History of timekeeping

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The origins of our current time measurement system, dates back to the Sumerian civilization of approximately 2000 BCE.^[1] The system produced then and still used today is known as the sexagesimal system which is based on the number 60. Numerous technologies have been developed to measure time in this system including devices measuring the path of the sun across the sky, small, intricate machines, and even water.

Modern calendars also have roots in the Sumerian civilization. Many ancient civilizations used the motion of the sun, moon, planets, and other stars to determine time. Ancient Sumerians used calendars which divided the year into 30-day segments, each day into 12 periods, and each of those periods into 30 parts. Ancient Egyptians originally developed a calendar system based on moon cycles, but realized that Sirius rose next to the sun every 365 days, coinciding with the flooding of the Nile River. The Ancient Mayans used not only the sun and moon, but also the planet Venus to develop 260- and 365-day calendars.^[2]

Contents 1 Early measurement devices 1.1 Sundials 1.2 Waterclocks 1.3 Hourglasses 1.4 Candle clocks 2 Modern devices 2.1 Clocks 2.2 Chronometers 2.3 Quartz oscillators 2.4 Atomic clocks 2.5 GPS 3 See also 4 References

[edit] Early measurement devices

[edit] Sundials

A sundial uses a gnomon to cast a shadow on a set of markings which are calibrated to the hour. The position of the shadow marked the hour in local time. Sundials in the form of obelisks (3500 BC) and shadow clocks (1500 BC) are known from ancient Egypt. It was further developed by other cultures, including the Greek (Greek hemyspherium),^[3] Chinese, Roman and Islamic cultures. Romans also built the largest sundial the world has known, the Solarium Augusti.^[4] Pliny the Elder records that the first sundial in Rome was looted from Catania, Sicily (264 BCE), which gave the incorrect time for a century, until the markings appropriate for the latitude of Rome were used (164 BCE).^[5]

Noontime was an event which could be marked by the time of the shortest shadow on a sundial. This was used in Rome to judge when a court of law was open; lawyers had to be at the court by that time. An earlier invention also using a cast shadow to determine time is an Egyptian device dating to c.1500 BCE, similar in shape to a bent T-square, which 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.^[6]

The mathematician and astronomer Theodosius of Bithynia (ca. 160 BC-ca. 100 BC) is said to have invented a universal sundial that could be used anywhere on Earth. Marcus Vitruvius Pollio the Roman author of De Architectura wrote on the mathematics of gnomons.^[7] The French astronomer Oronce Finé constructed a sundial of ivory in 1524. 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.

[edit] Waterclocks

The most accurate timekeeping devices of the ancient world were the waterclock or clepsydra, first found in Egypt. A waterclock was found in the tomb of pharaoh Amenhotep I (1525–1504 BCE). They could be used to measure the hours even at night, but required manual timekeeping to replenish the flow of water. Plato introduced the waterclock to Greece, ^[8] and invented a water-based alarm clock.^[9] 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).^[10] Another account says that it used two jars and a siphon. Water empties until it reaches the siphon, which transported the water via the siphon to the other jar. Water rising in the other jar forces air through a whistle, sounding the alarm. The Greeks and Chaldeans regularly maintained timekeeping records as an essential part of their astronomical observations. In particular, Arab engineers improved on the use of waterclocks up to the Middle Ages.^[11]

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.^[12][13]

[edit] Hourglasses

The hourglass (possibly invented by the Ancient Egyptians^[14]) uses the vertical flow of grains of sand from one chamber to another to measure the time. Ferdinand Magellan used 18 hourglasses on each ship for his circumnavigation of the globe (1522).^[15] 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. However, it is not until the 14th century that evidence of their existence was found, appearing in the painting Allegory of Good Government by Ambrogio Lorenzetti in 1338.^[16][17]

From the 15th century onwards they were being used in a wide range of applications at sea, in the church, in industry and in cookery. They were the first dependable, reusable and reasonably accurate measure of time.

[edit] Candle clocks

A candle clock is a thin candle with consistently spaced markings (usually with numbers), that when burned, indicate the passage of periods of time. While 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 became impractical.

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 (A.D. 520). Here, the graduated candle supplied a means of determining time at night. Similar candles were used in Japan until the early 10th century A.D. The most commonly mentioned candle clock is attributed to King Alfred the Great of England (c. 878 A.D.).^[18] His device consisted of six candles made from 72 pennyweights of wax, each being 12 inches high, of uniform thickness. At each inch, a mark was made, so there were twelve sections altogether. Each candle burned away completely in four hours, so each mark represented 20 minutes. The candles were placed for protection inside cases made of a wooden frame with transparent horn panels in the sides.

[edit] Modern devices

[edit] Clocks

Clocks can range from wristwatches, to more exotic varieties such as the Clock of the Long Now. The English word "clock" actually comes from French and Saxon words that mean bell.^[19] 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,^[20] or a pendulum.

The origins of the modern Longcase clock, also known as the Grandfather clock or Pendulum clock, are traced to the invention of the anchor escapement mechanism around 1670. Prior to that, pendulum clocks used the older verge escapement mechanism, which required very wide pendulum swings of about 100°. Such mechanisms with long pendulums could not be fitted within 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 metre (39 inches) long (to the centre 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.^[21]

[edit] 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. 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.

[edit] Quartz oscillators

The piezoelectric properties of quartz were discovered by Jacques and Pierre Curie in 1880. 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.^[22][23] 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.^[24] The National Bureau of Standards (now NIST) based the time standard of the United States on quartz clocks starting in the 1930s until the 1960s, when it changed to atomic clocks.^[25]

In 1969, Seiko produced the world's first quartz wristwatch, the Astron.^[26] 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 fuzes on munitions, from earlier mechanical balance wheel movements.

[edit] Atomic clocks

The most accurate type of timekeeping device is currently the atomic clock, which are accurate to seconds in many thousands of years, and are used to calibrate other clock and timekeeping instruments. Atomic clocks use the spin property of the caesium atom as its basis, and since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium. 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.

[edit] GPS

Today, the GPS global positioning systems in coordination with the network time protocol can be used to synchronize timekeeping systems across the globe. However, 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 Coordinated Universal Time (UTC), which GPS time was set to match UTC in 1980, but has since diverged due to the absence of corrections. This means that GPS time remains at a constant offset (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.

[edit] See also

• Time
• History of measurement
• Calendar
• Horology
• Clockmaker
• Watchmaker

[edit] References