HISTORY OF TIMEKEEPING
Early timekeeping devices
Many ancient
civilizations observed astronomical bodies, often the Sun and Moon, to determine times,
dates, and seasons. Methods of sexagesimal
timekeeping, now common in Western society, first originated nearly
4,000 years ago in Mesopotamia and Egypt; a similar system
was developed later in Mesoamerica. The first calendars
may have been created during the last glacial period, by hunter-gatherers
who employed tools such as sticks and bones to track the phases of the moon or the seasons. Stone circles,
such as England's Stonehenge, were built in various parts of the world,
especially in Prehistoric Europe, and are thought to have
been used to time and predict seasonal and annual events such as equinoxes
or solstices.
As those megalithic
civilizations left no recorded history, little is known of their
calendars or timekeeping methods.
Sundials have their origin in shadow clocks,
which were the first devices used for measuring the parts of a day. The oldest
known shadow clock is from Egypt,
and was made from green schist. Ancient Egyptian obelisks, constructed about
3500 BC, are also among the earliest shadow clocks.
Egyptian shadow clocks divided daytime into 10 parts, with an additional four
"twilight
hours" two in the morning, and two in the evening. One type of shadow
clock consisted of a long stem with five variable marks and an elevated
crossbar which cast a shadow over those marks. It was positioned eastward in
the morning, and was turned west at noon.
Obelisks functioned in much the same manner:
the shadow cast on the markers around it allowed the Egyptians to calculate the
time. The obelisk also indicated whether it was morning or afternoon, as well
as the summer and winter
solstices.
A third shadow clock, developed c. 1500 BC, was similar in shape to a bent T-square.
It measured the passage of time by the shadow cast by its crossbar on a
non-linear rule. The T was oriented eastward in the mornings, and turned around
at noon,
so that it could cast its shadow in the opposite direction.
Although
accurate, shadow clocks relied on the sun, and so were useless at night and in
cloudy weather. The Egyptians therefore developed a number of alternative
timekeeping instruments, including water clocks, hourglasses, and a system for
tracking star movements. The oldest description of a water clock is from the
tomb inscription of the 16th-century BC Egyptian court official Amenemhet,
identifying him as its inventor. There were several types of water clocks, some
more elaborate than others. One type consisted of a bowl with small holes in
its bottom, which was floated on water and allowed to fill at a near-constant
rate; markings on the side of the bowl indicated elapsed time, as the surface
of the water reached them. The oldest-known waterclock was found in the tomb of
pharaoh
Amenhotep I
(1525–1504 BC), suggesting that they were first used in ancient Egypt
Water clocks,
or clepsydrae, were commonly used in Ancient
Greece following their introduction by Plato, who also invented a
water-based alarm clock. One account of Plato's alarm clock
describes it as depending on the nightly overflow of a vessel containing lead
balls, which floated in a columnar vat. The vat held a steadily increasing
amount of water, supplied by a cistern. By morning, the vessel would have
floated high enough to tip over, causing the lead balls to cascade onto a
copper platter. The resultant clangor would then awaken Plato's students at the
Academy.
Another possibility is that it comprised two jars, connected by a siphon. Water
emptied until it reached the siphon, which transported the water to the other
jar. There, the rising water would force air through a whistle, sounding an
alarm. The Greeks and Chaldeans regularly maintained timekeeping
records as an essential part of their astronomical observations.
Greek
astronomer, Andronicus of Cyrrhus, supervised the
construction of the Tower of the Winds in Athens
In Greek
tradition, clepsydrae were used in court; later, the Romans
adopted this practice, as well. There are several mentions of this in
historical records and literature of the era; for example, in Theaetetus, Plato says that
"Those men, on the other hand, always speak in haste, for the flowing
water urges them on". Another mention occurs in Lucius
Apuleius': "The Clerk of the Court began bawling again, this
time summoning the chief witness for the prosecution to appear. Up stepped an
old man, whom I did not know. He was invited to speak for as long as there was
water in the clock; this was a hollow globe into which water was poured through
a funnel in the neck, and from which it gradually escaped through fine
perforations at the base". The clock in Apuleius' account was one of
several types of water clock used. Another consisted of a bowl with a hole in
its centre, which was floated on water. Time was kept by observing how long the
bowl took to fill with water.
Although
clepsydrae were more useful than sundials 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. Although still
not as accurate as sundials, Greek water clocks became more accurate around
325 BC, and they were adapted to have a face with an hour hand, making the
reading of the clock more precise and convenient. One of the more common
problems in most types of clepsydrae was caused by water
pressure: when the container holding the water was full, the
increased pressure caused the water to flow more rapidly. This problem was
addressed by Greek and Roman horologists beginning in 100 BC, and improvements
continued to be made in the following centuries. To counteract the increased
water flow, the clock's water containers usually bowls or jugs were given a
conical shape; positioned with the wide end up, a greater amount of water had
to flow out in order to drop the same distance as when the water was lower in
the cone. Along with this improvement, clocks were constructed more elegantly
in this period, with hours marked by gongs, doors opening to miniature
figurines, bells, or moving mechanisms. There were some remaining problems,
however, which were never solved, such as the effect of temperature. Water
flows more slowly when cold, or may even freeze.
Although the
Greeks and Romans did much to advance water clock technology, they still
continued to use shadow clocks. The mathematician and astronomer Theodosius of Bithynia, for example, is
said to have invented a universal sundial that was accurate anywhere on Earth,
though little is known about it. Others wrote of the sundial in the mathematics
and literature of the period. Marcus Vitruvius Pollio, the Roman author
of De Architectura, wrote
on the mathematics of gnomons, or sundial blades. During the reign of Emperor
Augustus, the Romans constructed the largest sundial ever built, the
Solarium
Augusti. Its gnomon was an obelisk from Heliopolis. Similarly, the obelisk from Campus
Martius was used as the gnomon for Augustus' zodiacal sundial. Pliny the
Elder records that the first sundial in Rome
arrived in 264 BC, looted from Catania,
Sicily;
according to him, it gave the incorrect time until the markings and angle
appropriate for Rome
Joseph
Needham speculated that the introduction of the outflow clepsydra to
China , perhaps from Mesopotamia,
occurred as far back as the 2nd millennium BC, during the Shang Dynasty,
and at the latest by the 1st millennium BC. By the beginning of the Han Dynasty,
in 202 BC, the outflow clepsydra was gradually replaced by the inflow
clepsydra, which featured an indicator rod on a float. To compensate for the
falling pressure head in the reservoir, which slowed
timekeeping as the vessel filled, Zhang Heng
added an extra tank between the reservoir and the inflow vessel. Around 550 AD,
Yin Gui was the first in China
... [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.
Between 270 BC
and 500 AD,
Hellenistic (Ctesibius,
Hero of Alexandria, Archimedes)
and Roman
horologists
and astronomers
were developing more elaborate mechanized water clocks. The added complexity
was aimed at regulating the flow and at providing fancier displays of the
passage of time. For example, some water clocks rang bells
and gongs,
while others opened doors and windows to show figurines of people, or moved
pointers, and dials. Some even displayed astrological
models of the universe.
Some of the most
elaborate water clocks were designed by Muslim engineers. In
particular, the water clocks by Al-Jazari in 1206 are credited for going "well beyond
anything" that had preceded them. In his treatise, he describes one of his
water clocks, the elephant clock. The clock recorded the passage
of temporal hours, which meant that the rate of flow had to be changed daily to
match the uneven length of days throughout the year. To accomplish this, the
clock had two tanks: the top tank was connected to the time indicating
mechanisms and the bottom was connected to the flow control regulator. At daybreak the
tap was opened and water flowed from the top tank to the bottom tank via a
float regulator that maintained a constant pressure in the receiving tank.
It is not known
specifically where and when candle clocks were first used; however, their
earliest mention comes from a Chinese poem, written in 520 by You Jianfu.
According to the poem, the graduated candle was a means of determining time at
night. Similar candles were used in Japan
The candle clock
most commonly mentioned and written of is attributed to King Alfred the
Great. It consisted of six candles made from 72 pennyweights
of wax, each 12 inches (30 cm) high, and of uniform thickness, marked
every inch (2.5 cm). As these candles burned for about four hours, each
mark represented 20 minutes. Once lit, the candles were placed in wooden
framed glass boxes.
The most
sophisticated candle clocks of their time were those of Al-Jazari
in 1206. One of his candle clocks included a dial to display the time and, for the first
time, employed a bayonet fitting, a fastening
mechanism still used in modern times. Donald Routledge Hill described
Al-Jazari's candle clocks as follows:
The candle,
whose rate of burning was known, bore against the underside of the cap, and its
wick passed through the hole. Wax collected in the indentation and could be
removed periodically so that it did not interfere with steady burning. The
bottom of the candle rested in a shallow dish that had a ring on its side
connected through pulleys to a counterweight. As the candle burned away, the
weight pushed it upward at a constant speed. The automata were operated from
the dish at the bottom of the candle. No other candle clocks of this
sophistication are known.
A
variation on this theme were oil-lamp
clocks. These early timekeeping devices consisted of a graduated
glass reservoir to hold oil usually
whale oil, which burned cleanly and evenly supplying the fuel for a built-in lamp. As the
level in the reservoir dropped, it provided a rough measure of the passage of
time.
In addition to
water, mechanical, and candle clocks, incense
clocks were used in the Far East,
and were fashioned in several different forms. Incense
clocks were first used in China
around the 6th century; in Japan
Several types of
incense clock have been found, the most common forms include the incense stick
and incense seal. An incense stick clock was an incense stick with
calibrations; most were elaborate, sometimes having threads, with weights
attached, at even intervals. The weights would drop onto a platter or gong
below, signifying that a certain amount of time had elapsed. Some incense
clocks were held in elegant trays; open-bottomed trays were also used, to allow
the weights to be used together with the decorative tray. Sticks of incense
with different scents were also used, so that the hours were marked by a change
in fragrance. The incense sticks could be straight or spiraled; the spiraled
ones were longer, and were therefore intended for long periods of use, and
often hung from the roofs of homes and temples.
In Japan China ,
but were produced in fewer numbers in Japan
While early
incense seals were made of wood or stone, the Chinese gradually introduced
disks made of metal, most likely beginning during the Song dynasty.
This allowed craftsmen to more easily create both large and small seals, as
well as design and decorate them more aesthetically. Another advantage was the
ability to vary the paths of the grooves, to allow for the changing length of
the days in the year. As smaller seals became more readily available, the
clocks grew in popularity among the Chinese, and were often given as gifts.
Incense seal clocks are often sought by modern-day clock collectors; however,
few remain that have not already been purchased or been placed on display at
museums or temples.
The earliest
instance of a liquid-driven escapement was described by the Greek
engineer Philo of Byzantium (fl. 3rd century BC) in his
technical treatise Pneumatics (chapter 31) where he likens the escapement
mechanism of a washstand automaton with those as employed in (water) clocks.
Another early
clock wich used escapements was built in Chang'an,
by Tantric
monk and mathematician, Yi Xing, and government official Liang Lingzan.
An astronomical instrument that served as a clock, it was discussed in a
contemporary text as follows:
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 Yi Xing's
clock was a water clock, it was affected by temperature
variations. That problem was solved in 976 by Zhang Sixun
by replacing the water with mercury,
which remains liquid down to −39 °C (−38 °F). Zhang implemented the
changes into his clock tower, which was about 10 metres
(33 ft) tall, with escapements to keep the clock turning and bells to
signal every quarter-hour. Another noteworthy clock, the elaborate Cosmic
Engine, was built by Su Song, in 1088. It was about the size of Zhang's tower, but
had an automatically rotating armillary
sphere also called a celestial globe from which the positions of the
stars could be observed. It also featured five panels with mannequins
ringing gongs or bells, and tablets showing the time of day, or other special
times. Furthermore, it featured the first known endless power-transmitting chain drive
in horology. Originally built in the capital of Kaifeng,
it was dismantled by the Jin army and sent to the capital of Yanjing
(now Beijing),
where they were unable to put it back together. As a result, Su Song's son Su
Xie was ordered to build a replica.
The clock towers
built by Zhang Sixun and Su Song, in the 10th and 11th centuries, respectively,
were also the first to incorporate a striking
clock mechanism, the use of clock jacks to sound the hours. The
earliest striking clock outside of China
was the clock tower near the Umayyad Mosque
in Damascus,
Syria,
which struck once every hour. It was constructed by the Arab engineer al-Kaysarani in 1154.
The first geared clock was invented
by the 11th-century Arab
engineer Ibn Khalaf al-Muradi in Islamic
Iberia; it was a water clock that employed both segmental and epicyclic
gearing. Other monumental water clocks constructed by medieval
Muslim engineers also employed complex gear trains
and arrays of automata.
Like the earlier Greeks and Chinese, Arab engineers at the time also developed
a liquid-driven escapement mechanism which they employed in some of their
water clocks. Heavy floats were used as weights and a constant-head system was
used as an escapement
mechanism, which was present in in the hydraulic controls they used to make
heavy floats descend at a slow and steady rate.
A mercury clock,
described in the Libros del saber de Astronomia, a Spanish
work from 1277 consisting of translations and paraphrases of Arabic works, is
sometimes quoted as evidence for Muslim knowledge of a mechanical clock.
However, the device was actually a compartmented cylindrical water clock, which
the Jewish
author of the relevant section, Rabbi Isaac, constructed using principles described by a
philosopher named "Iran", identified with Heron of Alexandria (fl. 1st century AD), on
how heavy objects may be lifted.
During the 11th
century in the Song Dynasty, the Chinese
astronomer, horologist and mechanical engineer Su Song
created a water-driven astronomical clock for his clock tower of Kaifeng
City. It incorporated an escapement mechanism as well as the earliest known
endless power-transmitting chain drive, which drove the armillary
sphere.
Contemporary Muslim
astronomers also constructed a variety of highly accurate
astronomical clocks for use in their mosques and observatories,
such as the water-powered astronomical clock by Al-Jazari
in 1206, and the astrolabic clock by Ibn al-Shatir
in the early 14th century. The most sophisticated timekeeping astrolabes were
the geared
astrolabe mechanisms designed by Abū Rayhān Bīrūnī in the 11th century and by
Muhammad ibn Abi Bakr in the 13th century. These devices functioned as
timekeeping devices and also as calenders.
The most
sophisticated water-powered astronomical clock was Al-Jazari's
castle clock,
considered to be an early example of a programmable analog
computer, in 1206. It was a complex device that was about 11 feet
high, and had multiple functions alongside timekeeping. It included a display
of the zodiac
and the solar and lunar orbits,
and a pointer in the shape of the crescent moon
which travelled across the top of a gateway, moved by a hidden cart and causing automatic
doors to open, each revealing a mannequin,
every hour.
It was possible to re-program the length of day and night everyday in order to
account for the changing lengths of day and night throughout the year, and it
also featured five robotic
musicians who automatically play music when moved by levers operated by a
hidden camshaft
attached to a water wheel. Other components of the castle
clock included a main reservoir with a float, a float chamber
and flow regulator, plate and valve trough, two pulleys, crescent
disc displaying the zodiac, and two falcon automata
dropping balls into vases.
Modern devices
Sundials were further developed by Muslim
astronomers. As the ancient dials were nodus-based with straight
hour-lines, they indicated unequal hours also called temporary hours that
varied with the seasons. Every day was divided into 12 equal segments
regardless of the time of year; thus, hours were shorter in winter and longer
in summer. The idea of using hours of equal length throughout the year was the
innovation of Abu'l-Hasan Ibn al-Shatir in 1371, based on earlier
developments in trigonometry by Muhammad ibn Jābir al-Harrānī
al-Battānī (Albategni). Ibn al-Shatir was aware that "using a gnomon that is
parallel to the Earth's axis will produce sundials whose hour lines indicate
equal hours on any day of the year". His sundial is the oldest polar-axis
sundial still in existence. The concept appeared in Western sundials starting
in 1446.
Following the
acceptance of heliocentrism and equal hours, as well as
advances in trigonometry, sundials appeared in their present form during the Renaissance,
when they were built in large numbers. In 1524, the French astronomer Oronce Finé
constructed an ivory
sundial, which still exists; later, in 1570, the Italian astronomer Giovanni
Padovani published a treatise including instructions for the
manufacture and laying out of mural (vertical) and horizontal sundials.
Similarly, Giuseppe Biancani's Constructio instrumenti ad
horologia solaria (c. 1620) discusses how to construct sundials. The Portuguese
navigator Ferdinand Magellan used 18 hourglasses on each
ship during his circumnavigation of the globe in 1522. Since the hourglass was
one of the few reliable methods of measuring time at sea, it is speculated that
it had been used on board ships as far back as the 11th century, when it would
have complemented the magnetic compass as an aid to navigation. However, the
earliest evidence of their use appears in the painting Allegory of Good
Government, by Ambrogio Lorenzetti, from 1338. 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, reasonably accurate, and easily constructed time-measurement devices.
The hourglass also took on symbolic meanings, such as that of death,
temperance, opportunity, and Father Time, usually represented as a bearded,
old man. Though also used in China
Clocks
Clocks encompass a wide
spectrum of devices, ranging from wristwatches
to the Clock of the Long Now. The English word
clock is said to derive from the Middle
English clokke, Old North French cloque, or Middle Dutch
clocke, all of which mean bell,
and are derived from the Medieval Latin clocca, also meaning bell.
Indeed, bells were used to mark the passage of time; they marked the passage of
the hours at sea and in abbeys.
Throughout
history, clocks have had a variety of
power sources, including gravity,
springs,
and electricity. The invention of mechanical
clockwork itself is usually credited to the Chinese official Liang Lingzan and
monk Yi Xing. However, mechanical clocks were not widely used in the West until
the 14th century. Clocks were used in medieval monasteries
to keep the regulated schedule of prayers. The clock continued to be improved,
with the first pendulum clock being designed and built in the
17th century by Christiaan Huygens, a Dutch scientist.
Early Western mechanical clocks
The earliest
medieval European clockmakers were Christian monks. Medieval religious
institutions required clocks because daily prayer and work schedules were
strictly regulated. This was done by various types of time-telling and
recording devices, such as water clocks, sundials and marked candles, probably
used in combination. When mechanical clocks were used, they were often wound at
least twice a day to ensure accuracy. Important times and durations were
broadcast by bells, rung either by hand or by a mechanical device, such as a
falling weight or rotating beater.
The religious
necessities and technical skill of the medieval monks were crucial factors in
the development of clocks, as the historian Thomas Woods
writes:
The monks also
counted skillful clock-makers among them. The first recorded clock 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 14th-century monk of Glastonbury,
built one of the oldest clocks still in existence, which now sits in excellent
condition in London's Science Museum.
The appearance
of clocks in writings of the 11th century implies that they were well-known in Europe in that period. In the early 14th century, the Florentine
poet Dante Alighieri referred to a clock in his Paradiso;
considered to be the first literary reference to a clock that struck the hours.
The earliest detailed description of clockwork was presented by Giovanni da
Dondi, Professor of Astronomy at Padua, in his 1364 treatise Il Tractatus
Astrarii. This has inspired several modern replicas, including some in London 's Science Museum and the Smithsonian Institution. Other notable
examples from this period were built in Milan (1335), Strasbourg (1354), Lund
(1380), Rouen
(1389), and Prague
(1462).
Salisbury cathedral clock, dating from
about 1386, is the oldest working clock in the world, still with most of its
original parts. It has no dial, as its purpose was to strike a bell
at precise times. The wheels and gears are mounted in an open, box-like iron
frame, measuring about 1.2 metres (3.9 ft) square. The framework is
held together with metal dowels and pegs, and the escapement is the verge and
foliot type, standard for clocks of this age. The power is 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, and the other drives the striking mechanism and the air
brake.
Peter
Lightfoot's Wells Cathedral clock, constructed
c. 1390, is also of note. The dial represents a geocentric
view of the universe, with the Sun and Moon revolving around a centrally fixed Earth. It is unique in
having its original medieval face, showing a philosophical model of the pre-Copernican
universe. Above the clock is a set of figures, which hit the bells, and a set
of jousting knights who revolve around a track every 15 minutes. The clock
was converted to pendulum and anchor
escapement in the 17th century, and was installed in London 's Science
Museum
The face of the Prague Astronomical Clock (1462)
One clock that
has not survived to the present-day is that of the Abbey of St Albans, built by the 14th-century
abbot Richard of Wallingford. It may have been
destroyed during Henry VIII's Dissolution of the Monasteries,
but the abbot's notes on its design have allowed a full-scale reconstruction.
As well as keeping time, the astronomical clock could accurately predict lunar
eclipses, and may have shown the Sun, Moon (age, phase, and node),
stars and planets, as well as a wheel of fortune, and an indicator of the
state of the tide at London Bridge. According to Thomas Woods,
"a clock that equaled it in technological sophistication did not appear
for at least two centuries". Giovanni de Dondi was another
early mechanical clockmaker, whose clock did not survive, but has been
replicated based on the designs. De Dondi's clock was a seven-faced
construction with 107 moving parts, showing the positions of the Sun,
Moon, and five planets, as well as religious feast days. Around this period,
mechanical clocks were introduced into abbeys and monasteries to mark important
events and times, gradually replacing water clocks which had served the same
purpose.
During the
Middle Ages, clocks were primarily used for religious purposes; the first
employed for secular timekeeping emerged around the 15th century. In Dublin, the
official measurement of time became a local custom, and by 1466 a public clock
stood on top of the Tholsel (the city court and council chamber). It was
probably the first of its kind in Ireland
Clock towers
in Western
Europe in the Middle Ages were also sometimes striking
clocks. The most famous original still standing is possibly St Mark's
Clock on the top of St Mark's Clocktower in St Mark's
Square, Venice,
assembled in 1493, by the clockmaker Gian Carlo Rainieri from Reggio Emilia.
In 1497, Simone Campanato moulded the great bell that every definite time-lapse
is beaten by two mechanical bronze statues (h. 2,60 m.) called Due Mori (Two Moors), handling a hammer.
Possibly earlier (1490 by clockmaster Jan Růže also called Hanuš) is the Prague Astronomical Clock, that according
to another source was assembled as early as 1410 by clockmaker Mikuláš of Kadaň and mathematician Jan Šindel.
The allegorical parade of animated sculptures rings at h. 12.00 every day.
Early clock
dials did not use minutes and seconds. A clock with a minutes dial is mentioned
in a 1475 manuscript, and clocks indicating minutes and seconds existed in Germany
in the 15th century. Timepieces which indicated minutes and seconds were
occasionally made from this time on, but this was not common until the increase
in accuracy made possible by the pendulum clock and, in watches, the spiral
balance spring. The 16th-century
astronomer Tycho Brahe used clocks with minutes and
seconds to observe stellar positions.
The Ottoman
engineer Taqi al-Din described a
weight-driven clock with a verge-and-foliot
escapement, a striking train of gears, an alarm,
and a representation of the moon's phases in his book The Brightest Stars for
the Construction of Mechanical Clocks (Al-Kawākib al-durriyya fī wadh'
al-bankāmat al-dawriyya), written around 1565. Similarly to earlier
15th-century European mechanical alarm clocks, the alarm was set by placing a
peg on the dial wheel at the appropriate time. The clock had three dials reading in hours, degrees and minutes.
Taqi al-Din later constructed a clock for the Istanbul Observatory,
where he used it to make observations of right
ascensions, stating: “We constructed a mechanical clock with three
dials which show the hours, the minutes, and the seconds. We divided each
minute into five seconds.” This was an important innovation in 16th-century
practical astronomy, as at the start of the century clocks were not accurate
enough to be used for astronomical purposes.
An example of a
watch which measured time in minutes was created by an Ottoman
watchmaker, Meshur Sheyh Dede, in 1702.
Pendulum clocks
Innovations to
the mechanical clock continued, with miniaturization leading to domestic clocks
in the 15th century, and personal watches in the 16th. In the 1580s, the
Italian polymath
Galileo
Galilei investigated the regular swing of the pendulum,
and discovered that it could be used to regulate a clock. Although Galileo
studied the pendulum as early as 1582, he never actually constructed a clock
based on that design. The first pendulum clock was designed and built by Dutch
scientist Christiaan Huygens, in 1656. Early versions
erred by less than one minute per day, and later ones only by 10 seconds,
very accurate for their time.
The Jesuits
were another major contributor to the development of pendulum clocks in the
17th and 18th centuries, having had an "unusually keen appreciation of the
importance of precision". 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".
They served a crucial role in spreading and testing the scientific ideas of the
period, and collaborated with contemporary scientists, such as Huygens.
The modern longcase
clock, also known as the grandfather clock, has its origins in the
invention of the anchor escapement mechanism in about 1670.
Before then, pendulum clocks had used the older verge
escapement mechanism, which required very wide pendulum swings of
about 100°. To avoid the need for a very large case, most clocks using the verge
escapement had a short pendulum. The anchor mechanism, however, reduced the
pendulum's necessary swing to between 4° to 6°, allowing clockmakers to use
longer pendulums with consequently slower beats. These required less power to
move, caused less friction and wear, and were more accurate than their shorter
predecessors. Most longcase clocks use a pendulum about a metre
(39 inches) long to the center of the bob, with each swing taking one
second. This requirement for height, along with the need for a long drop space
for the weights that power the clock, gave rise to the tall, narrow case.
In 1675,
18 years after inventing the pendulum clock, Huygens devised the spiral balance spring
for the balance wheel of pocket watches, an improvement
on the straight spring invented by English natural philosopher Robert Hooke.
This resulted in a great advance in accuracy of pocket watches, from perhaps
several hours per day to 10 minutes per day, similar to the effect of the
pendulum upon mechanical clocks.
Clockmakers
A pocket watch.
The first
professional clockmakers came from the guilds of locksmiths
and jewellers.
Clockmaking developed from a specialized craft into a mass production industry
over many years. Paris
and Blois
were the early centers of clockmaking in France France , possibly in Europe ".
He invented a special repeating mechanism which improved the precision of
clocks and watches, a face that could be opened to view the inside clockwork,
and made or supervised over 3,500 watches. The competition and scientific
rivalry resulting from his discoveries further encouraged researchers to seek
new methods of measuring time more accurately.
An antique pocket watch
movement, from an 1891 encyclopedia.
Between 1794 and
1795, in the aftermath of the French
Revolution, the French government briefly mandated decimal
clocks, with a day divided into 10 hours of 100 minutes
each. The astronomer and mathematician Pierre-Simon Laplace, among other
individuals, modified the dial of his pocket watch to decimal time. A clock in
the Palais des Tuileries kept decimal time as
late as 1801, but the cost of replacing all the nation's clocks prevented
decimal clocks from becoming widespread. Because decimalized clocks only helped
astronomers rather than ordinary citizens, it was one of the most unpopular
changes associated with the metric system, and it was abandoned.
In Germany , Nuremberg
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
Wristwatches
In 1904, Alberto Santos-Dumont, an early aviator,
asked his friend, a French watchmaker called Louis Cartier,
to design a watch that could be useful during his flights. The wristwatch had
already been invented by Patek Philippe, in 1868, but only as a
"lady’s bracelet watch", intended as jewelry. As pocket watches were
unsuitable, Louis Cartier created the Santos
Wristwatches
gained in popularity during World War I, when officers found them to be more
convenient than pocket watches in battle. Also, because the
pocket watch was mainly a middle class item, the enlisted men
usually owned wristwatches, which they brought with them. Artillery and
infantry officers depended on their watches as battles became more complicated
and coordinated attacks became necessary. Wristwatches 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 and pilots. In
World War II, the A-11 was a popular watch among American airmen, with its
simple black face and clear white numbers for easy readability.
Marine chronometers are clocks used at sea as time standards,
to determine longitude
by celestial navigation. They were first
developed by Yorkshire
carpenter John Harrison, who won the British government's
Longitude
Prize in 1759. Marine chronometers keep the time of a fixed location
usually Greenwich Mean Time allowing seafarers to
determine longitude by comparing the local high noon
to the clock.
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 certain 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 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. To meet this requirement, each movement is individually tested
for several consecutive days, in five positions, and at three temperatures. Any
watch with the designation chronometer has a certified movement.
Quartz oscillators
Internal construction of a modern
high performance HC-49 package quartz
crystal.
The piezoelectric
properties of crystalline 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. The
following decades saw the development of quartz clocks as precision time
measurement devices in laboratory settings—the bulky and 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. The National Bureau of Standards
(now NIST)
based the time standard of the United
States
A modern quartz watch
and chronograph
Atomic clocks
A
chip-scale atomic clock.
Atomic clocks
are the most accurate timekeeping devices known to date. Accurate to within a
few seconds over many thousands of years, they are used to calibrate other clocks
and timekeeping instruments. The first atomic clock, invented in 1949, is on
display at the Smithsonian Institution. It was based on
the absorption line in the ammonia molecule, but most are now based on the spin
property of the cesium
atom.
The International System of Units standardised
its unit of time, the second, on the properties of cesium in 1967. 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. The cesium atomic clock, maintained by the National Institute of Standards and
Technology, is accurate to 30 billionths of a second per year.
Atomic clocks have employed other elements, such as hydrogen
and rubidium
vapor, offering greater stability in the case of hydrogen clocks and smaller
size, lower power consumption, and thus lower cost (in the case of rubidium
clocks).
Radio clocks
A radio clock is
a clock that is synchronized by a time code
bit stream transmitted by a radio transmitter connected to a time standard
such as an atomic clock. Such a clock may be synchronized
to the time sent by a single transmitter, such as many national or regional
time transmitters, or may use multiple transmitters, like the Global Positioning System. Radio clocks
and watches have been very popular in Europe
since the late 1980s.
Global Positioning System
Artist Interpretation of GPS satellite,
image courtesy of NASA.
The Global Positioning System (GPS), in
coordination with the network time protocol, is a
radio-navigation system used to synchronize timekeeping systems across the
globe. GPS was developed by the US Department of Defense to provide
constant, all-weather navigation capabilities for military ground, sea, and air
forces. In 1983, following the shooting down of Korean Air Lines Flight 007 after it
entered Soviet
airspace, President Ronald Reagan issued a directive allowing the
free commercial use of GPS, to prevent further navigational errors. 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 applied 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. GPS time therefore
remains at a constant offset of 19 seconds from International Atomic Time (TAI). The
on-board satellite clocks are periodically corrected to compensate for relativistic effects, and to 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. In the United States, the Navstar
GPS system is maintained by 24 satellites circling the Earth every twelve
hours, travelling in 6 orbits; Russia operates
a system known as GLONASS
(Global Navigation Satellite System). In 2007, the European
Union approved funding for the Galileo navigation system
comprising 30 satellites scheduled to be operational by 2013. China
Conclusions
For thousands of years, devices
have been used to measure and keep track of time.
The current sexagesimal
system
of time measurement dates to approximately
2000 BC, in Sumer.
The Ancient
Egyptians divided the day into two 12-hour periods, and used large obelisks
to track the movement of the Sun. They also developed water clocks,
which were probably first used in the Precinct of Amun-Re, and later outside Egypt China , Japan , England
and Iraq ; the timestick, widely used in India
and Tibet , as well as some
parts of Europe ; and the hourglass,
which functioned similarly to a water clock.
The earliest clocks relied on shadows
cast by the sun, so they were not useful in cloudy weather or at night, and
required recalibration as the seasons changed if the gnomon was not
aligned with the Earth's axis. The first clock with an escapement
mechanism, which transferred rotational energy into intermittent motions, dates
back to 3rd century BC ancient Greece; Arabic engineers invented water clocks
driven by gears
and weights in the 11th century.
Mechanical
clocks employing the verge escapement mechanism were invented in
Europe at the turn of the 14th century, and became the standard timekeeping
device until the spring-powered clock and pocket watch
in the 16th century, followed by the pendulum
clock in the 18th century. During the 20th century, quartz
oscillators were invented, followed by atomic clocks.
Although first used in laboratories, quartz oscillators were both easy to
produce and accurate, leading to their use in wristwatches.
Atomic clocks are far more accurate than any previous timekeeping device, and
are used to calibrate other clocks and to calculate the proper time on Earth; a standardized civil
system, Coordinated Universal Time, is based on
atomic time.
BIBLIOGRAPHI:
WWW.CEASORNICAR.RO
WWW.SOFTPEDIA.COM
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