March 26, 2021 5 min read

The history of timekeeping is intrinsically linked to the development of human society. A carved bone found in the Semliki Valley in the Democratic Republic of Congo is thought to be the first instance of man recording the passage of days. If so, at 20,000 years old it is the earliest archaeological evidence of recording time, though this is disputed. Moon shaped pits have also been discovered in Scotland, dug 10,000 years ago, apparently to record the lunar phases. The earliest solid archaeological evidence of timekeeping, however, are the calendars used by the Egyptians and Babylonians 5,000 years ago. These were used to regulate and record religious and social events and track the passage of the seasons for farming.

Shadow Clocks and Sundials

Shadow clocks are thought to be the first attempt to divide the daylight hours into regular subdivisions. The earliest and simplest versions of these were large stone obelisks surrounded by 12 markers around them. As the sun moved through the sky the shadow would progress over the markers, allowing the time of day to be calculated. These were in use from 3500 BC, but by 1500 BC, several more accurate versions akin to modern sundials were in use. However, these had to be oriented Eastward in the mornings, then rotated at noon and were useless at night or on cloudy days. The first polar axis sundial that gave hours of equal length all year round was the invention of the Arab astronomer Abu’l-Hasan Ibn al-Shatir in 1371. The present form of sundials appeared during the Renaissance following the adoption of Heliocentrism.

Water Clocks

To effectively record the passage of time without sunlight, the ancient Egyptians also developed water clocks, whose use spread across the world. The earliest reference to these is in the tomb of Amenemhet from the 16th century BC, who is credited as its inventor. Plato introduced water clocks to ancient Greece, where they were known as clepsydrae. Several designs existed, becoming more complex with time. These used water flowing at a constant rate from a water vessel, or graduated bowls that slowly filled as they sank in water. By 325 BC, faces and hour hands were incorporated for more accurate timekeeping. Larger designs were less effective, as water pressure caused the water to flow faster when the vessel was full. Greek and Roman horologists circumvented this by using conical containers to equalise flow rate. Later models incorporated automata, gongs, bells and dials. From 500BC, water clocks were widely used in Persia, with designs so efficient they rivalled modern clocks in accuracy,

Candle Clocks

These are believed to have originated in China, where they are mentioned in records as far back as 520 AD. Candles of uniform weight and thickness that burned at a consistent rate were graduated to display the time as they burned. Candle clocks were particularly useful to determine the passage of time at night or indoors during daytime. The Mesopotamian polymath A-Jazari designed some of the most sophisticated candle clocks in history in the 13th century. These used a counterweight to exploit the diminishing weight of the burning candle to operate dials and automata. Oil lamp clocks also used burning fuel to record time but used a marked reservoir to record dropping levels of oil over time instead.

Incense Clocks

Incense clocks were unique to the Far East, being used in China from the 6th century, although it has been suggested that they originated in India. Some used incense sticks, marked at regular intervals, that burned at a constant rate. Sometimes these incorporated multiple fragrances to give a different scent each hour. Other suspended weights from threads at regular intervals. As the thread burned, the weight would drop into a metal tray or gong to sound the time. Incense seal clocks were also used, with a carved groove to hold powdered incense in a stone, wood, or metal seal. This groove would be marked to show the time elapsed as the incense burned. Large versions could burn for up to a month at a time.

Mechanical Clocks

While the first purely mechanical clock on record was built in 996 AD, mechanical elements had been incorporated for centuries. As far back as the 3rd century, the Greek engineer Philo of Byzantium recorded a liquid driven escapement in a water clock. A water driven astronomical clock incorporating an escapement was constructed in 7th century China by the mathematician Yi Xing.  The earliest purely mechanical European clocks were designed and built by Catholic monks. Strict observance of work schedules and prayer times were vital. By the 1300s mechanical clocks in town clock towers, churches and cathedrals were becoming widespread. The earliest of these had no face and simply sounded a bell each hour. Weights and springs provided power to the mechanism, but accuracy was limited. Often sundials would be needed to reset the early mechanical clocks that could lose as much as 15 minutes per day. The following century, coiled springs were incorporated into clock design to move the hour hand. The first recorded instance of a clock displaying minutes as well as hours was in 1475.

Domestic clocks first appeared in the 15th century as innovations in design allowed smaller mechanisms and thus smaller clocks. In 1582, the scientist Galileo Galilei hit upon the idea of incorporating pendulums to increase accuracy. However, despite working on the idea for many years, he was unsuccessful in his attempts. The first pendulum clock was invented by the Dutch astronomer Christian Huygens in 1656. This revolutionised accuracy and paved the way for widespread domestic clock ownership. Huygens is also credited with the invention of the spiral balance spring, allowing greater accuracy in portable clocks and watches. This system is still used in watchmaking today.

Quartz and Atomic Clocks

In 1927, engineers at the Bell Telephone Laboratories in Canada created the world’s first quartz clock. This used the regular vibration of electrically excited quartz crystals to create the most accurate timepiece in history. In 1939, the Royal Observatory installed its first quartz clocks that only varied by two thousandths of a second each day. The first quartz wristwatch was created by Seiko in 1969, making cheap, highly accurate timekeeping available to all.

Only 19 years later in 1948, the next great leap forward occurred. A prototype ammonia maser was built at the National Bureau of Standards in the USA to demonstrate measuring time using atomic transitions. In 1955 the caesium beam atomic clock was perfected, using the periodic oscillations between the two energy states of caesium atoms for even greater accuracy. Modern caesium clocks are accurate to 30 billionths of a second per year. The search for greater accuracy continues to this day. Future atomic clocks such as the hydrogen maser and frequency discriminators like the caesium fountain and optical clock are expected to possess an accuracy of 100 quadrillionths of a second per day.

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