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A clock is an instrument used to indicate, measure, keep, and co-ordinate time. The word clock is derived ultimately (via Dutch, Northern French, and Medieval Latin) from the Celtic words clagan and clocca meaning "bell". For horologists and other specialists the term clock continues to mean exclusively a device with a striking mechanism for announcing intervals of time acoustically, by ringing a bell, a set of chimes, or agong.[dubious – discuss] A silent instrument lacking such a mechanism has traditionally been known as a timepiece.[1] In general usage today a "clock" refers to any device for measuring and displaying the time. Watches and other timepieces that can be carried on one's person are often distinguished from clocks.[2]
The clock is one of the oldest human inventions, meeting the need to consistently measure intervals of time shorter than the natural units, the day, thelunar month, and the year. Such measurements require devices. Devices operating on several different physical processes have been used over the millennia, culminating in the clocks of today.
[edit]Sundials and other devicesThe sundial, which measures the time of day by the direction of shadows cast by the sun, was widely used in ancient times. A well-designed sundial can measure local solar time with reasonable accuracy, and sundials continued to be used to monitor the performance of clocks until the modern era. However, its practical limitations - it requires the sun to shine and does not work at all during the night - encouraged the use of other techniques for measuring time. In India, the King of Jaipur the Pink City, Jai Singh II constructed many instruments and sundials in the observatories in cities Jaipur, Varanasi, Ujjain, Mathura between 1724-1730 A.D. He had a good interest in astronomy and town planning.
Candle clocks, and sticks of incense that burn down at approximately predictable speeds have also been used to estimate the passing of time. In anhourglass, fine sand pours through a tiny hole at a constant rate and indicates a predetermined passage of an arbitrary period of time.
[edit]Water clocksWater clocks, also known as clepsydrae (sg: clepsydra), along with the sundials, are possibly the oldest time-measuring instruments, with the only exceptions being the vertical gnomon and the day-counting tally stick.[3] Given their great antiquity, where and when they first existed are not known and perhaps unknowable. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BC. Other regions of the world, including India and China, also have early evidence of water clocks, but the earliest dates are less certain. Some authors, however, write about water clocks appearing as early as 4000 BC in these regions of the world.[4]
Greek astronomer, Andronicus of Cyrrhus, supervised the construction of the Tower of the Winds in Athens in the 1st century B.C.[5]
The Greek and Roman civilizations are credited for initially advancing water clock design to include complex gearing,[6] which was connected to fancifulautomata and also resulted in improved accuracy. These advances were passed on through Byzantium and Islamic times, eventually making their way toEurope. Independently, the Chinese developed their own advanced water clocks, passing their ideas on to Korea and Japan.
Some water clock designs were developed independently and some knowledge was transferred through the spread of trade. It is important to point out that the need for the common person to 'know what time it is' largely did not exist until the Industrial Revolution, when it became important to keep track of hours worked. In the earliest of times, however, the purpose for using a water clock was for astronomical and astrological reasons. These early water clocks were calibrated with a sundial. While never reaching the level of accuracy based on today's standards of timekeeping, the water clock was the most accurate and commonly used timekeeping device for millennia, until it was replaced by the more accurate pendulum clock in 17th century Europe.
In 797 (or possibly 801), the Abbasid caliph of Baghdad, Harun al-Rashid, presented Charlemagne with an Asian Elephant named Abul-Abbas together with a "particularly elaborate example" of a water [7] clock.
In the 13th century, Al-Jazari, an engineer who worked for Artuqid king of Diyar-Bakr, Nasir al-Din, made numerous clocks of all shapes and sizes. The book described 50 mechanical devices in 6 categories, including water clocks. The most reputed clocks included the Elephant, the Castle and Scribe clocks, all of which have been successfully reconstructed. As well as telling the time, these grand clocks were symbols of status, grandeur and wealth of the Urtuq State.[9]
[edit]Early mechanical clocksNone of the first clocks survived from 13th century Europe, but various mentions in church records reveal some of the early history of the clock.[10]
Medieval religious institutions required clocks to measure and indicate the passing of time because, for many centuries, daily prayer and work schedules had to be 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. Important times and durations w
A clock is an instrument used to indicate, measure, keep, and co-ordinate time. The word clock is derived ultimately (via Dutch, Northern French, and Medieval Latin) from the Celtic words clagan and clocca meaning "bell". For horologists and other specialists the term clock continues to mean exclusively a device with a striking mechanism for announcing intervals of time acoustically, by ringing a bell, a set of chimes, or agong.[dubious – discuss] A silent instrument lacking such a mechanism has traditionally been known as a timepiece.[1] In general usage today a "clock" refers to any device for measuring and displaying the time. Watches and other timepieces that can be carried on one's person are often distinguished from clocks.[2]
The clock is one of the oldest human inventions, meeting the need to consistently measure intervals of time shorter than the natural units, the day, thelunar month, and the year. Such measurements require devices. Devices operating on several different physical processes have been used over the millennia, culminating in the clocks of today.
[edit]Sundials and other devicesThe sundial, which measures the time of day by the direction of shadows cast by the sun, was widely used in ancient times. A well-designed sundial can measure local solar time with reasonable accuracy, and sundials continued to be used to monitor the performance of clocks until the modern era. However, its practical limitations - it requires the sun to shine and does not work at all during the night - encouraged the use of other techniques for measuring time. In India, the King of Jaipur the Pink City, Jai Singh II constructed many instruments and sundials in the observatories in cities Jaipur, Varanasi, Ujjain, Mathura between 1724-1730 A.D. He had a good interest in astronomy and town planning.
Candle clocks, and sticks of incense that burn down at approximately predictable speeds have also been used to estimate the passing of time. In anhourglass, fine sand pours through a tiny hole at a constant rate and indicates a predetermined passage of an arbitrary period of time.
[edit]Water clocksWater clocks, also known as clepsydrae (sg: clepsydra), along with the sundials, are possibly the oldest time-measuring instruments, with the only exceptions being the vertical gnomon and the day-counting tally stick.[3] Given their great antiquity, where and when they first existed are not known and perhaps unknowable. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BC. Other regions of the world, including India and China, also have early evidence of water clocks, but the earliest dates are less certain. Some authors, however, write about water clocks appearing as early as 4000 BC in these regions of the world.[4]
Greek astronomer, Andronicus of Cyrrhus, supervised the construction of the Tower of the Winds in Athens in the 1st century B.C.[5]
The Greek and Roman civilizations are credited for initially advancing water clock design to include complex gearing,[6] which was connected to fancifulautomata and also resulted in improved accuracy. These advances were passed on through Byzantium and Islamic times, eventually making their way toEurope. Independently, the Chinese developed their own advanced water clocks, passing their ideas on to Korea and Japan.
Some water clock designs were developed independently and some knowledge was transferred through the spread of trade. It is important to point out that the need for the common person to 'know what time it is' largely did not exist until the Industrial Revolution, when it became important to keep track of hours worked. In the earliest of times, however, the purpose for using a water clock was for astronomical and astrological reasons. These early water clocks were calibrated with a sundial. While never reaching the level of accuracy based on today's standards of timekeeping, the water clock was the most accurate and commonly used timekeeping device for millennia, until it was replaced by the more accurate pendulum clock in 17th century Europe.
In 797 (or possibly 801), the Abbasid caliph of Baghdad, Harun al-Rashid, presented Charlemagne with an Asian Elephant named Abul-Abbas together with a "particularly elaborate example" of a water [7] clock.
In the 13th century, Al-Jazari, an engineer who worked for Artuqid king of Diyar-Bakr, Nasir al-Din, made numerous clocks of all shapes and sizes. The book described 50 mechanical devices in 6 categories, including water clocks. The most reputed clocks included the Elephant, the Castle and Scribe clocks, all of which have been successfully reconstructed. As well as telling the time, these grand clocks were symbols of status, grandeur and wealth of the Urtuq State.[9]
[edit]Early mechanical clocksNone of the first clocks survived from 13th century Europe, but various mentions in church records reveal some of the early history of the clock.[10]
Medieval religious institutions required clocks to measure and indicate the passing of time because, for many centuries, daily prayer and work schedules had to be 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. Important times and durations w
A Brief History of Time is a popular science book written by Stephen Hawking and first published by the Bantam Dell Publishing Group in 1988.[1] It became a best-seller and has sold more than 9 million copies. It was also on the London Sunday Times best-seller list for more than four years.[2]
There is also a documentary by the same name, directed by Errol Morris and released in 1991. Unlike the book, the documentary is primarily abiography of Stephen Hawking.
A Brief History of Time attempts to explain a range of subjects in cosmology, including the Big Bang, black holes, light cones and superstring theory, to the nonspecialist reader. Its main goal is to give an overview of the subject but, unusual for a popular science book, it also attempts to explain some complex mathematics. The author notes that an editor warned him that for every equation in the book the readership would be halved, hence it includes only a single equation: E = mc2. In addition to Hawking's abstinence from equations, the book also simplifies matters by means of illustrations throughout the text, depicting complex models and diagrams.
[edit]EditionsThe Introduction disappeared after the first edition. It was copyrighted by Sagan, rather than by Hawking or the publisher, and the publisher did not have the right to reprint it forever. Hawking wrote his own introduction for later editions.
September 2005 saw the release of A Briefer History of Time (a collaboration with Leonard Mlodinow), which is an abridged version of the original book. It was updated again to address new issues that have arisen due to further scientific development.
