History of timekeeping devices

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See also: Timeline of time measurement technology

The history of timekeeping devices encompasses the various methods used to measure time throughout history.

The origins of the current Western time measurement system date to approximately 2000 BC, in Sumer. The system developed then—which remains in use today—was sexagesimal. The Ancient Egyptians classified day and night as each being twelve hours long. A wide variety of timekeeping devices were engineered in ancient times, including some that measure the path of the sun across the sky, the flow of water or sand, or the burning of candles and incense. The Ancient Egyptians developed large obelisks to measure the movement of the sun, while later civilizations, such as the Roman Empire, improved on these early designs. Water clocks, called clepsydras by the Greeks, were also of early Egyptian design. They were probably first used in the Precinct of Amun-Re, however, their use continued outside of Egypt, especially in Greece. Around this period, the Chinese, ruled by the Shang dynasty, are thought to have used the outflow water clock, which was introduced from Mesopotamia as early as 2000 BC. Other ancient timekeeping devices include the candle clock, used in China, Japan, and England; the timestick, used in India and Tibet, as well as some parts of Europe; and hourglasses, which functioned similarly to a water clock.

While many ancient civilizations developed means of recording times and dates, conventional timekeeping as it is known today first began around the 11th century, prior to the wide introduction of the clock in Europe. Mechanical clocks became necessary because relying on the sun had two drawbacks: sundials worked only on clear days, cast no shadow at night, and the length of hours varied depending on the season.

Mechanical clocks, in all of their varieties, were the standard modern timekeeping device. From the turn of the 14th century, until the middle of the 20th, they contained various escapements, which transferred rotational energy into discrete motions. The first clock with escapements was invented in China in the 8th century BC. In the twentieth century, a variety of new methods were invented, including early quartz oscillators, atomic clocks, and the common wristwatch.

Contents 1 Early timekeeping devices 1.1 3500 BC – 500 BC 1.2 500 BC – 0 AD 1.3 0 AD – AD 1500 1.3.1 Water Clocks 1.3.2 Early Mechanical Clocks 1.3.3 Candle Clocks 1.3.4 Incense Clocks 1.3.5 Modern sundials 2 Modern devices 2.1 Modern clocks of ancient design 2.2 Clocks 2.2.1 Early mechanical clocks 2.2.2 Pendulum clocks 2.2.3 Clockmakers 2.2.4 Wristwatches 2.2.5 Marine chronometers 2.3 Chronometers 2.4 Quartz oscillators 2.5 Atomic clocks 2.6 Global Positioning System 3 Footnotes 4 References 5 Further reading 6 External links

[edit] Early timekeeping devices

See also: History of calendars

Many ancient civilizations observed astronomical bodies, often the Sun and Moon, to determine times, dates, and seasons.^[1][2] Methods of sexagesimal timekeeping, as are now prominent in Western society, date back to 200 BC.^[1][3] The first calendars may have been created by hunter-gatherers in the last glacial period, employing tools such as sticks and bones to track the phases of the moon or the seasons.^[2] Stone circles, such as England's Stonehenge, were used in various parts of the world, especially in Prehistoric Europe, for timing, and predicting seasonal and annual events—such as equinoxes or solstices.^[4][2] As these megalithic civilizations left no written records, little is known of their calendars or timekeeping methods, other than the significance of these dates.^[5]

[edit] 3500 BC – 500 BC

See also: History of timekeeping devices in Egypt

Shadow clocks are both the earliest known timekeeping devices and scientific instruments; they calculate the position of the sun to keep time.^[6] Among the first confirmed shadow clocks were the ancient Egyptian obelisks, first constructed around 3500 BC;^[2][7] the oldest existing sundial,—aside from obelisks—is made of green schist, and is also Egyptian.^[8][9]

Among the first to divide days into equal parts, the Egyptians used shadow clocks, among other instruments, to measure time.^[2] These clocks divided daytime into 10 parts, with an additional four "twilight hours"—two in the morning, and two in the evening. One type of shadow clock was made up of a long stem with 5 variable marks, and an elevated crossbar that cast a shadow over these marks. It was positioned eastward in the morning, and was turned west at noon. Obelisks functioned in much the same manner: when the sun cast a shadow on the markers around it, the Egyptians were able to calculate the time. The obelisk also indicated whether it was morning or afternoon, as well as the summer and winter solstices.^[10][2] A third shadow clock, c. 1500 BC, was similar in shape to a bent T-square. It measured the passage of time by the shadow cast by its crossbar on a non-linear rule. The "T" was oriented eastward in the mornings, and turned around at noon, so that it could cast its shadow in the opposite direction.^[11]

While accurate, shadow clocks relied upon the sun to keep time, and were therefore useless at night and cloudy weather.^[10][12] Various other instruments were invented by the Egyptians in order to fill this gap; for example, water clocks and hourglasses could be used anytime, while a system of tracking the stars' movements was also developed. The oldest documentation of the water clock is the tomb inscription of the 16th century BC Egyptian court official Amenemhet, which identifies him as the inventor of the clock.^[13] There were several types of water clocks, some more elaborate than others. One type consisted of a bowl with small holes in its bottom, which was floated on water and allowed to fill at a near-constant rate; markings on the side of the bowl indicated elapsed time, as the surface of the water reached them. One such clock was found in the tomb of pharaoh Amenhotep I (1525–1504 BC), proving that they were used in ancient Egypt, possibly earlier than anywhere else.^[10][14][15] Also believed to have been invented by the ancient Egyptians is the hourglass, which consisted of two vertically aligned glass chambers, with a small opening connecting them. When turned over, a flow of grains of sand passed from the upper one to the lower through the hole. As the downward current of sand was constant, it could be used to keep time; also, hourglasses could be reused by turning it over again.^[12] Another Egyptian method of determining the time during the night was using plumb-lines called merkhets, provided the stars were visible. Used since at least 600 BC, two of these instruments were aligned with Polaris, the north pole star, creating a north-south meridian. By observing certain stars as they crossed the line created with the merkhets, the time could be accurately calculated.^[10][16]

[edit] 500 BC – 0 AD

Water clocks were used to great extent in Greece, where they were introduced by Plato (424–348 BC); Plato also invented a water-based alarm clock.^[17][18] One account says it depended on the nightly overflow of a vessel containing lead balls, which would float in a columnar vat. The vat would hold a steadily increasing supply of water, supplied by a cistern. Eventually, the vessel would float high enough to tip over, causing the lead balls to cascade onto a copper platter. The resultant clangor would then awaken his students at the Academy.^[11] Another account says that it used two jars and a siphon were used. Water emptied until it reached the siphon, which transported the water to the other jar. There, the rising of the water would force air through a whistle, sounding the alarm.^[18] Another mention occurs in The Golden Ass: "The Clerk of the Court began bawling again, this time summoning the chief witness for the prosecution to appear. Up stepped an old man, whom I did not know. He was invited to speak for as long as there was water in the clock; this was a hallow globe into which water was poured through a funnel in the neck, and from which it gradually escaped through fine perforations at the base."^[19] The clock consisted of a bowl, with a hole in its center, which was floated on water. Time was measured by observing how long the bowl took to fill with water.^[20] Although clepsydras were more useful than sundials—as they could be used indoors, during the night, and also when the sky was cloudy—they were not as accurate. The Greeks, therefore, sought a way to improve their water clocks. It is possible, however, that their search for increased precision was not due to their interest in science, but rather their desire to imitate the heavens, which formed the basis of their religion.^[21] Regardless of the reason, Greek water clocks greatly improved in accuracy around 325 BC: they were adapted to have a face with an hour hand, making the reading of the clock more precise and facile.

Although water clocks were more practical than sundials, several problems arose with them. When the bowl was full, the increased water pressure caused the water to flow out of the holes more rapidly than if it was nearly empty. This problem was addressed by Greek and Roman horologists beginning in 100 BC, and improvements continued to be made in the following centuries. To counteract this increased water flow, the clocks were given a conical shape, with the wide end up, so that a greater amount of water would have to flow out to drop the same distance as when the water was lower in the cone. Along with this improvement, clocks were constructed more elegantly in this period, with hours marked by gongs, doors opening to miniature figurines, bells, or moving mechanisms.^[10] The second problem was temperature, as water flows more slowly when cold, or may even freeze. Also, the water clock did not account for the fact that the length of days and nights changes throughout the year. Because of this, the clocks' accuracy varied throughout the seasons.^[22]

The mathematician and astronomer Theodosius of Bithynia is said to have invented a universal sundial that could be used anywhere on Earth, but nothing is known about it.^[23] The sundial has also been discussed in mathematics and in older literature. Marcus Vitruvius Pollio, the Roman author of De Architectura, wrote on the mathematics of gnomons.^[24]

The shadow clocks were further developed by other cultures, including the Greeks, Chinese, Romans and Muslims.^[7] The largest sundial ever built, the Solarium Augusti, was constructed by the Romans during the rule of Emperor Augustus.^[25] Pliny the Elder records that the first sundial in Rome was looted in 264 BC, from Catania, Sicily, and gave the incorrect time for a century until the markings and angle appropriate for the latitude of Rome were used—a century later.^[11] Another example is the obelisk in Campus Martius in Rome, which acted as the gnomon for the great zodiacal sundial of Emperor Augustus.^[26]

[edit] 0 AD – AD 1500

[edit] Water Clocks

Historian Joseph Needham speculates that the introduction of the outflow clepsydra to China, perhaps from Mesopotamia, occurred as far back as the 2nd millennium BC, during the Shang Dynasty, and at the latest, by the 1st millennium BC. By the beginning of the Han Dynasty, in 202 BC, the outflow clepsydra was gradually replaced by the inflow clepsydra, that featured an indicator rod on a float. To compensate for the falling pressure head in the reservoir, which slowed timekeeping as the vessel filled, Zhang Heng (78–139 AD) added an extra tank between the reservoir and the inflow vessel. Around 550 AD, Yin Gui was the first in China to write of the overflow or constant-level tank added to the series, which was later described in detail by the inventor Shen Kuo (1031–1095). Soon after, this design was trumped by two Sui Dynasty (581–618) inventors, Geng Xun and Yuwen Kai, who, around 610 AD, were the first to create the balance clepsydra, with standard positions for the steelyard balance.^[27] Joseph Needham states that:

...[the balance clepsydra] permitted the seasonal adjustment of the pressure head in the compensating tank by having standard positions for the counterweight graduated on the beam, and hence it could control the rate of flow for different lengths of day and night. With this arrangement no overflow tank was required, and the two attendants were warned when the clepsydra needed refilling.^[27]

[edit] Early Mechanical Clocks

Many developments in horology, not limited to water clocks, occurred in China between 200 and 1300 AD.^[10] For example, the first mechanical clock—the first to use escapements—was built in Chang'an, by Tantric monk and mathematician, Yi Xing, and government official Liang Lingzan.^[28][29] An astronomical instrument that served as a clock, it was discussed in a contemporary text as follows:^[30]

[It] was made in the image of the round heavens and on it were shown the lunar mansions in their order, the equator and the degrees of the heavenly circumference. Water, flowing into scoops, turned a wheel automatically, rotating it one complete revolution in one day and night. Besides this, there were two rings fitted around the celestial sphere outside, having the sun and moon threaded on them, and these were made to move in circling orbit ... And they made a wooden casing the surface of which represented the horizon, since the instrument was half sunk in it. It permitted the exact determinations of the time of dawns and dusks, full and new moons, tarrying and hurrying. Moreover, there were two wooden jacks standing on the horizon surface, having one a bell and the other a drum in front of it, the bell being struck automatically to indicate the hours, and the drum being beaten automatically to indicate the quarters. All these motions were brought about by machinery within the casing, each depending on wheels and shafts, hooks, pins and interlocking rods, stopping devices and locks checking mutually.^[30]

Since it was a water clock, Xing's clock was easily affected by cold temperatures. This problem was solved by Zhang Sixun, in AD 976, by using mercury to operate the clock; unlike water, this element would not freeze under normal circumstances, due to its lower freezing point. Sixun implemented the changes into his clock tower, which was approximately ten meters tall, with escapements keeping the clock turning, and bells ringing to signal every quarter-hour. Another noteworthy clock, the elaborate "Cosmic Engine," was built by Su Song, in 1088 AD. It was around the size of Zhang's tower, but had an automatically rotating armillary sphere (also called a celestial globe), with which one could observe the positions of the stars. It also featured five panels with mannequins ringing gongs or bells, and tablets showing the time of day, or other special times.^[10] Originally built in the capital of Kaifeng, it was dismantled by the Jin army and sent to the capital of Yanjing (now Beijing), where they were unable to put it back together. As a result, Su Song's son Su Xie was ordered to build a replica.^[31]

[edit] Candle Clocks

It is unknown where and when candle clocks were first used. The earliest mention comes from Chinese poem by You Jiangu, 520 AD. Here, the graduated candle supplied a means of determining time at night. Similar candles were used in Japan until the early 10th century AD.^[32] The most commonly mentioned candle clock is attributed to King Alfred the Great of England (878 AD). His device consisted of six candles made from 72 pennyweights of wax, each 12 inches (30 cm) high, and of uniform thickness. At each inch, a mark was made. Each candle burned for approximately four hours, so each mark represented 20 minutes. The candles were placed in wooden framed glass boxes for protection when lit.^[33]

[edit] Incense Clocks

In addition to water clocks and clock towers, candle clocks were also used, as well as incense clocks, which came in several different forms.^[5] The hourglass was also in Chinese use, but its history in China and time of invention remain unknown.^[34]

The incense clock was invented in China around the Song Dynasty. It spread to neighboring countries such as Japan, along with Buddhism.^[35] Although similar to the candle clock, incense clocks burned evenly and without a flame; therefore, they were more accurate and safer to use in a building.^[36] Incense clocks came in two varieties: stick and powdered.^[35][37] Stick incense clocks at their most basic were simply incense sticks with calibrations.^[37] More elaborate incense clocks had threads with weights or bells tied to the end, which hung over an incense stick at desired intervals. As the incense burned, the threads would also burn one by one and the weights would drop onto a sounding plate, pan, or gong below.^[38] Sticks of incense with different scents were also used, so that the hours were marked by a change in fragrance.^[5]

In powdered incense clocks, a fine layer of white wood ash is first laid down in a small container and compacted.^[35][39] Seals in the form of patterned metal cutouts, or tracks, are simply laid down on the ash while the incense powder is poured into the cut out shapes.^[35] After a light compaction of the incense powder by a tamper, the shape is lifted, forming a trail of incense powder on top of the ash.^[35] In other clocks, seals are used with a protruded pattern that creates a negative indentation in the wood ash. The incense powder is carefully spooned into the indentation in the ash and then recompacted again with the seal. When the incense is burned, the time is shown by the grid marks created by the burned incense.^[35] In either case, depending on the size of the seal, the trails of incense may burn for long periods of time—anywhere from hours to days.^[40] To signal the passage of a specific amount of time, small pieces of fragrant woods, resins, or different scented incenses could be placed on the incense powder trails. Different powdered incense clocks had different formulas for the powdered incense, depending on how the clock was laid out.^[41]

[edit] Modern sundials

Sundials first appeared in their present form during the Renaissance with the acceptance of heliocentrism and equal hours, as well as applications of trigonometry. During the Renaissance, sundials were built in large numbers in many locations.^[42]

The French astronomer Oronce Finé constructed an ivory sundial, still in existence, in 1524.^[43] The Italian astronomer Giovanni Padovani published a treatise on the sundial in 1570, in which he included instructions for the manufacture and laying out of mural (vertical) and horizontal sundials. Giuseppe Biancani's Constructio instrumenti ad horologia solaria (ca. 1620) discusses how to make a perfect sundial, with accompanying illustrations.^[44]

Water clocks—and later, mechanical clocks, as well—were used to mark the events of the abbeys and monasteries of the Middle Ages. Richard of Wallingford (1292–1336 AD), abbot of St. Alban's abbey, built a mechanical clock as an astronomical orrery, around 1330.^[45][46] Arab engineers, in particular, improved water clocks, up until the Middle Ages.^[11]

Ferdinand Magellan used eighteen hourglasses on each ship for his circumnavigation of the globe (1522).^[47] Since the hourglass was one of the few reliable methods of measuring time at sea, it has been speculated that it was in use at sea as far back as the 11th century, where it would have complemented the magnetic compass as an aid to navigation, though it was not until the 14th century that evidence of their existence was discovered, appearing in the painting Allegory of Good Government by Ambrogio Lorenzetti in 1338.^[48] 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 and reasonably accurate measurement of time. The hourglass also took on symbolic meanings, such as that of death, temperance, opportunity, and Father Time, represented by the finite stream of sand.^[49]

[edit] Modern devices

[edit] Modern clocks of ancient design

A smaller model of the hourglass, the egg timer, is still in use in many homes today to measure the amount of time taken to boil an egg.^[50] It uses a special sand made from ground eggshells, which does not erode the hole as fast as ordinary sand.^[5]

Today's most common design, the horizontal sundial, consists of an upright right triangle, with an angle the same as the latitude, referred to as a gnomon. The gnomon casts a shadow onto a horizontal plane—often a table—which has markings used to measure the shadow, and thus determine the time. However, shadow clocks were often not designed to be round and were not to be found in open settings.^[51]

[edit] Clocks

Main article: Clock

Clocks encompass a wide spectrum of devices, ranging from wristwatches, to more exotic varieties such as the Clock of the Long Now. The English word clock is said to derive from the Middle English word clokke, Old North French cloque, or Middle Dutch term clocke, all of which mean bell, and are derived from the Medieval Latin clocca.^[52][53] The passage of the hours at sea were marked by bells; the same practice was common at abbeys. They can be powered by a variety of means, including gravity, springs, and electricity.^[54][55] The invention of mechanical clockwork itself is usually credited to the Chinese official Liang Lingzan and monk Yi Xing.^[29][28][56]

[edit] Early mechanical clocks

The earliest Medieval clockmakers were Christian monks.^[57] Medieval religious institutions required clocks because, for many centuries, daily prayer and work schedules were strictly regulated; often, this was done by a stationed clockkeeper. These clocks were wound twice a day, or more.^[58] This was done by various types of time-telling and recording devices, such as water clocks, sundials and marked candles, probably used in combination.^[58][55] Important times and durations were broadcast by bells, rung either by hand or by some mechanical device, such as a falling weight or rotating beater.

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

The monks also counted skillful clock-makers among them. The first clock of which we have any record was built by the future Pope Sylvester II for the German town of Magdeburg, around the year 996. Much more sophisticated clocks were built by later monks. Peter Lightfoot, a fourteenth-century monk of Glastonbury, built one of the oldest clocks still in existence, which now sits in excellent condition in London's Science Museum.^[59]

The appearance of clocks in writings of the 11th century implies that they were well known in Europe in that period.^[61] In the early 14th century, the Forentine poet Dante Alighieri referred to a clock in his Paradiso;^[62] this is considered the first literary reference to a clock that struck the hours.^[61] The earliest detailed description of clockwork was presented by Giovanni da Dondi, Professor of Astronomy at Padua, in his 1364 treatise Il Tractus Astarii.^[56] This has inspired several modern replicas, including some in London's Science Museum, and the Smithsonian Institution.^[56] Other notable examples from this period were built in Milan (1335), Strasbourg (1354), Lund (1380), Rouen (1389), and Prague (1462).^[56]

The oldest working clock in the world is Salisbury cathedral clock, which dates from about 1386, and has most of its original parts.^[63] This clock had no dial, as its purpose was to strike a bell at precise times.^[63] The wheels and gears are mounted in an open, box-like iron frame, measuring about 1.2 meters (3.9 ft) square. The framework was held together with metal dowels and pegs, and the escapement was the verge and foliot type, standard for clocks of this age. The power was supplied by two large stones, hanging from pulleys. As the weights fall, ropes unwind from the wooden barrels. One barrel drives the main wheel, which is regulated by the escapement, the other drives the striking mechanism and the air brake.^[63]

Pater Lightfoot's Wells Cathedral clock is also of note, and was constructed circa 1396.^[64] The clock was converted to pendulum and anchor escapement in the 17th century, and was installed in the London's Science Museum in 1884, where it continues to operate.^[65] The dial represents the geocentric view of the universe, with sun and moon revolving round a central fixed earth. It may be unique in showing a philosophical model of the pre-Copernican universe.^[66] Above the clock is a set of figures, which hit the bells, and a set of jousting knights who revolve around a track every 15 minutes.^[65] Similar astronomical clocks, or "horologes," can be seen at Exeter, Ottery St Mary, and Wimborne Minster.

One of the clocks that did not survive is that of the Abbey of St Albans, built by the 14th century abbot, Richard of Wallingford.^[67] It may have perished during Henry VIII's Dissolution of the Monasteries, but Richard's notes on its design have allowed a full-scale reconstruction. As well as timekeeping, the astronomical clock could accurately predict lunar eclipses. According to Thomas Woods, "a clock that equaled it in technological sophistication did not appear for at least two centuries."^[59][68]

Secular clocks emerged toward the end of the Middle Ages, and the increased luxury of castles sometimes brought the introduction of turret clocks.^[69] A 1435 example survives from Leeds castle, whose face is decorated with the images of Christ Crucified, Mary and St George.^[69] In Dublin, the official measurement of time was a local custom, and by 1466, a public clock stood on top of the Tholsel (the city court and council chamber).^[70] It was probably the first of its kind in Ireland, and would only have had an hour hand.^[70]

[edit] Pendulum clocks

Main article: Pendulum clock

Innovations continued, with miniaturization leading to domestic clocks in the 15th century, and personal watches in the 16th.^[56] In the 1580s, the Italian polymath Galileo Galilei investigated the regular swing of the pendulum, and discovered that it could be used to regulate a clock.^[71][55] Though Galilei studied the pendulum as early as 1582, he never actually constructed a clock based on these designs.^[55] The first "pendulum clock" was designed and built by Dutch scientist Christiaan Huygens, in 1656.^[55] This clock was accurate for its time, first erring by under a minute per day, and later versions by only ten seconds.^[55]

The Jesuits were another major contributor to pendulum clocks in the 17th and 18th centuries,^[72] having had an "unusually keen appreciation of the importance of precision."^[73] 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."^[73] They served a crucial role in spreading and testing the scientific ideas of the period, and collaborated with contemporary scientists, such as Huygens.^[72]

The modern Longcase clock, also known as the "Grandfather clock," traces its origins to the invention of the anchor escapement mechanism, around 1670.^[74] Before that, pendulum clocks had used the older verge escapement mechanism, which required very wide pendulum swings, of about 100°. As long pendulums of this type required exceedingly large cases, most clocks had short ones. The anchor mechanism, however, reduced the pendulum's swing to around 4° to 6°, allowing clockmakers to use longer pendulums, which had slower "beats." These required less power to move, caused less friction and wear in the movement, and were more accurate than their predecessors. Most longcase clocks use a pendulum where each swing takes one second. These are about a meter (39 inches) long, to the center of the bob. This requirement for height, along with the need for a long drop space for the weights that power the clock, gave rise to the design of the tall, narrow case.^[75]

In 1675, 18 years after inventing the pendulum clock, Huygens devised the spiral balance spring for the balance wheel of pocket watches, an improvement on the straight spring invented by English natural philosopher Robert Hooke.^[71] This resulted in a great advance in accuracy of pocket watches, from perhaps several hours per day^[76] to 10 minutes per day,^[10] similar to the effect of the pendulum upon clocks.

[edit] Clockmakers

The first professional clockmakers evolved out of the earlier guilds of locksmiths and jewellers. Clockmaking developed from a highly specialized craft, into a mass production industry. Paris and Blois were the early centers of clockmaking in France. French clockmakers were leaders in case design and ornamental clocks, with Julien Le Roy, clockmaker to Versailles, being one such example.^[77] 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 that improved the precision of clocks and watches, a face that could be opened to view the inside clockwork, and made or supervised over 3,500 watches. The competition and scientific rivalry from his discoveries further encouraged researchers to seek new methods to measure time more accurately.^[78]

Between 1794 and 1795, in the aftermath of the French Revolution, the French government briefly mandated decimal clocks, with a day divided into 100 minutes of 100 seconds each.^[79] The astronomer and mathematician Pierre-Simon Laplace, among some other individuals, modified the dial of his pocket watch to decimal time.^[79] A clock in the Palais des Tuileries kept decimal time as late as 1801, but the cost of replacing all the nation's clocks prevented it from becoming widespread.^[80] Because decimalized clocks only helped astronomers rather than ordinary citizens, it was one of the most unpopular changes associated with the metric system, and it was abandoned.^[80]

In Germany, Nuremberg and Augsburg were the early clockmaking centers, and the Black Forest came to specialize in wooden "cuckoo clocks." The English became the predominant clockmakers of the 17th and 18th centuries. Switzerland became established as a clockmaking center following the influx of Huguenot craftsmen, and in the 19th century, the Swiss industry "gained worldwide supremacy in high-quality machine-made watches." The leading firm of the day was Patek Philippe, founded by Antoni Patek of Warsaw and Adrien Philippe of Berne.^[77]

[edit] Wristwatches

Main article: Wristwatch

In 1904, Alberto Santos-Dumont, an early pioneer of aviation, asked his friend, Louis Cartier, a French watchmaker, to design a watch that could be useful during his flights.^[81] The wristwatch had already been invented by Patek Philippe, in 1868, but only as a "lady’s bracelet watch," intended as a jewel. Since pocket watches were unsuitable, Louis Cartier created the Santos wristwatch, which was the first wristwatch made for men, and for practical use.^[82]

Wristwatches gained more popularity in World War I, when officers realized that they were more convenient than pocket watches in battle. Also, because the pocket watch was more of a middle class item, the working class soldiers usually owned wristwatches, which they brought with them to their service. Artillery and infantry officers depended on these watches as battles became more complicated, especially as coordinated attacks became necessary. Wrist watches were found to be needed in the air as much as on the ground: military pilots found them more convenient than pocket watches for the same reasons as Santos-Dumont had. Eventually, army contractors manufactured watches en masse, for both infantry and pilots. In World War II, a popular watch of most American airmen was the A-11; it had a simple black face, and clear white numbers for easy readability, and it met the aviator’s basic needs.^[83]

[edit] Marine chronometers

Main article: Marine chronometer

Marine chronometers are clocks used at sea as time standards, to determine longitude by celestial navigation.^[84] First developed by Yorkshire carpenter John Harrison, who won the British government's Longitude Prize in 1759, marine chronometers keep the time of a fixed location—usually Greenwich Mean Time—allowing seafarers to compare the local high noon to the clock.^[84][85][86] This allowed for determination of longitude at sea.^[84]

[edit] Chronometers

Main article: Chronometer watch

A chronometer is a portable timekeeper that meets certain precision standards. Initially, the term was used to refer to the marine chronometer, a timepiece used to determine longitude by means of celestial navigation.^[84] More recently, the term has also been applied to the chronometer watch, a wristwatch that meets precision standards set by the Swiss agency COSC. Over 1,000,000 "Officially Certified Chronometer" certificates, mostly for mechanical wrist-chronometers—wristwatches—with sprung balance oscillators, are delivered each year, after passing the COSC's most severe tests, and being singly identified by an officially recorded individual serial number. According to COSC, a chronometer is a high-precision watch, capable of displaying the seconds and housing a movement that has been tested over several days, in different positions, and at different temperatures, by an official, neutral body. Each movement is individually tested for several consecutive days, in five positions, and at three temperatures. Any watch with the denomination "chronometer" is provided with a certified movement.^[87]

[edit] Quartz oscillators

Main article: Crystal oscillator

The piezoelectric properties of quartz, a crystal were discovered by Jacques and Pierre Curie in 1880.^[88][89][55] The first quartz crystal oscillator was built by Walter G. Cady in 1921, and in 1927 the first quartz clock was built by Warren Marrison and J.W. Horton at Bell Telephone Laboratories in Canada.^[90][91] The following decades saw the development of quartz clocks as precision time measurement devices in laboratory settings – the bulky 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.^[91] The National Bureau of Standards (now NIST) based the time standard of the United States on quartz clocks from late 1929 until the 1960s, when it changed to atomic clocks.^[92] In 1969, Seiko produced the world's first quartz wristwatch, the Astron.^[93] The inherent accuracy and low cost of production has resulted in the proliferation of quartz clocks and watches since that time.^[94][55]

[edit] Atomic clocks

Main article: Atomic clock

The most accurate timekeeping devices are atomic clocks, which are accurate to a few seconds over many thousands of years and are used to calibrate other clock and timekeeping instruments.^[95]The atomic clock was invented in 1949 and early models were based largely on the absorption line in the ammonia molecule.^[96][97] This first clock is on display at the Smithsonian Institution in Washington D.C.^[92]Atomic clocks use the spin property of the cesium atom as its basis and, since 1967, the International System of Units bases its unit of time, the second, on the properties of cesium.^[97] SI defines the second as 9,192,631,770 cycles of the radiation which corresponds to the transition between two electron spin energy levels of the ground state of the ¹³³Cs atom.^[98] The cesium atomic clock, maintained by the National Institute of Standards and Technology, is currently accurate to 30 billionths of a second per year.^[97] There are, however, atomic clocks in use that do not use cesium. These can use substances including hydrogen and rubidium vapor, offering more stability—in the case of hydrogen clocks—and smaller size, less power, and thus lower cost (in the case of rubidium clocks).^[97]

[edit] Global Positioning System

Main article: Global Positioning System

The Global Positioning System (GPS), in coordination with the network time protocol, is a radio-navigation system that can be used to synchronize timekeeping systems across the globe.^[99] GPS was developed by the US Department of Defense to provide all weather 24 hours a day navigation capabilities for military ground, sea, and air forces.^[100] GPS time is not corrected to match the rotation of the Earth, so it does not account for leap seconds or other corrections which are periodically employed to systems such as Universal Coordinated Time (UTC). GPS time was set to match UTC in 1980, but has since diverged because of the absence of corrections. This means that GPS time remains at a constant offset of 19 seconds with International Atomic Time (TAI). Periodic corrections are performed on the on-board clocks to correct relativistic effects and keep them synchronized with ground clocks. The GPS navigation message includes the difference between GPS time and UTC, which is 14 seconds, as of 2007. Receivers subtract this offset from GPS time to calculate UTC and specific timezone values.^[101] In ths US, GPS is regulated by 12 satellites in 6 orbits around the Earth on a 12-hourly schedule.^[99]

[edit] Footnotes

[edit] References

• Alder, Ken (2002). The Measure of All Things: The Seven-Year Odyssey and Hidden Error that Transformed the World. London: Little, Brown. ISBN 0743216768. 
• Bottomley, Frank (1983). The Castle Explorer's Guide. Crown Publishers. ISBN 0517421720. 
• Clarke, Howard B.; Dent, Sarah; Johnson, Ruth (2002). Dublinia: The Story of Medieval Dublin. Dublin: O'Brien. ISBN 0862787858. 
• Davies, Norman (1996). Europe: A History. Oxford: Oxford University Press. ISBN 0-19-820171-0. 
• Frugoni, Chiara (1988). Pietro et Ambrogio Lorenzetti. Scala Books. ISBN 0935748806. 
• Marrison, Warren (1948). "The Evolution of the Quartz Crystal Clock". Bell System Technical Journal 27: 510–588. AT&T. 
• Reid, Thomas (1832). Treatise on Clock and Watch Making: Theoretical and Practical. Carey and Lea. 
• Woods, Thomas (2005). How the Catholic Church Built Western Civilization. ISBN 0-89526-038-7. 

[edit] Further reading

[edit] External links

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

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