Clock

A clock is a mechanical or electrical device, other than a watch, for displaying time. It is a machine in which a device that performs regular movements in equal intervals of time is linked to a counting mechanism that records the number of movements.

The first device for indicating the time of day was probably the gnomon, dating from about 3500 BCE, a vertical stick or pillar whose shadow length gave an indication of the time of day. The oldest known sundial is a simple L-shaped piece of stone from the reign of Thutmosis III, who ruled Egypt around 1500 BCE; at dawn it would have been aligned towards the rising Sun so that a bar on the short arm cast a shadow on the first hour line, while at noon the sundial was reversed so the same six hour lines counted out the afternoon hours. Obelisks, the slender tapering four-sided monuments built as early as 3500 BCE, cast moving shadows that acted as a kind of sundial, letting people partition the day into morning and afternoon and showing the year's longest and shortest days at noon. Another Egyptian shadow clock or sundial, possibly the first portable timepiece, came into use around 1500 BCE and divided a sunlit day into 10 parts plus two twilight hours in the morning and evening.

The water clock, or klepsydra, probably developed because the sundial could not work when there was no sun and could not maintain a constant division of time. The first and simplest water clocks were small bowls with a small opening, set in a basin of water so that they sank as water leaked in, and an attendant announced when each interval had passed and reset the bowl to sink again. This kind of clock was used above all for timing speeches in law courts, most notably in Athens, where judicial water clocks were in common usage around 430 B.C.. Vitruvius attributes more elaborate water clocks to Ctesibius of Alexandria, who used an aperture made of gold or a perforated gem to admit water evenly; the water raised an inverted bowl, or phellos, whose bar of teeth engaged a revolving drum. Ctesibius made a perforation in a piece of gold or a smooth gem, because these materials are not worn by the action of the water nor collect filth that might obstruct its passage. The water flowing equably through the hole raises an inverted bowl called by the workmen the phellos, or tympanum, to which are connected a rule and revolving drum wheels with perfectly equal teeth that, acting on one another, produce revolutions and measured motion.

Greek and Roman horologists and astronomers developed more elaborate mechanized water clocks between 100 BCE and 500 CE, regulating the pressure to keep the flow more constant and adding displays such as bells and gongs, moving figures, and astrological models of the universe. The Greeks constructed sundials of considerable complexity; the octagonal Tower of the Winds in Athens, dating from about 100 BCE, contains eight planar sundials facing various cardinal points of the compass. The single greatest literary source for the sundials of Greece and Rome is Vitruvius's Ten Books on Architecture, whose ninth book lists dials such as the semicircular one attributed to Berosus the Chaldaean and the hemisphere, or scaphê, credited to Aristarchus of Samos. The curves traced out by the tip of a gnomon's shadow on the dial may have led to the discovery of the conic sections attributed to Menaechmus, since the shadow path is the intersection of the dial plane with a cone whose vertex is the gnomon's tip. The first official timepiece of Rome was a sundial captured in a war on Sicily in 264 B.C., and although it differed by about four degrees of latitude it served the city for almost one hundred years.

Vitruvius treats the rules of dialling, which are deducible from the shadow produced by the rays of the sun from a gnomon, and explains in what proportions that shadow is lengthened and shortened. The equinoctial gnomons are of different lengths at Athens, Alexandria, Rome and Piacenza, and hence the construction of dials varies according to the places in which they are to be erected; from the size of the equinoctial shadow analemmata are formed, by means of which the shadows of gnomons and the lines which mark the hours are adjusted to the situation of the place. An analemma is a rule deduced from the sun's course and founded on observation of the increase of the shadow from the winter solstice, by means of which, with mechanical operations and the use of compasses, an accurate knowledge of the true shape of the world is reached. At the equinoxes, when the sun is in Aries or Libra, it casts a shadow at Rome equal to eight ninths of the length of the gnomon, at Athens three fourths, at Rhodes five sevenths, at Tarentum nine elevenths and at Alexandria three fifths; elsewhere the shadow of the gnomon at the equinoxes naturally differs. Lines may be projected on the analemma according to the place, as winter lines, summer lines, equinoctial lines or lines of the months, and by this ingenious method as many varieties and species of dials as can be desired may be constructed; in these figures the equinoctial as well as the solstitial days are always divided into twelve equal parts.

A clock needs a regular, constant or repetitive process to mark off equal increments of time, and early examples include the sun's movement across the sky, candles marked in increments, oil lamps with marked reservoirs, sand glasses, knotted cords, and incense mazes burning at a certain pace. Ice-age hunters in Europe more than 20,000 years ago scratched lines and gouged holes in sticks and bones, possibly counting the days between phases of the moon. Five thousand years ago, Sumerians in the Tigris-Euphrates valley in today's Iraq used a calendar that divided the year into 30 day months, the day into 12 periods each corresponding to 2 of our hours, and each period into 30 parts. There are no written records of Stonehenge, built over 4000 years ago in England, but its alignments show that its purposes apparently included determining seasonal and celestial events such as lunar eclipses and solstices. The earliest Egyptian calendar was based on the moon's cycles, but the Egyptians later realized that Sirius, the Dog Star in Canis Major, rose next to the sun every 365 days, and they devised a 365 day calendar that seems to have begun around 3100 BCE.

The early tower clocks were large, iron-framed structures driven by falling weights attached to a cord wrapped around a drum and regulated by a mechanism known as a verge, or crown wheel, escapement, and their errors probably were as large as a half hour per day. The oldest surviving clock in England is that at Salisbury Cathedral, which dates from 1386; a clock erected at Rouen, France, in 1389 is still extant, and one built for Wells Cathedral is preserved in the Science Museum in London. Small clocks driven by a spring began to be made around 1450 by clockmakers working probably in southern Germany or northern Italy, and these were the first portable timepieces.

The anchor escapement, an improvement invented in England in the 17th century, works with a pendulum and allows much smaller arcs of swing than the verge escapement; its pallets are shaped like an inverted anchor and lie in the same plane as the wheel. The length of a pendulum with a period of one second is about 39 inches, and an increase in length of 0.001 inch will make the clock lose about one second per day.

The pendulum's characteristic timekeeping property was noticed as early as about 1581 by Galileo, but it was Christiaan Huygens, the Dutch astronomer and physicist, who made the pendulum a practical controller of clocks from 1656 onward, an invention that greatly increased the importance and extent of clock making. Huygens was a Dutch mathematician, astronomer and physicist who founded the wave theory of light, discovered the true shape of Saturn's rings, and made original contributions to dynamics. He based his explanations of reflection and refraction solely on the principle of secondary wave fronts, which is known as Huygens' principle.

Huygens published his Horologium Oscillatorium in 1673; that work contained a theory of the mathematics of curvatures and complete solutions to problems of dynamics, including the derivation of the formula for the time of oscillation of the simple pendulum. The same work proves that the cycloid is tautochronous, a result of great theoretical importance but of little practical application to the pendulum.

The first pendulum clock, regulated by a mechanism with a natural period of oscillation, kept time with an error of less than one minute a day, the first time such accuracy had been achieved. Around 1675 Huygens developed the balance wheel and spring assembly, still found in some of today's wristwatches, which let portable 17th-century watches keep time to 10 minutes a day. The Shortt clock, demonstrated in 1921, contained a slave pendulum that drove the hands and gave a master pendulum the gentle pushes to maintain its motion, letting the master remain free from mechanical tasks that would disturb its regularity, and it almost immediately replaced Riefler's clock as a supreme timekeeper in many observatories.

Quartz crystal oscillators and clocks, developed in the 1920s and onward, operate on the piezoelectric property of quartz crystals, whose shape changes when an electric field is applied, improving timekeeping far beyond pendulum and balance-wheel escapements. Quartz clocks and watches continue to dominate the market in numbers because their performance is excellent for their price, even though their timekeeping has been substantially surpassed by atomic clocks.

Several unfortunate disasters at sea, caused ostensibly by poor navigation, prompted the British government to create a Board of Longitude, which was empowered to award a prize of GBP 20,000 to the first person who developed a chronometer that could calculate longitude within half a degree at the end of a voyage to the West Indies; Harrison completed his first chronometer in 1735 and submitted it for the prize. John Harrison was a carpenter by trade who was self-taught in clock making, and his precision longcase clocks of the mid-1720s achieved an accuracy of one second in a month. To solve the problem of longitude he set out to build a portable clock that kept time to within three seconds a day.

Until the mid-19th century, towns relied on local time set by the Sun, and the time in London was ahead of that of any town further west. A railway network whose trains could cross the country within a day, however, needed standard time to ensure passengers departed on schedule and to avoid collisions. The Royal Observatory at Greenwich provided the standard for London time, counting noon from the Sun's zenith over the 0° meridian, and in 1852 its timekeepers introduced equipment that transmitted accurate time signals across the country over the electric telegraph network, so that by 1855 nearly all public authorities set their clocks to railway time.

Representatives from different nations gathered in Washington D.C. in 1884 to choose one single meridian as a common reference, and after weeks of discussion voted to make the Greenwich Meridian the world's Prime Meridian. Greenwich was chosen as the Prime Meridian because it offered the least disruption, since nearly two-thirds of the world's ships were already using charts based on the Greenwich Meridian. Conference delegates also recommended a new global time zone system based on Greenwich Mean Time.

Every chemical element and compound absorbs and emits electromagnetic radiation at its own characteristic frequencies, and because such resonances are inherently stable over time and space, atoms offer a potential pendulum whose rate can be reproduced, promising clocks of greater accuracy. Research aimed at developing an atomic clock focused first on microwave resonances in the ammonia molecule; NIST built the first atomic clock based on ammonia in 1949, but its performance was not much better than the existing standards, so attention shifted almost immediately to cesium atomic-beam devices. In 1967 the cesium atom's natural frequency was formally recognized as the new international unit of time, redefining the second as exactly 9,192,631,770 oscillations of the cesium atom's resonant frequency and replacing the old second that was defined in terms of the Earth's motions.

Fountain clocks such as NIST-F3 and NIST-F4 use a fountain-like movement of atoms to calibrate the offset of a microwave frequency from the unperturbed cesium clock transition frequency that defines the SI second. Six infrared laser beams crossing at right angles slow a cloud of cesium atoms to temperatures near absolute zero, and two vertical lasers then toss that cloud upward about a meter high through a microwave-filled cavity.

• about 3500 BCE
  Gnomon
  The gnomon, a vertical stick or pillar, is among the earliest devices used to track the Sun's shadow.
• 3500 BCE
  Obelisks
  Obelisks served as monumental shadow-casting markers in ancient Egypt.
• about 3100 BCE
  Egyptian 365-day calendar
  The Egyptians devised a 365-day calendar after observing that Sirius rose next to the sun every 365 days.
• around 1500 BCE
  Egyptian shadow clock and sundial of Thutmosis III
  A portable Egyptian shadow clock divided a sunlit day into 10 parts plus two twilight hours; the oldest known sundial, an L-shaped stone from the reign of Thutmosis III, used a bar's shadow to mark six hour lines, reversed at noon to count the afternoon.
• 264 BCE
  Rome's captured sundial
  Rome acquired a sundial as war spoils, marking the city's first known use of such an instrument.
• about 100 BCE
  Tower of the Winds
  The Tower of the Winds in Athens was built as a combined sundial and water clock.
• around 430 B.C.
  Athenian judicial water clocks
  Athenian courts used water clocks to time legal proceedings.
• late 13th century
  First mechanical clocks
  The earliest mechanical clocks appeared in Europe, marking a shift from shadow and water-based timekeeping.
• 1386
  Salisbury Cathedral clock
  The clock at Salisbury Cathedral is among the oldest surviving mechanical clocks.
• 1389
  Rouen clock
  The Rouen clock was installed, adding to the growing tradition of public mechanical timepieces.
• around 1450
  Spring-driven portable clocks
  The introduction of spring-driven mechanisms made portable clocks possible.
• 1656
  Huygens's first pendulum clock
  Christiaan Huygens built the first pendulum clock, greatly improving timekeeping accuracy.
• 1673
  Horologium Oscillatorium
  Huygens published his work on pendulum clocks, detailing the mathematics of their motion.
• 1714
  Longitude Act
  The British Parliament passed the Longitude Act, offering a reward for a practical method of determining longitude at sea.
• 1728
  Harrison's interest in an accurate chronometer
  John Harrison began pursuing the development of a precise marine chronometer.
• 1735
  Harrison's first chronometer
  Harrison completed his first chronometer, a significant step toward solving the longitude problem.
• 1749
  Copley Medal
  Harrison was awarded the Copley Medal for his work on timekeeping.
• 1884
  Prime Meridian vote
  An international conference voted to adopt the Greenwich meridian as the prime meridian.
• 1921
  Shortt clock
  The Shortt clock, a highly accurate pendulum clock, was introduced.
• 1949
  First atomic clock
  The first atomic clock was built, using the vibrations of atoms to measure time.
• 1967
  Redefinition of the second
  The second was redefined in terms of the vibration of the cesium atom.
• 1972
  UTC effective
  Coordinated Universal Time (UTC) became the international standard for timekeeping.