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.
Sun Clocks
With the disappearance of any ancient civilisation, such as the Sumerian culture, knowledge is also lost. Whilst we can but hypothesise on the reasons of why the equivalent to the modern wristwatch was never completed, we know that the ancient Egyptians were next to layout a system of dividing the day into parts, similar to hours.
'Obelisks' (tall four-sided tapered monuments) were carefully constructed and even purposefully geographically located we believe around 3500 BC. A shadow was cast as the Sun moved across the sky by the obelisk, which it appears was then marked out in sections, allowing people to clearly see the two halves of the day. Some of the sections have also been found to indicate the 'year's longest and shortest days', which it is thought were developments added later to allow identification of other important time subdivisions.
Another ancient Egyptian 'shadow clock' or 'sundial' has been discovered to have been in use around 1500 BC, which allowed the measuring of the passage of 'hours'. The sections were divided into ten parts, with two 'twilight hours' indicated, occurring in the morning and the evening. For it to work successfully then at midday or noon, the device had to be turned 180 degrees to measure the afternoon hours.
The Egyptians also used the 'Merkhet', the oldest known astronomical tool, which is believed to have been developed around 600 BC. Two merkhets were used to establish a north-south line which was achieved by lining them up with the 'Pole Star'. This enabled the measurement of night-time hours, when certain stars crossed the marked meridian. By 30 BC, 'Vitruvius' describes thirteen different sundial styles being used across Greece, Asia Minor, and Italy, inherently demonstrating how the development must have grown to be more complex.
Water Clocks
'Water clocks' were among the earliest time keeping devices that didn't use the observation of the celestial bodies to calculate the passage of time. The ancient Greeks, it is believed, began using water clocks around 325 BC. Most of these clocks were used to determine the hours of the night, but may have also been used during daylight. An inherent problem with the water clock was that they were not totally accurate, as the system of measurement was based on the flow of water either into, or out of, a container which had markers around the sides. Another very similar form was that of a bowl that sank during a period as it was filled of water from a regulated flow. It is known that water clocks were common across the Middle East, and that these were still being used in North Africa during the early part of the twentieth-century.
In the Far East, mechanised 'astronomical' and 'astrological' clock-making is known to have developed between 200-1300 AD. In 1088 AD, 'Su Sung' and his colleagues designed and constructed a highly complex mechanism that incorporated a water-driven escapement, invented about 725 AD. It was over seven metres in height and had all manor of mechanisms running simultaneously. During each hour an observer could view the movement of a power-driven armillary sphere, constructed of bronze rings, an automatically rotating celestial globe, together with five doors that allowed an enticing glimpse of seeing individual statues, all of which rang bells, banged gongs or held inscribed tablets showing the hour or a special time of the day. The appearance and actions would have appeared similar to the automaton we know so well today.
Candle clocks
The earliest mention of candle clocks 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 until the early 10th century.
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 (2.54 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, to prevent the flame from extinguishing.
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.
An oil-lamp clock
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.
Incense clocks
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, one still exists in the Shōsōin, although its characters are not Chinese, but Devanagari.Due to their frequent use of Devanagari characters, suggestive of their use in Buddhist ceremonies, Edward H. Schafer speculated that incense clocks were invented in India.Although similar to the candle clock, incense clocks burned evenly and without a flame; therefore, they were more accurate and safer for indoor use.
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, a geisha was paid for the number of senkodokei (incense sticks) that had been consumed while she was present, a practice which continued until 1924.Incense seal clocks were used for similar occasions and events as the stick clock; while religious purposes were of primary importance, these clocks were also popular at social gatherings, and were used by Chinese scholars and intellectuals.The seal was a wooden or stone disk with one or more grooves etched in it into which incense was placed.These clocks were common in China, but were produced in fewer numbers in Japan. To signal the passage of a specific amount of time, small pieces of fragrant woods, resins, or different scented incenses could be placed on the incense powder trails. Different powdered incense clocks used different formulations of incense, depending on how the clock was laid out.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; all burned for long periods of time, ranging between 12 hours and a month.
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
Mechanical Clocks
In 1656, 'Christian Huygens' (Dutch scientist), made the first 'Pendulum clock', with a mechanism using a 'natural' period of oscillation. 'Galileo Galilei' is credited, in most historical books, for inventing the pendulum as early as 1582, but his design was not built before his death. Huygens' clock ,when built, had an error of 'less than only one minute a day'. This was a massive leap in the development of maintaining accuracy, as this had previously never been achieved. Later refinements to the pendulum clock reduced this margin of error to 'less than 10 seconds a day'.
Huygens, in 1657, developed what is known today as the 'balance wheel and spring assembly', which is still found in some of today's wrist watches. This allowed watches of the seventeenth-century to keep accuracy of time to approximately ten minutes a day. Meanwhile, in London, England (UK) in 1671, 'William Clement' began building clocks with an 'anchor' or 'recoil' escapement, which interfered even less with the perpetual motion of the pendulum system of clock.
'George Graham', in 1721, invented a design with the degree of accuracy to 'one second a day' by compensating for changes in the pendulum's length caused by temperature variations. The mechanical clock continued to develop until they achieved an accuracy of 'a hundredth-of-a-second a day', when the pendulum clock became the accepted standard in most astronomical observatories.
Quartz Clocks
The running of a 'Quartz clock' is based on the piezoelectric property of the quartz crystal. When an electric field is applied to a quartz crystal, it actually changes the shape of the crystal itself. If you then squeeze it or bend it, an electric field is generated. When placed in an appropriate electronic circuit, this interaction. between the mechanical stress and the electrical field. causes the crystal to vibrate, generating a constant electric signal which can then be used for example on an electronic clock display. The first wrist-watches that appeared in mass production used 'LED', 'Light Emitting Diode' displays. By the 1970's these were to be replaced by a 'LCD', 'Liquid Crystal Display'.
Quartz clocks continue to dominate the market because of the accuracy and reliability of the performance, also being inexpensive to produce on mass scale. The time keeping performance of the quartz clock has now been surpassed by the 'Atomic clock'.
Digital clocks
Rise of the digital era enabled us to gain access to very precise and reliable electronic clocks that display time with numeric displays. The two most common display formats are 24-hour notation (from 00-23) and in 12-hor notation where clock must also show AM/PM indicator. With each passing year, digital clocks gain ground over slowly disappearing analogue clocks. Display surface of this clock does not need to be inside of small LCD, LED or VFD screens, but they can also be projected on either very large public surfaces or indoors for persons with imperfects vision.
Modern Time Device
Electronic Word clocks
Word clocks are not using numeric display, but instead they are writing natural sentences on the screen that tells us time. Sentences can be recorded either via software or with hardware.
Auditory clocks
Clocks that used recording of human voice or computer generated voice to tell time. Alternatively, instead of voice time can be presented as auditory codes. This type of clocks is often used for announcing time in large areas (church bells that tool specific number of times at the start of each hour), telephony or for blind people.
Tactile clocks
People with imperfect vision or blindness can also use clocks that are producing physical representation of the numbers on their surfaces, either as standard numbers or in blind text code.
Multi Display clocks
This type of clock can be either analogue or digital, and their main feature is ability to show multiple time zones, have multiple faces, or use several time standards.
