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(originally published to Helium writing site, now gone)

In prehistoric times, people used the sun to tell the time of day. When the sun rose in the morning, they knew it was time to rise and get on with their daily tasks. They knew that the middle of the day was when the sun was highest in the sky. As the sun sank towards the horizon, they knew it was time to return to their camp and prepare for nightfall. Accurate time keeping wasn’t necessary.

With the development of bureaucracies, religion and other activities, a need emerged to better organise time, and civilisations in the Middle East and North Africa started dividing the day into parts.

An obelisk, a type of sundial, was the first device to tell the time. These were made by the Egyptians as early as 3500 BCE and were tall, thin, tapering, four sided structures made from a single piece of stone. Often over 70 feet high and weighing many tons, their moving shadow formed a kind of clock, dividing the daylight hours into two parts either side of midday. These structures showed the year’s longest and shortest days, when the shadow at noon was shortest and longest respectively. Later examples included markings around the base to show further divisions of day. Twenty-six ancient Egyptian obelisks survive but they are scattered around the world, with 11 in Italy, eight in Egypt, three in England and one each in France, Israel, Turkey and the United States.

In about 1500 BCE, the Egyptians developed a more accurate type of sundial, the shadow clock. This was the first portable timepiece. It divided daylight hours into ten parts plus a twilight hour at each end of the day. An elevated crossbar cast a moving shadow over a long stem that had five spaced marks. But it had to be turned the other way at noon and was no good on overcast days. Various improvements were made over the centuries and by 30 BCE, Roman writer Vitruvius identified 13 different sundial types used in Greece, Asia Minor and Italy. The Romans had pocket sundials only a few centimetres in diameter.

The star clock or merkhet, developed by the Egyptians around 600 BCE, is the oldest known astronomical tool and was used to align the foundations of pyramids and temples with the compass points. It was made from the central rib of a palm leaf and used a string with a weight on the end to obtain a vertical line. Using the Pole Star, two of them could be used to determine a north-south line and thus night-time hours when other stars crossed the meridian.

Another Egyptian invention was the water clock. It was more accurate than the obelisk and perhaps the earliest timekeeping device that didn’t depend on using celestial bodies. One was found in the tomb of pharaoh Amenhotep I, who was buried around 1500 BCE. The Greeks used them from about 325 BCE, calling them clepsydras, meaning ‘water thieves’. They were stone vessels with sloping sides and a tiny hole in the bottom that allowed water to drip out at a constant rate. Others were cylindrical or bowl shaped, designed to slowly fill with water through a tiny hole, with markings on the inside measuring the number of hours it took. They could be used by day or night and were often set up alongside a sundial for use on gloomy days and at night. Similar bowls, made of metal, were still in use in North Africa in the 20th century.

Greek and Roman horologists and astronomers developed more elaborate, mechanised water clocks from 100 BCE to 500 CE. They regulated pressure, had bells and gongs (probably the first alarm clock), or had doors and windows that opened to show little figures of people, or moved pointers or dials. Some included astrological models of the universe.

Macedonian astronomer Andronikos built the Tower of the Winds in Athens in about 50 BCE, to show scholars and customers mechanical hour indicators and sundials. It had a 24 hour mechanised clepsydra and indicators for the eight winds, as well as showing the seasons and astrological information. The building still stands, near the Acropolis.

Astronomical and astrological clock making developed in China between 200 and 1300 CE. A clock tower built by Su Sung in 1088 CE shows the lengths people went to in order to tell the time. An elaborate contraption over thirty feet high, it had a water driven escapement, a power driven armillary sphere (a ball representing the earth and showing the poles, equator, meridians, parallels and apparent path of the sun) for observations, an automatically rotating celestial globe, and manikins that rang bells or gongs and held tablets showing the hour.

However, water flow rates are hard to control and a high degree of accuracy of time is not possible with water clocks. People looked for other ways. But in Europe there wasn’t much technological advancement in the Middle Ages, spanning about 500–1500 CE. Some people had simple sundials above doorways to show noon and the four ‘tides’ of time that governed the working day in the medieval period. Candle clocks were also used. Several types of pocket sundials were common from the 10th century. An English version took account of the sun’s altitude in different seasons.

Hourglasses were the first dependable and fairly accurate timepieces. Although a number of ancient civilisations had the technology to make them, such as the Egyptians who invented glassmaking around 1500 BCE, the first definite evidence was when one appeared in a painting by Italian Ambrogio Lorenzetti in 1328. They were used on ships from the 14th century and perhaps as earlier as the 11th century. Portuguese explorer Ferdinand Magellan had 18 hourglasses on each of his five ships for his voyage around the world from 1519. From the 15th century, they were used in churches, in industry and for cooking. They are still commonly used as eggtimers, in board games, on computer screens, and as shower-timers. The Australian Parliament uses one to time certain procedures.

Large mechanical clocks, with an hour hand only, began appearing in the towers of English and Italian cities as early as the 1270s. The clocks were weight-driven and had verge and foliot escapement mechanisms, usually involving weighted rope unwinding from the barrel, turning a toothed escape wheel. The oscillation period was hard to regulate as the mechanism depended on the amount of driving force and friction. Clocks could be out by plus or minus an hour a day.

The first portable clock came around 1500 CE when German locksmith Peter Henlein invented a spring-powered clock. This allowed for smaller clocks that people could put on a table or shelf rather than on the wall. Using the same technology, Henlein created the first pocket watch in 1524, although the first wristwatch wasn’t until the late 19th century. It is thought that Henry VIII wore a pocket watch on a chain around his neck. But the clocks and watches slowed as the mainspring unwound, making them too inaccurate to worry about a minute hand. The first clock with a minute hand was by Swiss clockmaker Jost Burgi in 1577 but it wasn’t precise.

Greater accuracy of clocks was achieved in the 17th century. Dutch scientist Christian Huygens built the first pendulum clock in 1656, based on a design by Galileo. The clock was accurate to within a minute a day and soon to less than 10 seconds. A pendulum clock’s escapement usually involves a weight or spring on the gear, forcing it to rotate. The gear pushes against an arm that is connected to the pendulum, making it move from side to side. Minute hands and then second hands were introduced in the 1670s, but the addition of these hands was gradual, and a hundred years later some town clocks still only had an hour hand.

Huygens also developed the balance wheel and spring assembly in 1675, allowing watches to be out by no more than 10 minutes a day. The mechanism is still used in some wristwatches. The accuracy of pendulum clocks improved to one second a day in 1721 thanks to English clockmaker George Graham when he found a way to compensate for changes in the length of a pendulum caused by temperature variation. Pendulum clocks remained the most accurate clock type through to about 1930.

In the 18th century, English carpenter and self-taught clockmaker John Harrison refined Graham’s method and also found new ways of reducing friction. He revolutionised sea travel by inventing a chronometer, a maritime clock that could accurately assess longitude, and show the time. On the strength of an offer of £20,000 by the British Government to the person who found a way of determining longitude to within half a degree and coming up with a clock accurate to two seconds a day on a long sea voyage, Graham spent thirty years of trial and error to perfect his chronometer. It easily met the criteria but he spent many more years fighting the government before he was finally paid.

Our pursuit of accuracy and perfection continued. Siegmund Riefler of Germany developed a clock in 1889 with an almost free pendulum and accurate to a hundredth of a second a day. The first free pendulum clock was invented around 1898 by R. J. Rudd. The pendulum swings freely for one minute without control by the escapement, while he used a subsidiary or ‘slave’ clock to send an impulse to the pendulum every minute and to keep time between impulses.

But it was W. H. Shortt’s free pendulum clock, first demonstrated in 1921, that replaced Riefler’s clock as the standard in astronomical observatories. The gravity arm or slave pendulum pushes the timekeeping or master pendulum to maintain its motion and drives the clock’s hands. Thus the timekeeping pendulum has no mechanical tasks to disturb its regularity. This clock remained the norm until the development of quartz crystal clocks in the 1930s and 1940s.

Crystals generate voltage when mechanical stress is applied, allowing crystal clocks to operate. A crystal changes shape if an electric field is applied to it. By squeezing or bending it, the crystal generates an electric field. The interaction between mechanical stress and electric field makes the crystal vibrate. This gives a constant frequency electric signal to operate an electronic clock. Canadian Warren Marrison of Bell Laboratories made the first quartz clock in 1927. The clocks are far more accurate than any previous clock as they have no gears or escapements to upset their regular frequency. However, they do rely on mechanical vibration, whose frequency depends on the size and shape of the crystal. Quartz clocks still dominate the market as they are reliable, accurate enough for most purposes, and cheap.

Most car clocks now have quartz movements. In earlier times, travellers used a range of portable timepieces, including pocket sundials and pocket watches. Pendulum clocks only work when they are stationary and are no use in vehicles because motion and a change in speed will affect the movement and pace of the pendulum. In the 1790s, Swiss watchmaker Abraham-Louis Breguet, living and working in France, made the first carriage clock, selling it to Napoleon. Placing pocket watches in leather holders and attaching them to a carriage’s front board was popular. Mechanical car clocks could be bought as an accessory by 1908 and electric clocks from the 1930s.

Atomic clocks are considerably more accurate again than quartz. The first one, based on ammonia, was built in 1949 by the US National Institute of Standards and Technology. In 1955, Louis Essen of the National Physical Laboratory in the United Kingdom came up with a caesium based atomic clock. NIST completed a caesium clock in 1957. With the high degree of accuracy of these clocks, it was decided in 1967 to base the definition of a second on atomic time rather than on the earth’s revolution around the sun, which had been the case from 1956. A second is thus defined as 9,192,631,770 cycles of the caesium atom’s resonant frequency. Claims as to the accuracy of atomic clocks include a billionth of a second per day and one second in six million years.

We have come to depend on very precise time. Gone are the days when people only needed to know the time to the nearest hour, minute or even second. Technology and industry need extremely accurate clocks. Demand continues to drive the search for ever greater accuracy of time. Global positioning systems and network time protocol are used to synchronise timekeeping systems around the world.

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