Timeline of time measurement technology

The history of timekeeping devices dates back to when ancient civilizations first observed astronomical bodies as they moved across the sky. Devices and methods for keeping time have since then improved through a long series of new inventions and ideas. Sundials and water clocks originated from ancient Egypt, and were later used by the Babylonians, the Greeks and the Chinese; medieval Islamic water clocks were unrivalled in their sophistication until the mid-14th century. Incense clocks, which may have been invented in India, were being used in China by the 6th century. The hourglass, one of the few reliable methods of measuring time at sea, was a European invention and does not seem to have been used in China before the mid-16th century.

In medieval Europe, purely mechanical clocks were developed after the invention of the bell-striking alarm, used to warn a man to toll the monastic bell. The weight-driven mechanical clock, controlled by the action of a verge and foliot, was a synthesis of earlier ideas derived from European and Islamic science, and one of the most important inventions in the history of timekeeping. The most famous mechanical clock was designed and built by Henry de Vick in 1360—for the next 300 years, all the improvements in timekeeping were essentially developments based on it. The invention of the mainspring in the early 15th century allowed small clocks to be built for the first time.

From the 17th century, the discovery that clocks could be controlled by harmonic oscillators led to the most productive era in the history of timekeeping. Leonardo da Vinci had produced the earliest known drawings of a pendulum in 14931494, and in 1582 Galileo Galilei had investigated the regular swing of the pendulum, discovering that frequency was only dependent on length. The pendulum clock, designed and built by Dutch polymath Christiaan Huygens in 1656, was so much more accurate than other kinds of mechanical timekeepers that few clocks have survived with their verge and foliot mechanisms intact. Other innovations in timekeeping during this period include inventions for striking clocks, the repeating clock and the deadbeat escapement. Errors in early pendulum clocks were eclipsed by those caused by temperature variation, a problem tackled during the 18th century by the English clockmakers John Harrison and George Graham; only the invention of invar in 1895 eliminated the need for such innovations.

From the 18th century, a succession of innovations and inventions led to timekeeping devices becoming increasingly accurate. Following the Scilly naval disaster of 1707, after which governments offered a prize to anyone who could discover a way to determine longitude, Harrison built a succession of accurate timepieces. The electric clock, invented in 1840, was used to control the most accurate pendulum clocks until the 1940s, when quartz timers became the basis for the precise measurement of time and frequency. The wristwatch, which had been recognised as a valuable military tool during the Boer War, became a symbol of masculinity and bravado after World War I. During the 20th century the non-magnetic wristwatch, battery-driven watches, the quartz wristwatch, and transistors and plastic parts were all invented. The most accurate timekeeping devices in practical use today are atomic clocks, which can be accurate to within a few billionths of a second per year. They are used to calibrate other clocks and timekeeping instruments.

Continuous timekeeping devices

Ancient civilizations observed astronomical bodies, often the Sun and Moon, to determine time. According to the historian Eric Bruton, Stonehenge is likely to have been the Stone Age equivalent of an astronomical observatory, used to seasonal and annual events such as equinoxes or solstices. As megalithic civilizations left no recorded history, little is known of their timekeeping methods.

Mesoamericans modified their usual vigesimal (base-20) counting system when dealing with calendars to produce a 360-day year. The Aboriginal Australians understood the movement of objects in the sky well, and used their knowledge to construct calendars and aid navigation; most Aboriginal cultures had seasons that were well-defined and determined by natural changes throughout the year, including celestial events. Lunar phases were used to mark shorter periods of time; the Yaraldi of South Australia being one of the few people recorded as having a way to measure time during the day, which was divided into seven parts using the position of the Sun.

All timekeepers before the 13th century relied upon methods that used something that moved continuously. No early method of keeping time changed at a steady rate. Devices and methods for keeping time have improved continuously through a long series of new inventions and ideas.

Shadow clocks and sundials

The first devices used for measuring the position of the Sun were shadow clocks, which later developed into the sundial. Egyptian obelisks, constructed  BC, are among the earliest shadow clocks. The oldest of all known sundials dates back to  BC (during the 19th Dynasty), and was discovered in the Valley of the Kings in 2013. Obelisks could indicate whether it was morning or afternoon, as well as the summer and winter solstices. A kind of shadow clock was developed  BC that was similar in shape to a bent T-square. It measured the passage of time by the shadow cast by its crossbar, and was oriented eastward in the mornings, and turned around at noon, so it could cast its shadow in the opposite direction.

A sundial is referred to in the Bible, in 2 Kings 20:911, when Hezekiah, king of Judea during the 8th century BC, is recorded as being healed by the prophet Isaiah and asks for a sign that he would recover:

A clay tablet from the late Babylonian period describes the lengths of shadows at different times of the year. The Babylonian writer Berossos () is credited by the Greeks with the invention of a hemispherical sundial hollowed out of stone; the path of the shadow was divided into 12 parts to mark the time. Greek sundials evolved to become highly sophisticated—Ptolemy's ''Analemma'', written in the 2nd century AD, used an early form of trigonometry to derive the position of the sun from data such as the hour of day and the geographical latitude.
The Romans borrowed the idea of the sundial from the Greeks. The military commander Pliny the Elder recorded that the first sundial in Rome arrived in 264 BC, looted from Catania in Sicily; according to him, it gave the incorrect time for a century, until the markings and angle appropriate for Rome's latitude were used.

According to the German historian of astronomy Ernst Zinner, sundials were developed during the 13th century with scales that showed equal hours. The first based on polar time appeared in Germany ; an alternative theory proposes that a Damascus sundial measuring in polar time can be dated to 1372. European treatises on sundial design appeared .

An Egyptian method of determining the time during the night, used from at least 600 BC, was a type of plumb-line called a merkhet. A north–south meridian was created using two merkhets aligned with Polaris, the north pole star. The time was determined by observing particular stars as they crossed the meridian.

Water clocks

The oldest description of a clepsydra, or water clock, is from the tomb inscription of an early 18th Dynasty ( BC) Egyptian court official named Amenemhet, who is identified as its inventor. It is assumed that the object described on the inscription is a bowl with markings to indicate the time. The oldest surviving water clock was found in the tomb of pharaoh Amenhotep III ( 14171379 BC). There are no recognised examples in existence of outflowing water clocks from ancient Mesopotamia, but written references have survived.

The introduction of the water clock 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. Around 550 AD, Yin Gui was the first in China to write of the overflow or constant-level tank. Around 610, two Sui Dynasty inventors, Geng Xun and Yuwen Kai, created the first balance clepsydra, with standard positions for the steelyard balance. In 721 the mathematician Yi Xing and government official Liang Lingzan regulated the power of the water driving an astronomical clock, dividing the power into unit impulses so that motion of the planets and stars could be duplicated. In 976, the Song dynasty astronomer Zhang Sixun addressed the problem of the water in clepsydrae freezing in cold weather when he replaced the water with liquid mercury. A water-powered astronomical clock tower was built by the polymath Su Song in 1088, which featured the first known endless power-transmitting chain drive.

The Greek philosophers Anaxagoras and Empedocles both referred to water clocks that were used to enforce time limits or measure the passing of time. The Athenian philosopher Plato is supposed to have invented an alarm clock that used lead balls cascading noisily onto a copper platter to wake his students.

A problem with most clepsydrae was the variation in the flow of water due to the change in fluid pressure, which was addressed from 100 BC when the clock's water container was given a conical shape. They became more sophisticated when innovations such as gongs and moving mechanisms were included. There is evidence that the 1st century BC Tower of the Winds in Athens once had eight sundials, a water clock, and a wind vane. In Greek tradition, clepsydrae were used in court, a practise later adopted by the Ancient Romans.

The first geared clock, invented in the 11th century by the Arab engineer Ibn Khalaf al-Muradi in Islamic Iberia, was a water clock that employed both segmental and epicyclic gearing. Islamic water clocks, which used complex gear trains and included arrays of automata, were unrivalled in their sophistication until the mid-14th century. Liquid-driven mechanisms (using heavy floats and a constant-head system) were developed that enabled water clocks to work at a slower rate.

The 12th-century Jayrun Water Clock at the Umayyad Mosque in Damascus was constructed by Muhammad al-Sa'ati, and was later described by his son Ridwan ibn al-Sa'ati in his ''On the Construction of Clocks and their Use'' (1203). A sophisticated water-powered astronomical clock was described by Al-Jazari in his treatise on machines, written in 1206. This castle clock was about high, and included a display of the zodiac and the solar and lunar paths, and doors that opened on the hour, to reveal a mannequin. In 1235, a water-powered clock that "announced the appointed hours of prayer and the time both by day and by night" stood in the entrance hall of the Mustansiriya Madrasah in Baghdad.

Chinese incense clocks

Incense clocks were first used in China around the 6th century, mainly for religious purposes, but also for social gatherings or by scholars. Due to their frequent use of Devanagari characters, American sinologist Edward H. Schafer has speculated that incense clocks were invented in India. As incense burns evenly and without a flame, the clocks were safe for indoor use. To mark different hours, differently scented incenses (made from different recipes) were used.

The incense sticks used could be straight or spiralled; the spiralled ones were intended for long periods of use, and often hung from the roofs of homes and temples. Some clocks were designed to drop weights at even intervals,

Incense seal clocks had a disk etched with one or more grooves, into which incense was placed. The length of the trail of incense, directly related to the size of the seal, was the primary factor in determining how long the clock would last; to burn 12 hours an incense path of around has been estimated. The gradual introduction of metal disks, most likely beginning during the Song dynasty, allowed craftsmen to more easily create seals of different sizes, design and decorate them more aesthetically, and vary the paths of the grooves, to allow for the changing length of the days in the year. As smaller seals became available, incense seal clocks grew in popularity and were often given as gifts.

Astrolabes

Sophisticated timekeeping astrolabes with geared mechanisms were made in Persia. Examples include those built by the polymath Abū Rayhān Bīrūnī in the 11th century and the astronomer Muhammad ibn Abi Bakr al‐Farisi

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