A clock is any free-standing device or instrument for measuring or displaying the current time. The English word “clock” comes from the Celtic words clocca and clogan, both meaning “bell”. A chronometer is an exceptionally precise mechanical timepiece, designed to be accurate in all conditions of temperature, pressure, etc, especially one used at sea. Watches are sometimes distinguished from clocks in general, but really a watch is just a portable clock, usually worn in a pocket or on the wrist.
Really, a clock can be anything that repeats itself in a predictable way. The rotation of the Earth is a good example of something that is both repetitive and predictable, and indeed the turning of the Earth, as measured by the position of the Sun in the sky, was the first method mankind used to estimate the time of day. Other kinds of clock mechanisms, from the passage of water out of a vessel to the swinging of a pendulum to the vibration of a quartz crystal or the oscillation of a microscopic atom, are really just more manageable and more accurate variations on the same idea.
If the principle behind a clock is relatively simple, the technological and engineering challenges involved in building a clock of any accuracy are relatively complex, and much of the history of clocks in the last two millennia has revolved around the invention of more and more complex machines that are able to overcome the intrinsic problems of friction, temperature differences, movement, magnetic fields, size, etc, in search of the ultimate in accurate, reliable and practical timekeeping devices.
Division of Time
We use clocks to divide the day into smaller increments. Although a 10-hour clock was briefly popular during France’s experiment with metric time measurement after the French Revolution, the 12-hour clock – a convention dating back to ancient Egypt and Babylon – has continued to be the norm, along with 24-hour clocks for some (mainly military and astronomical) applications. The standard sexagesimal system of time measurement – 60 minutes in an hour, 60 seconds in a minute – also owes its ancestry to the sexagesimal system used in Sumer and Babylonia from around 2000BCE, in which 60 was the base number for most mathematical and counting purposes.
Our present system of dividing the day and night into 24 equal hours was instituted around the 14th Century. Prior to that, other than for some technical astronomical purposes, the day was usually split into 12 equal hours and the night into 12 equal hours, a practice dating back to ancient Egypt (although in practice, the night was often divided into 3 or 4 “watches” for security reasons). This meant of course that daytime hours were not necessarily of the same length as nighttime hours (except at the equinoxes), and hours at different times of the year would also vary in length. The advent of mechanical clocks institutionalized the use of 24 equal hours per day.
The first reference to clocks using 12 hours before noon and 12 hours after noon (all equal) comes from England in about 1380, although splitting the day into AM (ante meridiem, or before midday), and PM (post meridiem, or after midday) actually dates back to Roman days. The modern convention of starting the day at midnight, in the Roman style, is perhaps an arbitrary one, although a widely used one for all that, and the hour at which the day is considered to begin has varied throughout history and across different cultures. The ancient Greeks and Babylonians counted the day from sunset, as do orthodox Jews and Muslims even today; the Egyptians reckoned the hours from sunrise; in ancient Umbria the new day began at noon.
The clock is one of the oldest human inventions, and a bewildering array of different mechanisms have been employed over the millennia.
The sundial, or shadow-clock, which measures the time of day by means of a shadow cast by the Sun onto a cylindrical stone, was widely used in ancient times, and can give at least a reasonably accurate reading of the local solar time. In a typical horizontal sundial, the Sun casts the shadow of a gnomon (a thin vertical rod or shaft) onto a horizontal surface marked with lines indicating the hours of the day. Some sundials may also be set vertically on the sides of buildings, and a wide variety of different designs have been employed over the centuries. As the Sun moves across the sky, the edge of the shadow aligns with the different hour markings. Sundials are therefore only of use during daylight hours, and when sufficient sun is shining, although in the Middle Ages an instrument called a nocturnal was developed which measured the nighttime hours according to the positions of the North Star or some other combination of stars (although again only when atmospheric conditions allowed).
Obelisks may have been used for shadow timekeeping in ancient Egypt as early as 3500BCE, but the first purpose-built sundials date from around 1500BCE, also in Egypt, and in Babylonia not too much later. The ancient Greek, Roman and Chinese civilizations all used sundials extensively, mainly based on earlier Egyptian and Babylonian designs. The idea of using a gnomon parallel to the Earth’s axis so that the hour lines indicate equal hours on any day of the year was first developed in the Islamic Caliphate in the 14th Century, and began to be used throughout Europe and elsewhere thereafter to give a better and more accurate reading of the time regardless of the seasons. Sundials were still being widely used as late as the 19th Century. Our use of time measurement units like minutes and seconds, which were originally radial angle measurements in geometry, probably arises from this method of measuring the time of day.
The water clock, or clepsydra, is, along with the sundial, probably the oldest time-measuring instrument. A simple water clock measures time by measuring the regulated flow of water into or out of a vessel of some sort. Water clocks existed in ancient Egypt and Babylonia at least as early as 1600BCE, and possibly significantly earlier in India and China, although the documentation is vague. Early water clocks were probably not very accurate, and even differences in ambient temperatures could result in significant errors. The ancient Greeks and Romans are usually credited with improving their accuracy by the use of complex gearing and escapement mechanisms (which transfer rotational energy into intermittent motions) around the 3rd Century BCE. They passed these designs on to Byzantium and the Islamic world, and thence back to Europe, although the Chinese also independently made similar improvements to their water clocks by about the 8th Century CE. Water clocks were usually calibrated using a sundial and, while never reaching a level of accuracy comparable to today’s standards of timekeeping, they were the most accurate and commonly-used timekeeping devices for many centuries, until replaced by more accurate pendulum clocks in 17th Century Europe.
Candle clocks have also been used since antiquity, although it is difficult to establish any firm dates. A candle clock utilizes a slow-burning thin candle, with markings that indicate the passing of the hours as the candle burns down throughout the day (or night). A series of sticks of incense that burn down at a reasonably predictable speed were used in ancient Sparta, with each different smell denoting a particular hour of the day. In ancient China, the practice was to burn a knotted rope, noting the length of time required for the fire to travel from one knot to the next. Perhaps the most sophisticated candle clocks date from the Islamic Golden Age (12th – 13th Century).
The hourglass or sandglass, in which fine sand pours through a tiny hole at a constant rate, has long been used to indicate the passage of a predetermined period of time, even if it could not be used to tell the absolute time of day. Typically, an hourglass has two connected vertical glass bulbs, which allow a regulated trickle of sand (or sometimes crushed eggshell, which does not erode the glass as much) from the top to the bottom bulb. Once the top bulb is empty it can be inverted to begin timing again. Although the concept goes back to antiquity, the hourglass as we know it appears to have been invented in medieval Europe, and was in common use in the Middle Ages, particularly on board ships.
Mechanical clocks, governed by continually repeated mechanical (“clockwork”) motion, began to be developed independently in China, the Middle East and Europe during the early Middle Ages. They used a simple controlled release of power or escapement mechanism (a method of gradually and smoothly translating rotational energy into an oscillating motion that can be used to count time), as well as all manner of toothed wheels, ratchets, gears and oscillating levers. Early mechanical clocks were huge devices housed in church towers, and most early models still did not utilize a clock face or hands, but just struck the hour for religious and administrative purposes.
By the late 14th Century, the convention of a rotating hour hand on a fixed dial was adopted. Initially, accuracy was low, and errors of 15 minutes to an hour each day were common (minute hands were therefore not used). Gradually, though, these clocks began to acquire more and more extravagant features such as automata (moving figures) and complex astronomical depictions of the phases of the Moon, star maps, etc. The abbot of St. Alban’s abbey, Richard of Wallingford, built a famous mechanical clock as an astronomical orrery (a mechanical model of the solar system) as early as about 1330. Another famous astronomical clock was built in Strasbourg in 1352.
Spring-driven clocks began to appear in the 15th Century in Europe, and gradually new innovations were developed in order to keep the clock movement running at a constant rate as the spring ran down. Once wound, a spring-driven clock conserves energy by means of a gear train, with a balance wheel regulating the motive force. Around 1500, Peter Henlein, a locksmith in Nürnberg, Germany, began producing portable timepieces known popularly as ”Nürnberg eggs”. As accuracy increased (correct to within a minute a day), clocks began to appear with minute hands in the 16th Century, principally in Germany and France, one of the first being a 1577 clock made by Jost Burgi for the astronomer Tycho Brahe, who needed an accurate clock for his stargazing. However, minute hands only came into regular use around 1690. By the time of the scientific revolution, clocks and their workings had become miniaturized enough for sufficiently wealthy families to share a personal clock, or perhaps even a pocket watch.
In 1656, the Dutch scientist Christiaan Huygens developed the pendulum clock, following earlier ideas of Galileo, who had discovered the isochronism, or constant period, of a pendulum’s motion as early as 1583. The pendulum clock used a swinging bob to regulate the clock motion, achieving an accuracy of within 10 seconds per day. Such accuracy made the use of minute hands and even second hands a practical proposition. It had been known since 1644 that the period of a pendulum with a shaft length of precisely 0.994 metres (about 39.1 inches) had a period of precisely two seconds, one second for a swing in one direction and one second for the return swing. During the 17th Century, the centre of clock-making production and innovation moved to England and, in 1670, William Clement created the anchor escapement as an improvement on the old verge (or crown wheel) escapement, which had been used since the 14th Century, and he also encased the pendulum in the classic long-case or “grandfather” clock.
In 1675, Huygens and Robert Hooke made another crucial advance with the spiral balance, or hairspring, to control the oscillating speed of the balance wheel. Englishman Thomas Tompion also successfully used this mechanism in pocket watches or fob watches (a fob is a pocket designed the hold a watch, or the chain or ribbon that attaches it). In 1761, another Englishman, John Harrison, made various improvements which allowed accurate clocks to be used at sea, an instrument known as the marine chronometer, which provided an important boost for navigation (the measurement of longitude requires an accurate knowledge of time). Harrison received a handsome £20,000 reward (equivalent today to around $4.5 million) from the British government for his solution to the intractable problem of longitude. Second hands began to be commonly added to long-case clock dials around 1780, and jewelled bearings to reduce friction and prolong the life of clockworks were introduced in the 18th Century. The cuckoo clock (containing a carved wooden bird that emerges and “sings” to tell the time) made its first appearance in the Black Forest region of Germany in the mid-17th Century, although the classic “chalet-style” cuckoo clock originated in Switzerland in the late 19th Century.
The accuracy and reliability of pendulum and spring-driven clocks, and their increasing cheapness and ubiquity, marked a big shift in everyday life in much of the developed world, as people moved from telling the time by natural signs or events to measuring it by the clock, with all the repercussions this had for work practices, productivity, industrial development, etc. Pendulum clocks continued to be widely used in the 18th and 19th Century, and Switzerland gradually established itself as the pre-eminent clock-making centre during the 19th Century.
Alarm clocks have been around almost as long as clocks themselves. Some water clocks in classical times were adapted to strike an alarm, and some medieval mechanical clocks were also capable of chiming at a fixed time every day. Early user-settable mechanical alarm clocks, in which the alarm was set by placing a pin in the appropriate hole of a ring of holes in the clock dial, date back at least to 15th Century Europe. But the traditional mechanical wind-up alarm clock, that could be set for any time and was small enough to use on a bedside table, was patented by the American Seth E. Thomas in 1875.
Mass production of clocks with interchangeable parts began in the United States in the late 18th Century, and in 1836 the Pitkin brothers of Connecticut produced the first American-designed watch, and the first containing machine-made parts. New innovations and the economies of mass production, soon made the United States the leading clock-making country of the world, and competition reduced the price of a clock to $1 or less, so that for the first time most families could afford a clock. In 1884, at the International Meridian Conference, it was decided to place the Prime Meridian at Greenwich, England, establishing the international baseline of Greenwich Mean Time (GMT).
Electric clocks, which wind the mainspring using an electric motor, first arrived in 1840, patented by Scottish clockmaker Alexander Bain, and were further developed commercially by an American, Henry E. Warren, in the early 1900s. By the end of the 19th Century, the invention of the dry cell battery made electric clocks a practical proposition, and mechanical clocks gradually came to be largely powered by batteries, removing the need for daily winding. By the 1930s, electric clocks were the most widely-used type of clock.
Meanwhile, pocket watches began to be replaced by wristwatches after the First World War, when the success of wristwatches in military operations finally made them acceptable fashion accoutrements. The Swiss further developed their market dominance at this time through their mastery of the intricacies high quality wristwatches. The British Broadcasting Company began broadcasting its famous hourly time signal of six pips over the radio in 1924, allowing normal people to synchronize their watches with great accuracy.
The development of electronics in the early decades of the 20th Century led to electronic clocks with no clockwork parts at all, with the timekeeping regulated by methods as varied as the vibration of a tuning fork, the piezoelectric behaviour of quartz crystals, and even the quantum vibration of atoms of caesium or rubidium (all of which are however examples of oscillatory motion, the same general method as that employed by simple pendulum clocks).
The development of the quartz clock in the late 1920s finally made electronic clocks more accurate and reliable than pendulum clocks. Using vibrating quartz crystals as oscillators allowed the production of clocks that were accurate to a few ten-thousandths of a second per day. The advances in microelectronics in the 1960s, particularly in Japan, made quartz clocks both compact and cheap to produce, so that by the 1980s they had become the dominant timekeeping technology for both clocks and wristwatches. Even in the digital age, though, Switzerland has managed to retain its reputation for quality mechanical watches, although ironically more as prestige jewellery items and status symbols than as ultra-accurate timepieces.
Atomic clocks, which use the oscillation frequencies of the electromagnetic spectrum of atoms (principally caesium atoms) to regulate their timekeeping, are currently the most accurate clocks available (with an accuracy of around 10−9 seconds per day, or about 1 second in 316,000 years). Indeed, they keep time better and more consistently than the rotation of the Earth and the movement of the stars, and they have been used as primary standards for international time distribution services since the 1960s. A hydrogen maser clock at the US Naval Research Laboratory in Washington DC, which uses hydrogen atoms instead of caesium, is believed to be accurate to 1 second in 1.7 million years, currently the most accurate clock in use, although it is believed that super-cooled hydrogen maser clocks could reach accuracies approaching 1 second in 300 million years.
The electronic revolution of the 20th Century has also made possible digital clocks, which display a numeric representation of the time using LCD, LED or VCF displays, and analog clocks (with hands) have declined in popularity ever since. Digital clocks are now found on all computers, cellphones, etc, as well as on the electronic timers for central heating systems, ovens, VCRs, etc. Indeed, the youth of today are more likely to use their cellphone to tell the time than to wear a wristwatch. Many newer clocks and computer-based applications even reset themselves based on radio or internet time servers that are tuned to ultra-accurate atomic clocks. Auditory clocks, tactile clocks and Braille watches are also available for those with limited sight.
Satellite navigation systems like the Global Positioning System (GPS) require an unprecedentedly accurate knowledge of time. For example, a time error of just 1 microsecond can translate to a spatial error of about 300 metres. GPS signals are now provided directly from a network of 31 Earth-orbit satellites and linked to atomic clocks.
Atomic clocks and modern time standards (see the section on Time Standards) are able to keep incredibly precise time measurements consistent across the world. They are also able to account for the tiny and gradual changes needed to keep the time compatible with astronomical data. For example, the Earth’s motion is slightly perturbed by the gravitational attraction of the other planets, and there is a gradual shift in the orientation of Earth’s axis of rotation (precession), so that the length of the tropical year is slowly decreasing: at the end of the 19th Century, the tropical solar year was 365.242196 days; at the end of the 20th Century, it was 365.242190 days. Also, tidal friction is gradually slowing the Earth’s rotation, and lengthening the day by almost 2 milliseconds every century (so that, in a few hundred million years, a day will actually be 25 hours long), and large natural cataclysms like earthquakes and hurricanes can also have perceptible effects on the Earth’s rotation.
The Clock of the Long Now is a recent project to design and build a mechanical clock that will function and keep accurate time for at least 10,000 years, and that will tell the time in a way that would be intelligible to any future civilization. While not as accurate as an atomic clock, it a good example of the long-term view of the importance of timekeeping that we humans are now starting to take.