The history of timekeeping devices encompasses the various methods used to measure time across history, falling under the study of horology and chronometry, as well as covering both the science of timekeeping and that of making timekeeping devices. The origins of the current Western time measurement system date to approximately 2000 BCE, in Sumer.^[1] The system developed then—still in use today—was sexagesimal.^[2] A wide variety of timekeeping devices was engineered in ancient times, including some that measure the path of the sun across the sky, the flow of water, or the burning of candles.

Ancient civilizations developed simple systems of time measurement, which were often based on the environments around them, especially the sun. The Ancient Egyptians classified day and night as each being twelve hours long, developing large obelisks to measure the movement of the sun. These obelisks are considered to be the earliest sundials. Later civilizations, such as the Roman Empire, improved on these early designs. Water clocks, or clepsydras, were also of early Egyptian design, probably first used in the Precinct of Amun-Re; their use continued, especially in Classical Greece. The Shang Dynasty of China is thought to have used the outflow water clock, introduced from Mesopotamia, as early as 2000 BCE.^[3] Other timekeeping devices of the ancient world include candle clocks, which were used in China and Japan.

Clocks, in all of their varieties, have become the standard, modern timekeeping device, and are usually small, containing various escapements. Several varieties of clocks have been created, including early quartz oscillators, and atomic clocks. Early church records indicate water clocks were in use, until the turn of the 14th century, when they were replaced by "horologes"—clocks that made use of escapements, rather than water.

Early timekeeping devices

Many ancient civilizations used astronomical bodies to determine times, dates, and seasons.^[4] The first calendars may have been created by hunters in the last glacial period, employed to track the phases of the moon. Stone circles were used in various parts of the world, especially in Prehistoric Europe, for timing seasonal and annual events, such as equinoxes or solstices. 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]

Timekeeping devices in Egypt

The Ancient Egyptians were among the first to divide days into generally agreed-upon equal parts; they used devices such as sundials and shadow clocks to measure time. It is thought that obelisks were built to serve as sundials; the first of these were constructed and used around 3500 BCE. Shadow clocks, another form of Egyptian sundial, came into use around 1500 BCE.^[6][7] The shadow clock divided daytime into 10 parts, with an additional four "twilight hours"—two in the morning, and two in the evening. It was made up of a long stem with 5 variable marks, and an elevated crossbar that cast a shadow over the marks. The device was positioned eastward in the morning, and was turned west at noon, marking the afternoon. Obelisks functioned in much the same manner: markers around it would mark units of time, and indicated morning and afternoon, as well as the summer and winter solstices.^[6]

There is also evidence showing the usage of water clocks in ancient Egypt. An early Egyptian water clock, dating back to approximately 1500 BCE, was found in the tomb of Pharaoh Amenhotep I. Early water clocks, such as these, consisted of a bowl with a small hole in the bottom, which was floated on water, and allowed to fill. The water level reached equally-spaced markings on the side, indicating the passage of time.^[6]

Using plumb-lines called "merkhets", the Egyptians could determine the time at night, provided the stars were visible.^[6][8] Used since at least 600 BCE, two of these instruments were aligned with Polaris, the North pole star, creating a North-South meridian.^[6][8] By observing certain stars as they crossed the line created with the merkhets, the time could be accurately calculated.^[6][8] The Egyptians also developed an early solar calendar; it was first used when it was discovered that Sirius rose by the sun once in 365 days, coinciding with the annual inundation of the Nile River, Egypt's lifeblood.^[5]

Timekeeping devices in China

By at least the Shang Dynasty or before, China had found the clepsydra, brought from Mesopotamia. Several innovations were added since then, and several additions had been added to it that made it more accurate.

File:Susong.gif

The first mechanical clock was built in China by inventor, Tantric monk, and mathematician Yi Xing and government official Liang Lingzan. It was an astronomical instrument that served as a clock, described by a contemporary text this way:^[9]

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

Since it was a water clock, it was affected by cold temperatures that froze the water. This problem was sidestepped by using mercury to operate the clock, which would not freeze. Such a clock was built by Zhang Sixun in 976 CE. The clock tower was around 10 meters tall, with escapements keeping the clock turning and ringing bells to signal every quarter-hour. Another noteworthy clock, the "Cosmic Engine", was built by Su Song in 1092. It was around the size of Zhang's tower, but had an armillary sphere with which one could observe the positions of the stars. Su Song's tower eventually led to the development of mechanical clocks in Europe.^[9]

In addition to water clocks and clock towers, candle clocks were also used, as well as special incense clocks. Special incense sticks were slowly burnt, and the time could be told by the smell of the incense.^[5]

Sundials

A sundial is a device used to measure time by the position of the sun, and is currently the oldest known timekeeping device and the most ancient of scientific instruments. The most common and simple design, the horizontal sundial, consists of an upright right triangle with an angle the same as the latitude (referred to as a gnomon) that casts a shadow onto a horizontal table with markings placed at hourly intervals. However, sundials were often not designed to be round and were not to be found in open settings.^[10]

Sundials in the form of obelisks (3500 BCE) and shadow clocks (1500 BCE) were used in ancient Egypt.^[6][11] The sundial was further developed by other cultures, including the Greek, Chinese, Roman and Islamic cultures.^[7] Romans also built the largest sundial the world has known, the Solarium Augusti.^[12] Pliny the Elder records that the first sundial in Rome was looted from Catania, Sicily (264 BCE), and gave the incorrect time for a century until the markings and angle appropriate for the latitude of Rome were used (164 BCE).^[13]

Noontime was usually marked by the time of the shortest shadow on a sundial. This was used in Rome to determine when a court of law was open; lawyers had to be at the court by noon. An earlier Egyptian invention (c. 1500 BCE), also using a cast shadow to determine time, is similar in shape to a bent T-square, that measured the passage of time from the shadow cast by its crossbar on a non-linear rule. The T was oriented Eastward in the mornings. At noon, the device was turned around so that it could cast its shadow in the evening direction.^[13]

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.^[14]

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.^[15] 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.^[16] The French astronomer Oronce Finé constructed an ivory sundial, still in existence, in 1524.^[17] 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.^[18]

Water clocks

Water clocks were the most accurate timekeeping devices of the ancient world. They were able to measure time even at night, when sundials became impractical, though they required regular replenishing of water. These clocks later developed a common name of clepsydra (Κλεψύδρα Greek "water thief") by the Greeks, who used them since at least 235 BCE.^[19] The earliest uses of water clocks were in Egypt and China.^[5]

The oldest documentation of the water clock is the tomb inscription of the 16th century BCE Egyptian court official Amenemhet, and identifies him as its inventor.^[20] These clocks were bowls with small holes in the bottom which were placed floating on water and was allowed to fill with water at a near constant rate, with markings on the side to indicate time as the surface of the water reached them.^[6] One such clock was found in the tomb of pharaoh Amenhotep I (1525–1504 BCE), solid proof of ancient Egyptian use.^[19][21]

Historian Joseph Needham speculates that the introduction of the outflow clepsydra to China from perhaps Mesopotamia occurred as far back as the 2nd millennium BCE during the Shang Dynasty, and certainly by the 1st millennium BCE. By the beginning of the Han Dynasty in 202 BCE, the outflow clepsydra was gradually replaced by the inflow clepsydra that featured an indicator rod on a float.^[3] To compensate for the falling pressure head in the reservoir, which slowed timekeeping as the vessel filled, Zhang Heng (78–139 CE) added an extra tank between the reservoir and the inflow vessel.^[3] In about 550 CE, 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).^[3] Soon after, this design was trumped by two Sui Dynasty (581–618) inventors Geng Xun and Yuwen Kai, who in about 610 CE were the first to create the balance clepsydra with standard positions for the steelyard balance.^[3] 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.^[3]

Water clocks were used to great extent in Greece, introduced by Plato (424–348 BCE), who also invented a water-based alarm clock.^[22][23] 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 an increasing supply of water supplied by a cistern. Eventually the vessel would float high enough to tip over. The lead balls would then cascade onto a copper platter. The resultant clangor would then awaken his students at the Academy (378 BCE).^[13] Another account says that it used two jars and a siphon. Water emptied until it reached the siphon, which transported the water via the siphon to the other jar. Water rising in the other jar forced air through a whistle, sounding the alarm.Cite error: A <ref> tag is missing the closing </ref> (see the help page). 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.^[24] 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.^[25] It is possible, however, that their search for increased precision was not because of their interest in science, but rather to imitate the heavens, which formed the basis of their religion.^[26] No matter their reason, Greek water clocks greatly improved in accuracy around 325 BCE.^[25] They were adapted to have a face with an hour hand, making the reading of the clock more precise and facile.^[25]

Although water clocks were more practical than sundials, several problems arose with them. When the water clock was full, the weight of the water caused it to flow out of the holes faster than when it was nearly empty. To counteract this, water 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. The second problem was temperature: Water flows more slowly when it is cold. Sometimes the water in the clepsydra even froze. Also, the water clock did not account for the fact that the length of the days and nights changes throughout the year.^[27]

Waterclocks (and later, mechanical clocks) were used to mark the events of the abbeys and monasteries of the Middle Ages. Richard of Wallingford (1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical orrery about 1330.^[28][29] In particular, Arab engineers improved on the use of waterclocks up to the Middle Ages.^[13]

Hourglasses

The hourglass (commonly referred to as a sand timer or sand clock) is believed to have been invented by the Ancient Egyptians, and uses a vertical flow of grains of sand from one chamber of the glass to another to measure time.^[30] Ferdinand Magellan used eighteen hourglasses on each ship for his circumnavigation of the globe (1522).^[31] 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.^[32] 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.^[33]

A smaller version 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.^[34] It uses a special sand made from ground eggshells, which does not erode the hole as fast as ordinary sand.^[5]

Candle clocks

A candle clock is a thin candle with consistently spaced markings (usually numbered), that indicated the passage of time when burned.^[35] No longer used today, candle clocks provided an effective way to determine the passage of time indoors, at night, or on a cloudy day, when sundials lost their functionality.

It is unknown where and when candle clocks were first used. The earliest reference to their use occurs in a Chinese poem by You Jiangu, 520 CE. Here, the graduated candle supplied a means of determining time at night. Similar candles were used in Japan until the early 10th century CE.^[36] The most commonly mentioned candle clock is attributed to King Alfred the Great of England (878 CE). His device consisted of six candles made from 72 pennyweights of wax, each 12 inches (30 cm) high, 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.^[37]

Modern devices

Clocks

Clocks encompass a spectrum of devices, from wristwatches, to more exotic varieties such as the Clock of the Long Now. The English word "clock" is variously said to derive from Saxon clugga, Old North French cloque or Middle Dutch clocke, all of which mean bell.^[38][39] The passage of the hours at sea were marked by bells, and denoted the time (see ship's bells). The hours were marked by bells in the abbeys as well as at sea. They can be powered by a variety of means, including gravity, springs, electrical power,^[40] or a pendulum. The invention of mechanical clockwork itself is usually credited to Liang Lingzan, an Chinese government official, and Yi Xing, a Chinese monk.^[41]

Early mechanical clocks

The earliest Medieval clockmakers were Christian monks.^[42] Medieval religious institutions required clocks to measure and indicate the passing of time because, for many centuries, daily prayer and work schedules had to be strictly regulated; often this was done by a stationed clockkeeper. These clocks also often had to be wound twice a day, or more.^[43] This was done by various types of time-telling and recording devices, such as water clocks, sundials and marked candles, probably used in combination.^[43] 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 thus 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.^[44]

This was the Wells Cathedral clock,^[46] constructed circa 1396. The clock was converted to pendulum and anchor escapement in the 17th century. It was installed in the London's Science Museum in 1884, where it continues to operate.^[47] 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.^[48] Above the clock is a set of figures, which hit the bells, and a set of jousting knights who chase each other every 15 minutes.^[47] Similar astronomical clocks or "horologes" can be seen at Exeter, Ottery St Mary, and Wimborne Minster.

The oldest working clock in the world is Salisbury cathedral clock, which dates from about 1386,^[49] and most of the parts are original. It had no dial, as its purpose was to strike a bell at precise times.^[49] The wheels and gears are mounted in an open box-like iron frame about 1.2 meters (3.9 ft) square. The framework was held together with metal dowels and pegs. 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.

Other examples from the period were built in Milan (1335), Strasbourg (1354), Lund (1380), Rouen (1389) and Prague (1462). A clock had been mentioned before this allegorically, in Dante Alighieri's Paradiso.^[50] The earliest detailed description of clockwork was presented by Giovanni da Dondi, Professor of Astronomy at Padua, in his 1364 treatise Il Tractus Astarii.^[41]

One clock that has not survived is that of the Abbey of St Albans, built by the 14th century abbot, Richard of Wallingford. 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."^[44]

Pendulum Clock

Innovations continued, with miniaturization leading to domestic clocks in the 15th century, and personal watches in the 16th.^[41] In the 1580s, the Italian physicist Galileo Galilei investigated the regular swing of the pendulum, and discovered that it could be used to regulate a clock.^[51][52] The first such "Pendulum clock" was invented by Dutch scientist Christiaan Huygens in 1657.^[51] The Jesuits were also a major contributor to pendulum clocks in the 17th and 18th centuries,^[53] having had an "unusually keen appreciation of the importance of precision".^[54] 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."^[54] They served a crucial role in spreading and testing the scientific ideas of the period, and collaborated with contemporary scientists such as Huygens.^[53]

The modern Longcase clock, also known as the "Grandfather clock", traces its origins to the invention of the anchor escapement mechanism around 1670. Before that, pendulum clocks had used the older verge escapement mechanism, which required very wide pendulum swings of about 100°. Such mechanisms with long pendulums could not be fitted in a case, so most clocks had short pendulums. The anchor mechanism reduced the pendulum's swing to around 4° to 6°, allowing clockmakers to use longer pendulums, which had slower "beats". These needed less power to move, caused less friction and wear in the movement, and were more accurate. 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 which power the clock, gave rise to the design of the long narrow case.^[55]

In 1675, 18 years after inventing the pendulum clock, Huygens devised the spiral balance, which would eventually allow for greater accuracy and miniaturization of pocket watches.^[56] The English natural philosopher Robert Hooke had also invented a balance spring for regulating watches,^[56] and there is some dispute as to which of the two had originated the idea.

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.^[57] 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 3500 watches. The competition and scientific rivalry from his discoveries further encouraged researchers to seek new methods to more accurately measure time.^[58]

In Germany, Nuremburg 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 Hugenot 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.^[57]

Wristwatches

File:Relogio stDumont.jpg

In 1904, 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.^[59] The wristwatch had already been invented by Patek Philippe in 1868, but only as a "lady’s bracelet watch" (a jewel). Since pocket watches were not suitable, Louis Cartier created the Santos wristwatch, which was also the first wristwatch made for men and for practical use.

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 wrist watches, which they brought with them to their service. Artillery and infantry officers depended on these watches as battles became more complicated, because attacks needed to be coordinated at precise moments. 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 soldiers 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.

Pilot watches

The "Pilot's watch" was invented by Rolex in collaboration with Pan Am Airlines in the 1950s. Eventually these were issued to all pilots, first officers, and navigators on Pan Am's fleet of Boeing 707's. This watch was a modification of an existing Rolex "Turn-O-Graph" model with the addition of a hand that rotates once every 24 hours, and a rotating 24 hour bezel. This watch was first called the Rolex GMT, as it displays a total of three time zones, including the time indicated by the standard hands, Greenwich Mean Time (when properly set), and another timezone found by rotating the bezel to the appropriate offset from GMT.

Chronometers

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.^[60] 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 being 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 (COSC). 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.

Quartz oscillators

The piezoelectric properties of quartz were discovered by Jacques and Pierre Curie in 1880.^[61] 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.^[62][63] 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.^[63] 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.^[64]

In 1969, Seiko produced the world's first quartz wristwatch, the Astron.^[65] The inherent accuracy and low cost of production has resulted in the proliferation of quartz clocks and watches since that time. By the 1980s quartz technology had taken over applications such as kitchen timers, alarm clocks, bank vault time locks, and time fuses on munitions, from earlier mechanical balance wheel movements.

Atomic clocks

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. The atomic clock was invented in 1949, and is based largely on the absorption line in the ammonia molecule.^[66] 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. 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.^[67] The world's first atomic clock, created in 1949, is on display at the Smithsonian Institution, at Washington D.C.^[64]

Global Positioning System

Today, the Global Positioning System (GPS), in coordination with the network time protocol, can be used to synchronize timekeeping systems across the globe. GPS was developed by the US Department of Defense to provide all weather twenty-four hours a day navigation capabilities for military ground, sea, and air forces.^[68] 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.

As opposed to the year, month, and day format of the Gregorian calendar, the GPS date is expressed as a week number and a day-of-week number. The week number is transmitted as a ten-bit field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal. To address this concern, the modernized GPS navigation messages use a 13-bit field, which only repeats every 8192 weeks (157 years), and will not return to zero until near the year 2137.

References

Further reading

External links

• Eternalegypt.org — "Measuring time in ancient Egypt"
• Physics.nist.gov — "A walk through time"