were shorter than Italian ones. The oldest Roman sundial, erected in the 3rd century BC in front of the Temple of Quirinus, was, according to tradition, a piece of booty from the First Punic War . Due to its relocation in 262 BC, it showed the wrong time for 100 years until this was noticed and the markings and angles were adjusted to the longitude of Rome. ^{[ 35 ] [ 36 ]}

Joseph Needham speculates that the introduction of the outlet water clock in China dates back to the 2nd millennium BC, during the Shang Dynasty , and at the latest to the 1st millennium BC. With the beginning of the Han Dynasty in 202 BC, the outlet water clock was gradually replaced by the inlet water clock. The inlet water clock had an indicator rod on which a float weight was placed. To compensate for the falling pressure head in the container, Zhang Heng had installed an additional tank between the reservoir and the inlet. This adjusted the flow rate of the water to the time measurement. Around 550 AD, Yin Gui described the first water clock in operation in China with a constant liquid level. The details of this water clock were later described by the inventor Shen Kuo. In 610 AD, during the Sui Dynasty, two inventors, Geng Xun and Yuwen Kai, invented the balanced water clock. By shifting the weight of the float on the float arm, the pressure on the water surface in the compensating tank was changed. Markings of standard positions on the float arm made it possible to regulate the varying lengths of day and night based on the flow rate of the water. This allowed this balanced water clock to be used throughout the year. ^{[ 37 ]}

Between 270 BC and 500 AD, Greek and Roman clockmakers and astronomers were busy developing elaborate mechanized water clocks. This was made possible by the inventions of Euclid , who established the theorems of geometry, and Archimedes , who taught the laws of the lever and pulley, the gear and the endless screw , as well as the fundamental laws of hydraulics. A student of Archimedes, a barber named Ctesibius, who applied the laws of hydraulics and mechanics to water clocks, built a water clock with a dial and hands. In addition, the regulation of the water flow was adjusted, which significantly improved the accuracy of the water clocks. ^{[ 22 ]} Various types of water clocks were built. For example, there were water clocks with bells and gongs, while others opened windows and doors behind which figures appeared. Still others displayed astrological models of the universe. The monastery water clock functions similarly to the hourglass: water flows from an upper sphere through a tube into a lower sphere and is rotated after a calibrated unit. The water pendulum clock utilizes the properties of the pendulum. The compensation water clock of the Greek Pyrlas worked in conjunction with mercury and compensated for temperature fluctuations. Finally, the whale clock should be mentioned, which consists of a frame in which a drum can move up and down depending on the water level in the container. An axle rod runs through the center of the drum, which moves across a time scale so that the time can be read. ^{[ 22 ]}

Some of the most elaborate water clocks were developed by Muslim engineers. Notably, the water clocks of Al-Jazari, built in 1206, or the so-called Elephant Clock. This water clock recorded the position of the temporal hours, which meant that the flow rate of the water could be changed. This allowed it to be adjusted daily to account for the unequal length of days throughout the year. To achieve this, the clock had two tanks; the upper tank indicated the time. This was connected to mechanisms via a flow rate regulator and the lower tank. At daybreak, the upper tank was opened and the water flowed via a float into the lower tank, maintaining a constant pressure in the receiving container. ^{[ 38 ]}

The earliest example of a fluid-powered escapement was described by the Greek engineer Philo of Byzantium (3rd century BC) in his technical treatise Pneumatics (Chapter 31). ^{[ 39 ]} Another early escapement clock was built by the tantric monk and mathematician Xing Yi and government official Liang Lingzan in Chang'an. ^{[ 40 ] [ 41 ]} It was an astronomical instrument that also served as a clock. This water clock was created in the image of a celestial sphere and showed the equator and the lunar orbits in their sequence. The water flowed in spheres and automatically turned a wheel. One full revolution of the wheel corresponded to a day and a night. Fixed outside around the celestial sphere were two rings. On these rings, the model of the sun was mounted on one and a model of the moon on the other. These rings orbited the celestial sphere as the so-called orbits of these two planets. The celestial sphere was half-submerged in a wooden case, the surface of which represented the horizon. This astronomical instrument allowed the precise determination of time, sunrises and sunsets, as well as the full and new moons. In addition, there were two wooden jacks attached to the horizon surface. The first wooden jack struck a bell, indicating the full hours with the chime, while the second struck a drum, announcing a new quarter. All functions were accomplished by wheels and shafts, hooks, pins and locking rods, as well as braking devices within the case. ^{[ 42 ]}

The use of Xing Yi's water clock was hampered by temperature fluctuations. This problem was solved in 976 AD by Zhang Sixun, who replaced the water with mercury, as mercury remains liquid down to minus 39 °C. ^{[ 43 ]} Zhang Sixun implemented this change in a tower clock approximately ten meters tall, which was equipped with an escapement and rang a bell every quarter hour. The Chinese mathematician and engineer Han Kung-Lien also built a water clock with an escapement in 1088 AD. A wheel with scooping chambers was built into a wooden frame. Every 24 seconds, a scooping chamber was filled with water. The weight of the filled water scoop depressed a trigger. This pulled a chain, which released the escapement and allowed the wheel to advance one notch before the lock re-engaged. ^{[ 44 ]} A Chinese manuscript from 1090 AD, kept in the National Library in Beijing, tells of a water clock that Su Song built for the palace gardens in Kai-Feng. The "heavenly machine", which was built in 1088 AD, was ten meters high. The cladding of the cylindrical structure had five openings in which tablets and figures with cymbals and gongs displayed the time. A wheel with a diameter of about four meters was driven by a steadily flowing jet of water so that containers attached to the wheel's circumference were filled. Once a container reached a certain weight, a device released an escapement until the next cup was under the water jet; then the wheel was locked again. This regulating mechanism anticipated the mechanical escapements that were later implemented in wheel clocks. A lever mechanism moved the figures and tablets that displayed the time. ^{[ 17 ]} This water clock had the first known endless power-transmitting chain drive in clockmaking. ^{[ 45 ]} This clock originally stood in the capital of Kaifeng. There it was dismantled by the Jin army and taken to the capital Yanjing (now Beijing), where it could not be reassembled. Therefore, Su Song's son So Xie was commissioned to make a replica. ^{[ 45 ]}

The clock towers of Zhang Sixun and Su Song, built in the 10th and 11th centuries, were the first clocks with a striking mechanism . These clocks struck every hour using bushings. ^{[ 46 ]} The first striking clock outside China was in the bell tower near the Umayyad Mosque in Damascus . It was built by the Arab engineer al-Kaysarani in 1154 and announced the hour with a chime. ^{[ 47 ]}

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 worked with range andplanetary gears . ^{[ 6 ]} Other monumental water clocks with complex gear trains and ranges of automatons were built by Muslim engineers. ^{[ 48 ]} Like the Greeks and Chinese, Arab engineers also built water clocks with a fluid-driven escapement. Heavy floats were used as weights. The clock's constant head system was used with an escapement mechanism. This hydraulic control is still used to lift heavy loads slowly and steadily. ^{[ 48 ]}

The water clock was considered a royal gift for centuries. As early as 507 AD, Theodoric, then regent of Italy, presented the Burgundian king Sigmund with a shadow clock and a water clock. In Baghdad , Muslim culture had reached its peak when Caliph Harun al-Rashid presented Charlemagne with a water clock made of bronze and damascened gold as a coronation gift. It was the most magnificent water clock with automatons and chimes ever known. ^{[ 49 ] [ 50 ]}

In 1277, a mercury clock was first described in the Libros del saber de Astronomia , a Spanish work consisting of scientific translations of Arabic and Jewish texts. This mercury clock already possessed the essential characteristics of a mechanical clock. It was driven by weights. A rope moved a drum containing mercury. Mercury is a viscous substance whose inertia was used as an escapement. The rotating drum was divided into sectors by built-in perforated plates. As the drum rotated, the mercury flowed from one chamber through the perforation into the next. Due to its inertia, it slowed the rotation of the drum. By appropriately adjusting the driving weights, the drum completed one revolution in four hours. By using a gear reduction ratio of 6:1, the drum completed one revolution in 24 hours. This meant that the time and other astronomical data could be read directly from the display plate. ^{[ 51 ]}

A stick coated with pitch and sawdust was cut to a specific length. Small metal balls were attached to the stick at regular intervals with threads. The stick protruded above a gong. When the stick was lit at sunrise, the flame would eat its way down the stick.

In doing so, she burned the threads from which the metal balls hung. They fell onto the gong, striking it, and the people could hear that another unit of time had passed.

It is not known where and when candle clocks were first used; their oldest mention comes from a Chinese poem written by Jianfu in 520 AD. According to the poem, the candle was a means of determining the time of night. Similar candles were also used in Japan until the beginning of the 10th century. ^{[ 52 ]} History mentions that King Alfred the Great of England invented the candle clock in Europe in the 9th century. It consisted of six wax candles, each 30 centimeters high and 2.5 centimeters thick. The burning time of one wax candle was four hours. His chronicler recorded that he devoted exactly eight hours to his public duties, eight hours to studying, eating and sleeping, and eight hours to prayer. In order to maintain his structured daily routine, he needed six wax candles every day, which he kept in a lantern to optimize the even burning. ^{[ 53 ]}

The most modern candle clocks of their time were those of Al-Jazari in 1206. One of his candle clocks featured a dial with a time display, held for the first time by a bayonet fitting. This fastening mechanism was still in use in modern times. ^{[ 54 ]} Donald Routledge Hill described the Al-Jazari candle clocks as follows:

The candle, whose burning rate was known, had a hole in the underside of the cap through which the wick was passed. The burned wax was collected in the inlet and could be periodically removed so that it did not come into contact with the constantly burning candle. The lower part of the candle lay in a shallow bowl, which was connected to a ring on its side via rollers with a counterweight. As the candle burned, the weight moved upwards at a constant speed. The time display was operated by the bowl on the underside of the candle. ^{[ 55 ]}

A variation of the early timekeeping devices were oil lamp clocks. These consisted of a graduated glass chamber with a vertical scale. This glass chamber served as a reservoir for the fuel supply of the lamp, which was installed on the side of the reservoir. Oil or blubber served as the fuel. Usually, blubber was used because it burned cleaner and more evenly than oil. As the lamp consumed fuel, the oil or blubber level in the glass reservoir sank, allowing the time to be read on the scale. With the oil lamp clock, rough timekeeping was possible at night. ^{[ 22 ]}

In the Far East, incense clocks were used in various forms, alongside water clocks, mechanical clocks, and candle clocks. ^{[ 56 ]} Incense stick clocks were first used in China around the 6th century. In Japan, incense clocks are still used in the Shōsōin (treasury of the Tōdai-ji ), ^{[ 57 ]} although the characters are not Chinese, but Devanagari. ^{[ 58 ]} Due to their frequent use of Devanagari characters and their use in Buddhist ceremonies, Edward H. Schäfer speculates that incense clocks were invented in India. ^{[ 58 ]} Although similar to the candle clock, the incense clock burned evenly and without flame, making it more accurate and safer for indoor use. ^{[ 59 ]}

Various types of incense clocks have been found; the most common forms are incense sticks and incense seals. ^{[ 60 ] [ 61 ]} One type of incense clock was filled with calibrated incense sticks, ^{[ 61 ]} others had an elaborate mechanism. For example, weights were attached at regular intervals with a thread. When the incense stick burned, the weight fell onto a gong after a certain time. Some incense clocks were built into elegant bowls, in which the weights fell through an open base plate into a decorative compartment. ^{[ 62 ] [ 63 ]} There were incense sticks with different scents, so that the hours were marked by a change in the scent. ^{[ 64 ]} The incense sticks were used in stick form or as spirals. The spiral shape was often hung on the roofs of houses and temples and had a longer burning time than sticks. ^{[ 65 ]} Until 1924, incense sticks were a special type of timepiece used exclusively in Japanese geisha houses (Okiya). The geisha were paid for their entertainment according to the number of senkodokei (incense sticks for a fee) they burned. ^{[ 66 ]} Incense seal clocks were used for official occasions and events and were of primary importance for religious purposes. The seal was etched from wood or a stone slab with one or more grooves. They were filled with incense and used predominantly by Chinese scholars and intellectuals. ^{[ 60 ] [ 67 ]} These clocks were common in China, but were also produced in smaller numbers in Japan. ^{[ 68 ] [ 69 ]} To mark the passage of a particular hour, various resins or fragrant incense sticks, as well as incense powder, were applied to the clock face. This gave rise to a variety of incense clocks, depending on the fragrance used. ^{[ 70 ]} The length of the incense path directly determined the burning time of the clock. There were incense clocks for short periods and those that burned for between twelve hours and a month. ^{[ 71 ] [ 72 ] [ 73 ]}

While initially incense holders were made of wood or stone, the Chinese gradually introduced metal plates. This made incense clocks easier to manufacture and more decorative. Another advantage was that the paths of the grooves could be varied, allowing the clocks to be adjusted to the changing length of the day throughout the year. As smaller holders became available, the clock gained popularity among the Chinese and was a popular gift. ^{[ 74 ]} Incense seal clocks are sought after by many clock collectors, but few are available because they have often been sold or are in the possession of museums or temples. ^{[ 75 ]}

An astrolabe is a scientific astronomical device used by Muslims to determine the time of prayer, for simple surveying purposes, and for navigation. It provided, among other things, accurate time to Arab and European astronomers until the 17th century. The astrolabe consisted of a ring in which a disk with a rotating radius was suspended. Contemporary Muslim astronomers used a variety of very accurate astronomical clocks for use in their mosques and observatories, ^{[ 76 ]} such as the water-powered astronomical clock of Al-Jazari from 1206 ^{[ 77 ] [ 78 ]} and the astronomical clock of Ibn al-Shatir in the early 14th century. ^{[ 79 ]} The most modern astrolabes for timekeeping were the aligned astrolabes of Al-Biruni in the 11th century and of Muhammad ibn Abi Bakr in the 13th century. These were used as timekeeping devices and as calendars. ^{[ 6 ]} Al-Jazari's castle clock of 1206 was the most modern water-powered astronomical clock. It is considered an early example of a programmable analog computer. ^{[ 80 ]} This clock was a complex device, about 11 meters high, that had several functions besides timekeeping. It contained a representation of the zodiac and the solar and lunar orbits and had a hand shaped like a crescent moon. This hand moved with its tip over a gate that opened automatically every hour and revealed a figure. ^{[ 81 ] [ 82 ]} The length of the days and nights could be reprogrammed according to the seasons. In the front stood five musician figures, connected to a lever by a hidden camshaft. This lever was moved by the rotating waterwheel, causing music to play automatically at a specific time. ^{[ 80 ]} Other components of the castle clock were: a reservoir with a float device, a float chamber and a flow regulator, as well as two automatons from which balls fell into vases to serve as an alarm device. ^{[ 83 ]}

With sundials, the shadow of a point-like body ( nodus ) is read on a clock face. The length of the temporal daytime hours used in antiquity depended on the time of year. The day was divided into two parts of twelve hours: the light day, which runs from sunrise to sunset, and the night from sunset to sunrise. In summer, the daytime hours were longer than the night hours, and in winter it was the other way around. The idea of using hours of equal length throughout the year was applied in 1371 by Abul-Hasan Ibn al-Shatir. His idea was based on earlier developments in trigonometry by Muhammad ibn al-Jabir al-Harrānī Battani (Albategni), who was known as Ibn al-Shatir. The gnomon (shadow indicator) was aligned parallel to the Earth's axis, so the hour lines showed the same time every day of the year. His sundial is the oldest surviving sundial aligned with the Earth's axis. This concept was applied in Western sundials from 1446 onwards. ^{[ 84 ] [ 85 ]}

Following the adoption of heliocentrism and equal hours, as well as advances in trigonometry, sundials in their present form were built in large numbers during the Renaissance . ^{[ 86 ]} In 1524, the French astronomer Oronce Finé built an ivory sundial, which still exists. ^{[ 87 ]} In 1570, the Italian astronomer Giovanni Padovani published a treatise, including instructions for making and laying murals of vertical and horizontal sundials. Around 1620, Giuseppe Biancani also describes how to construct sundials in Constructio instrumenti ad horologia solarien . ^{[ 88 ]} During his circumnavigation of the world in 1522, the Portuguese navigator Ferdinand Magellan used 18 hourglasses on each of his ships. ^{[ 89 ]} Since the hourglass was one of the few reliable methods of measuring time at sea, it is speculated that it was already used on board ships as a supplementary navigation aid in the 11th century. However, the earliest evidence of its use in paintings appears in 1338 (Allegory of Good Government by Ambrogio Lorenzetti). ^{[ 90 ]} Hourglasses were based on the principle of sand running through a narrow channel from an upper to a lower chamber as a measure of time. The passing time was usually limited to half an hour. From the 15th century onwards, hourglasses were used in a wide range of applications, mainly to measure short periods of time, for example as a pulpit clock to determine the length of a sermon or even the speaking time in court. In seafaring, they determined the watch cycle in four-hour watches of eight bells. A cabin boy had to turn the hourglass every half hour. The hourglass was also used in industry and in the kitchen. They were the first reliable, reusable, sufficiently accurate, and easy-to-build timekeeping devices. In the Middle Ages, the hourglass was considered a symbol of the passing of time and the transience of humanity. ^{[ 91 ]} Although the hourglass was also used in China, it is unknown when it was first used there. ^{[ 92 ]}

The earliest medieval European clockmakers were Christian monks. ^{[ 93 ]} Medieval monasteries and learning institutions required clocks because daily prayer and work schedules were regulated exclusively by them. This was done with various types of timekeeping devices, such as the sundial, the water clock, or the candle clock. These types of clocks were also used in combination. ^{[ 7 ] [ 94 ]} When mechanical clocks were used, they had to be adjusted twice a day to ensure accuracy. ^{[ 95 ]} Important times were announced by bell sounds or by a mechanical device, such as a falling weight or a rotating knocker. The needs of piety and the technical skills of medieval monks were decisive factors in the development of clocks. Among the monks were also talented clockmakers. In 996 AD, the first recorded clock was erected in the city of Magdeburg by the future Pope Sylvester II. Peter Lightfoot, a 14th-century monk from Glastonbury, constructed one of the oldest clocks, which can still be seen in the Science Museum in London . ^{[ 96 ]} The mention of clocks in 11th-century writings suggests that they were widely known in Europe by this period. ^{[ 97 ]} In the early 14th century, the Florentine poet Dante Alighieri referred to a clock in his Paradiso , which contained the first literary reference to an hour-bell mechanism. ^{[ 97 ] [ 98 ]} The earliest detailed description of the clock mechanism was presented by Giovanni Da Dondi, professor of astronomy at Padua , in his 1364 treatise IL Tractatus Astrarii . ^{[ 99 ]} This mechanism has had some modern replicas. Other notable examples during this period were built in Milan (1335), Strasbourg (1354), Lund (1380), Rouen (1389) and Prague (1410). ^{[ 99 ]}

The Salisbury Cathedral Clock ( Wiltshire , England), dating from 1386, is the oldest clock in the world made almost entirely from original components. This clock has no dial, as it only strikes one bell at the exact times. ^{[ 100 ]} The wheels and gears were mounted in an open, box-like iron frame, 1.20 meters on a side. The frame is held together with metal dowels and pegs. The clock is driven by the gravity of two large stones suspended from a pulley. As the weights fall, ropes unwind from the wooden barrels. One barrel drives the driving wheel, which is regulated by the escapement, and the other drives the striking mechanism and the pneumatic brake. ^{[ 100 ]} Peter Lightfoot's cathedral clock, built in 1390, works on the same system. ^{[ 101 ] [ 102 ]} By adjusting the dial, a view of the medieval universe was displayed, with the sun and moon rotating around the centrally mounted Earth. Above the clock are figures striking the bell, while a set of jousting knights rotate on a track every 15 minutes. ^{[ 103 ] [ 104 ]} This clock was refitted with a pendulum and anchor escapement in the 17th century. It was brought to the London Science Museum in 1884, where it is still in working order. ^{[ 104 ]} Similar astronomical clocks can still be seen at the cathedrals of Exeter , Ottery Saint Mary, and Wimborne Minster .

A clock that no longer exists was built in the 14th century by Abbot Richard of Wallingford in the Abbey of Saint Albans. ^{[ 105 ]} When Henry VIII dissolved the monasteries, this clock was destroyed, but records made a complete reconstruction possible. This clock could not only accurately predict the time, but also the phases of the sun and moon, as well as the positions of the stars and planets. A display made it possible to read the current state of the tides at London Bridge . ^{[ 106 ]} The clockmaker Giovanni de Dondi built an astronomical clock (astrarium) between 1348 and 1364, which was technically unparalleled for the next two centuries. ^{[ 96 ] [ 107 ]} This clock also no longer exists, but was rebuilt based on existing drawings by Leonardo da Vinci and, above all, on precise technical drawings and descriptions that de Dondi wrote down during construction. In contrast to many other astronomical clocks, this clock was small and made entirely of brass in 107 individual parts. De Dondi's clock had seven faces that displayed not only the time but also the positions of the sun, the moon, and five other planets, as well as all religious holidays. The accuracy and versatility of this mechanism were previously unknown. Reconstructions can be seen in the National Museum of American History of the Smithsonian Institution in Washington D.C., the Paris Observatory, the Science Museum in London, and the Beyer Clock Museum in Zurich. ^{[ 106 ]}

Medieval life was regulated by a multitude of bells ringing from church and city towers. Prayer times in monasteries, opening times of city gates, court and market times, and other important times were announced by the bell ringers. This required a reliable time display; a necessity that sundials and water clocks did not meet. ^{[ 108 ] [ 109 ]}

While in the Middle Ages clocks were mainly used for religious purposes, from the 15th century onwards they were also used for secular timekeeping. In Dublin, official timekeeping became a local custom, and by 1466 a public clock stood on the City Court and Town Hall. This clock was the first of its kind in Ireland and had only one hour hand. ^{[ 110 ]} The increasing splendour in the castles led to the construction of large tower clocks. A tower clock from Leeds Castle has survived. The clock, dating from 1435, was decorated with images of the crucifixion of Jesus, as well as images of Mary and Saint George. ^{[ 111 ]}

In the Middle Ages, mechanical clocks were used in many bell towers in Western Europe. The clock of St. Mark's in Venice was assembled in 1493 by the clockmaker Gian Carlo Rainieri from Reggio Emilia. In 1497, Simone Campanato sculpted a large bell, also called a Marangona, for the St. Mark's bell tower. This bell was installed on December 1, 1497, and had a diameter of 1.27 meters and a height of 1.56 meters. The start and end of a working day are announced by two mechanical bronze statues, 2.60 meters high, which strike the bell with a hammer. In 1410, the Prague Astronomical Clock was constructed by the two clockmakers Mikulas of Kadan and Jan Šindel . It consists of three main components: 1. the astronomical hand, which represents the position of the sun and moon in the sky and shows other astronomical details; 2. the clock mechanism, called "The Walk of the Apostles," which displays figures of the apostles and some other sculptures every hour; and 3. the calendar hand, which displays the months with medallions. Around 1490, the remaining hands were added by the clockmaker Jan Růže, and the clock received its Gothic design. The Prague astronomical clock was the third of its kind. The first was made in Padua, Italy, in 1344. ^{[ 112 ] [ 113 ]}

Early mechanical clocks did not use minute and second displays. A manuscript from 1475 mentions a minute display for the first time. ^{[ 114 ]} Clocks with minute and second displays had existed in Germany since the 15th century. ^{[ 115 ]} Clocks with minute and second displays were still in the minority, and their displays were inaccurate. Only with the development of the pendulum did greater display accuracy become possible. In the 16th century, the astronomer Tycho Brahe used clocks with minute and second displays to observe stellar positions. ^{[ 114 ]}

Between 1794 and 1795, the French government demanded the introduction of decimal clocks. A day was divided into ten hours, and the hour had 100 minutes. The astronomer and mathematician Pierre-Simon Laplace and other intellectuals then changed the clock setting to decimal time. ^{[ 116 ]} A clock in the Palais des Tuileries continued to show decimal time until 1801. The cost associated with replacing all the clocks in France prevented the widespread use of decimal clocks. Because decimal clocks only helped astronomers rather than ordinary citizens, it was one of the most unpopular changes associated with the metric system, and so it was rejected. ^{[ 117 ]}

In 1565, the Ottoman engineer Taqi ad-Din described a weight-driven mechanical clock in his book The Brightest Stars for the Construction of Mechanical Clocks (al-Kawākib ad-durriyya fī waḍʿ al-bankāmat ad-dauriyya). This clock had a foliot (German: balance, beam balance or spoon balance) ^{[ 118 ]} with an escapement, gears, an alarm mechanism and a display of the moon phase. ^{[ 119 ]} Similar to the European mechanical alarm clocks of the 15th century, the alarm time was set by moving a plug. ^{[ 120 ] [ 121 ]} The clock had three displays: for the hours, the minutes and the degree. ^{[ 122 ]} Taqi al-Din later constructed a mechanical clock for the Istanbul Observatory, which he used to observe the right ascension . In astronomy, right ascension is the angle between the longitude of the vernal equinox and the longitude above which the observed object is located, measured on the equator. It is the equivalent on the (imaginary) celestial sphere to the geographical longitude on Earth. This clock had an hour, minute, and second display. Each minute was divided into five seconds. This was an important innovation in practical astronomy of the 16th century, as mechanical clocks at the beginning of the century were not accurate enough for astronomical purposes. ^{[ 123 ]}

Mechanical clocks made of brass or iron with verge escapements were introduced to Japan in the 16th century by Jesuit missionaries. They founded a missionary school in what is now Nagasaki Prefecture, which provided general and vocational education. There, students learned how to build clocks, organs, and astronomical instruments. From 1635 onwards, Japan began to isolate itself from foreign influence ( sakoku ), and travel abroad was also prohibited by the Tokugawa Shogunate . But while the country became increasingly isolated from the outside world, Japanese clockmaking experienced a golden age for three hundred years. The royal city of Edo (present-day Tokyo ) became the center of Japanese clockmaking. Soon after, the first Japanese timepieces, called wadokei, were created. These differed from Western clocks in that they displayed temporal hours until the second half of the 19th century. For timekeeping, each day was divided into a daytime period and a nighttime period from sunrise to sunset. The day and night were each divided into six periods. Since the length of day and night varied constantly throughout the year, the lengths of these periods also changed daily. The Wadokei clocks had to cope with these daily variations. This worked satisfactorily; later versions were equipped with alarm mechanisms. The Wadokei clocks were produced until 1872, when the Japanese cabinet decided to introduce the equinoctial hours at the same time as adopting the Gregorian calendar . The old Japanese time system was abandoned, the Wadokei clocks lost their usefulness, and from then on, Japanese clockmakers built clocks according to the Western system. ^{[ 124 ]}

Since Japan lacked experience with clocks for the new time system, clocks were initially imported from the West. Western technologies were initially used in the construction of wall clocks . The complex Japanese technology culminated in 1850 in the "10,000-Year Clock" by Tanaka Hisashige (1799–1881), the later founder of the Toshiba Group. In 1881, Hattori Kintarō founded a watch and jewelry business (now the Seiko Group) and later his own watch manufacturer, Seikōsha , thus giving the starting signal for the development of a Japanese watch industry. Seikōsha became one of the most important watch manufacturers in the world and was significantly involved in many technical developments, such as quartz movements. ^{[ 125 ]}

In the 1970s, a new wristwatch from Japan came onto the market that did not have a mechanical movement, but rather a computerized control. This wristwatch's features included accurate timekeeping, date and day-of-the-week displays, and leap years. ^{[ 126 ]}

Clocks have been built for a wide variety of uses, ranging from wristwatches to atomic clocks. Throughout the history of timekeeping devices, clocks have used a variety of energy sources, such as the sun, water, gravity, electricity, and even the atom. ^{[ 7 ] [ 127 ]} The Chinese official Liang Lingzan and the monk Yi Xing are credited with the invention of the mechanical clockwork. ^{[ 40 ] [ 99 ] [ 128 ]} However, mechanical clocks were not used in the Western world until the 14th century. From 1550 onwards, the precision of clock mechanics reached a remarkable level. Inspired by the spirit of the Renaissance, mechanical clocks with striking mechanisms and astronomical information for periods of up to 20 years were created. However, differences in the clock's time of up to one hour per day had to be accepted. ^{[ 129 ]}

With the miniaturization of clocks in the 15th century and the manufacture of personal clocks in the 16th century, the innovations of mechanical clocks continued. ^{[ 99 ]} In 1581, the famous astronomer and physicist Galileo Galilei proposed his pendulum theory. It states that the oscillation period of a pendulum is hardly determined by its amplitude, but primarily by its length. ^{[ 7 ] [ 130 ]} Although Galileo studied the pendulum, he never constructed a clock based on this principle. The first pendulum clock was not designed and built until 1656 by the Dutch scientist Christiaan Huygens . Early versions had a time deviation of less than one minute per day, which was soon improved to a few seconds. ^{[ 7 ]}

In the 17th and 18th centuries, the Jesuits also contributed to the development of the pendulum clock, as they had an unusually keen understanding of the importance of accuracy. For example, the Italian priest Giovanni Battista Riccioli developed an accurate one-second pendulum that ideally produced 86,400 oscillations per day. Jesuits played a crucial role in the dissemination of scientific ideas, collaborating with contemporary scientists such as Huygens. ^{[ 131 ]}

The invention of the anchor escapement in 1670 made the development of modern pendulum clocks possible. ^{[ 132 ]} Previous long pendulum clocks had used the verge escapement, which required a very large pendulum swing of about 100 degrees (?). To reduce this large swing, most pendulum clocks with verge escapements used a short pendulum. However, these short pendulums had the disadvantage of inaccurate timekeeping, required more kinetic energy, and caused more friction and wear than long pendulums. By using the anchor escapement, the swing could be reduced to four to six degrees, allowing the use of a long pendulum again. Most pendulum clocks were built so that the pendulums were tuned to a time interval of one second per pendulum swing, resulting in a pendulum length of about one meter. Due to the length of the pendulum and the long drop space of the driving weights, tall and narrow pendulum clocks had to be manufactured. ^{[ 133 ]}

The accuracy of astronomical timekeeping reached tenths of a second with second pendulums as early as the 18th century, which inspired the construction of temperature-compensated pendulum rods. The Riefler pendulum, developed around 1880, further improved observatory time systems to within a few 0.01 seconds, and in 1921, the Shortt clock improved them to milliseconds. These precision pendulum clocks served as the basis for timekeeping services until around 1960 (when they were replaced by high-precision quartz clocks and later atomic clocks ).

Since pendulum clocks had to be permanently mounted, marine chronometers often served as working clocks on the telescope. Synchronization was initially achieved through electrical contacts and later by radio technology or time signal transmitters .

A further technical refinement was the retrograde hook escapement, invented in 1676 by the English philosopher Robert Hooke , which was used particularly in grandfather clocks. This invention enabled Christiaan Huygens to insert a spiral spring (balance) into the regulators of conventional clocks, which enabled them to oscillate naturally. ^{[ 130 ]} This reduced the disturbances caused by the uneven drive and by external influences when wearing pocket watches, while at the same time achieving a major advance in the accuracy of pocket watches. Huygens' contributions to improving the reliability of timepieces were the decisive prerequisite for the mass production of clocks. ^{[ 17 ] [ 134 ]}

In 1904, aviator Alberto Santos-Dumont asked his friend Louis Cartier , a French watchmaker, to design a watch that could be useful to him during his flights. ^{[ 135 ]} In 1868, Patek Philippe had already invented a wristwatch, but it was intended more as a piece of jewelry for women. Since pocket watches were unsuitable for pilots, Louis Cartier produced the Santos wristwatch. This was the first wristwatch for men intended for practical use and is still in production. ^{[ 136 ]} During the First World War, the wristwatch gained popularity. Since wristwatches were more convenient than pocket watches during combat, they were particularly favored by officers. Artillery and infantry officers depended on their watches because coordinated operations were necessary during combat. The so-called trench watch was developed during the First World War. In this watch, the glass was protected by a steel grille to prevent the glass from breaking. To make reading the time as easy as possible, these early military watches had particularly large hour numerals on the dial and very large hands, which were also coated with a luminous material made of radium , so that soldiers could read the time even in the dark. ^{[ 137 ] [ 138 ]}

In the 1920s, the process developed by Abraham-Louis Perrelet around 1770, using a rotating rotor, became widespread and enabled automatic winding of watches. Between 1770 and the beginning of the 20th century, this technology did not gain widespread acceptance because it was mainly used in pocket watches, which were not moved sufficiently. It was only the wristwatch that helped Perrelet's process achieve a breakthrough, as the wearer's arm movements were sufficient to operate the automatic winding mechanism. ^{[ 139 ]} Due to the changing wearing habits of wristwatches, shocks, vibrations and temperature fluctuations had to be taken into account during production in order to ensure a long service life of the wristwatch. A pioneer in this field was the German watchmaker Hans Wilsdorf , who subjected some of his watch creations to very specific tests as early as the beginning of the century. ^{[ 140 ]} A decisive innovation in the field of wristwatches in the 1930s was the Nivarox hairspring developed by Reinhard Straumann. It was made of a special alloy, was independent of temperature, elastic, non-magnetic and rust-proof. ^{[ 141 ]} In 1931, a company in La Chaux-de-Fonds, Switzerland, developed the so-called Incabloc system, in which shocks and blows were converted into a guided mechanical movement. This system is still in use. Since the 1950s, the German watchmaker Helmut Sinn has also been producing wristwatches for the blind.

A marine chronometer is a precise timepiece used at sea to determine longitude and astronomical locations. Marine chronometers were first developed in 1759 by the English watchmaker John Harrison . In 1761, John Harrison won the 20,000 pound sterling prize offered by the British Parliament in 1741 for solving the longitude problem with the watch he developed. This chronometer, called the H.4, achieved an accuracy of 5.1 seconds on a five-month voyage to Jamaica and back in stormy seas. The Swiss watchmaker Louis Berthoud (1753–1813) produced a precision pocket chronometer , which Alexander von Humboldt tested on his voyages in 1799. This marine chronometer was able to determine longitude very accurately. Humboldt was thus able to make precise descriptions of ocean currents and, by comparing these with the ship's displacement, calculate their direction and strength. ^{[ 144 ] [ 145 ]} Marine chronometers continued to be used in the navy even after the Second World War. The decline of chronometers only came in the second half of the 20th century with the invention of the quartz clock, whose accuracy improved by three orders of magnitude. Thus, there was no longer any need for chronometers as navigation instruments.

Chronometers are particularly precise mechanical watches that were once needed for timekeeping and navigation at sea and in aviation. Even today, mechanical chronometers are still produced for collectors and enthusiasts. A mechanical watch is called an official chronometer if the movement meets certain precision standards. Each individual movement is subjected to a precisely defined test and individually certified. Worldwide, this testing may only be carried out by the independent Swiss observatory, the Contrôle officiel suisse des chronomètres (COSC). ^{[ 146 ]} Over a million chronometers are tested and certified with a serial number every year. ^[

In 1880, the piezoelectric properties of crystalline quartz were discovered by Jacques and Pierre Curie . ^{[ 7 ] [ 148 ]}

In 1921, the first quartz oscillator was built by Walter Guyton Cady .

In 1927, Warren Marrison and JW Horton built the first quartz clock at the Bell Telephone Laboratories in Canada . ^{[ 149 ] [ 150 ]}

The following decades saw the development of quartz clocks as precision time measuring devices in laboratories, their practical use being limited to the calibration of sensitive counting electronics.

It was not until 1932 that the German physicists Adolf Scheibe and Udo Adelsberger developed a quartz clock capable of measuring small weekly fluctuations in the Earth's rotation rate. ^{[ 150 ]}

In the mid-1930s, the quartz watch was developed to series production by the German company Rohde & Schwarz and was manufactured in 1938 as the world's first portable quartz watch; it weighed 36 kg.

In 1969, Seiko in Japan produced the first quartz wristwatch, called the "Astron", for the mass market. ^{[ 151 ]} The "Astron" had a battery life of one year; its accuracy and low manufacturing costs led to the subsequent widespread use of quartz wristwatches. ^{[ 7 ]}

Digital technology was already being used in timekeeping in the 19th century. The display variants used up to that point, such as " drop dials ", "disks" or "cylinders" with printed numbers to show the time, were replaced by light-emitting diodes ( LEDs ) and liquid crystal elements ( LCDs ) in the digital clocks used today. So-called seven-segment displays and matrix displays are usually used to show the numbers. For LED displays to function, driver modules are required that control individual LEDs or seven-segment displays. The display data is transferred to the module via a serial three-wire interface; in addition, a decoding mode can be activated for convenient control of seven-segment displays. Oscillating quartz crystals are mainly used to control the clock frequency of digital clocks. 

In 1972, the Swiss company Longines developed the prototype of a digital watch, called the "Clepsydre", which was equipped with a liquid crystal display (LCD). This watch consumed up to 30,000 times less power than LED watches and, thanks to the use of mercury batteries, had a service life of more than a year. Due to the falling price of LCD elements, Japanese watch manufacturers turned the once high-tech product into a cheap mass product. Later in the history of the digital watch, additional options such as computers, databases, heart rate monitors, cameras, compasses and TV reception were introduced. ^{[ 152 ] [ 154 ]} Digital wristwatches are now being replaced by so-called smartwatches , watches which have LCD displays and can communicate with the Internet.

Atomic clocks are the most accurate timekeeping devices known. Because of their accuracy of only a few seconds over millions of years, they are used to calibrate other clocks and timekeeping instruments. The principles of the atomic clock were developed by the American physicist Isidor Isaac Rabi at Columbia University, who received the Nobel Prize in Physics for this in 1944. ^{[ 155 ]} The first atomic clock was invented in 1949 and is located in the Smithsonian Institution (the largest museum complex in the world). ^{[ 156 ]} It was based on the absorption line in the ammonia molecule, ^{[ 157 ]} but most atomic clocks are based on the spin of the cesium atom. ^{[ 158 ]} The International System of Units standardized its unit of time for the second time in 1967, based on the properties of the isotope Cs-133. Due to the excellent accuracy of these clocks, atomic time was defined as the international standard for the second. ^{[ 159 ]} One application is caesium atomic clocks, in which the oscillation of the caesium atom is used as a very accurate timepiece. Atomic clocks also work with other elements, such as hydrogen and rubidium vapor. The hydrogen atomic clock has major advantages over the rubidium clock , such as greater stability, small size, low power consumption, and consequently, it is cheaper. ^{[ 159 ]}

Researchers at the National Institute of Standards and Technology (NIST) in Boulder (USA) developed the optical atomic clocks. These are considered the successors to the 50-year-old cesium clocks that set the time all over the world. The mercury atomic clock was first presented in 2000 and has been continuously improved since then. This optical atomic clock uses the rapid oscillations of a mercury ion that sits in an ultra-cold magnetic trap. The excited mercury ion emits a light pulse with a frequency of more than one quadrillion oscillations per second. ^{[ 160 ]} The second optical atomic clock uses an aluminum ion. Since it is hardly affected by electric and magnetic fields or temperature fluctuations, it has a high accuracy . The cesium atomic clock has an accuracy of one femtosecond , which is 15 decimal places after the decimal point. In experiments at the National Institute of Standards and Technology (NIST), the time of the optical atomic clocks could be measured to within a few attoseconds (18 decimal places). These experiments prove that both the mercury atomic clock and the newly developed aluminum ion clock are ten times more accurate than the world's best cesium-based atomic clocks.

A radio-controlled clock is a clock synchronized by a bit stream of time code transmitted by a signal transmitter connected to a time reference, such as an atomic clock. This method of transmitting time via radio was invented and patented by the Telefunken company in 1967. A radio-controlled clock can be synchronized with national or regional signal transmitters, or it can utilize a multiple signal transmitter, such as theGlobal Positioning System . Radio-controlled clocks have been widely used in Europe as wall clocks and wristwatches since the 1980s.

The Global Positioning System (GPS), in coordination with the Network Time Protocol, is a radio navigation system used to synchronize timekeeping systems across the globe. ^{[ 163 ]} GPS was developed by the U.S. Department of Defense to provide constant all-weather navigation capabilities for the Army , Navy , and Air Force . ^{[ 164 ]} Between February 22, 1978, and October 9, 1985, the first generation of 24 satellites comprising the Global Positioning System were launched from Vandenberg Air Force Base in California . In 1983, after Korean Air Lines Flight 007 was shot down while crossing Soviet airspace, President Ronald Reagan issued a directive allowing free commercial use of GPS to prevent further navigational incidents. ^{[ 165 ]} GPS time cannot be accurately aligned with the Earth's rotation, and therefore does not take into account leap seconds or other corrections regularly applied to systems such as Coordinated Universal Time (UTC). This is the reason why the connection between GPS time and UTC has diverged. GPS time therefore remains at a constant offset of 19 seconds from International Atomic Time (TAI). The satellite clocks of the GPS system are regularly synchronized with atomic clocks on Earth to correct for relativistic effects. Since 2007, the time difference between GPS time and UTC is only 14 seconds, which is taken into account by GPS navigation. Receivers subtract this offset from GPS time, thus allowing specific UTC time zone values to be calculated. ^{[ 166 ]} In the United States, the Navstar-GPS system is maintained by 24 satellites that orbit the Earth in six orbits every twelve hours. Russia operates a system known as GLONASS (Global Navigation Satellite System). In 2007, the European Union approved funding for the Galileo system . This system consists of 30 satellites, which are expected to be operational by 2018. China has two Earth-orbiting satellites in orbit out of the planned 35 for its Beidou system. ^{[ 163 ]}

The first clockmakers were blacksmiths, cannon founders, metalworkers, goldsmiths, and silversmiths. Craftsmen who could make clocks in the Middle Ages were travelers who moved from town to town taking on commissions. Over the years, clockmaking evolved from a specialist skill into a mass-production industry. Paris and Blois were the early centers of clockmaking in France . Other clockmaking centers were Augsburg and Nuremberg in Germany , Geneva in Switzerland , and London in England. French clockmakers such as Julien Le Roy of Versailles were leaders in the design of decorative clocks. ^{[ 167 ]} Le Roy was the fifth generation of a family of clockmakers and was described by his contemporaries as 'the most talented clockmaker in France'. He invented a special repeating mechanism that improved the precision of clocks. He built two clocks for Louis XV whose dials could be opened, thus revealing the inner workings. During his lifetime, he produced over 3,500 clocks in his workshop, an average of 100 per year. By comparison, other clockmakers produced about 30–50 per year. The competition and scientific rivalry resulting from his discoveries continued to inspire researchers to search for new methods of accurate timekeeping. ^{[ 168 ]}

In Germany, Nuremberg and Augsburg were the early centers of watchmaking. Meanwhile, in the first half of the 19th century, lacquer shield clocks were produced in the Black Forest . Lacquer shield clocks get their name from their shield-shaped, painted and varnished dial, which covers everything else. ^{[ 169 ]} The majority of watchmakers in the 17th and 18th centuries came from England. Switzerland established its own center of watchmaking in Geneva in the 19th century. The influx of Huguenot craftsmen enabled Switzerland to manufacture watches by machine, and as a result, the Swiss industry achieved world dominance in the manufacture of high-quality machine-made watches. The leading company during this period was Patek Philippe . It was founded by Antoni Patek of Warsaw and Adrien Philippe of Bern.