Synchronization of Digital Telecommunications Networks

6

PHYSICAL PRINCIPLES ANDTECHNOLOGY OF CLOCKS

The oldest historical examples of clocks are mostly dated back to 3500 BC: Egyptian obelisks and sundials. Across several technical improvements, sundials (based on measuring the rotation of a shadow with the sun) and clepsydrae (based on measuring the level of water in a vessel with a regulated flow of water at the input or output) have been the clocks in use during the Middle Ages. On the other hand, early mechanical clocks date back to the thirteenth century, while the first pocket clocks were constructed in the fifteenth century, based on a spring mechanism. However, a true milestone was the invention of pendulum clocks, due to Galileo and Huygens (sixteenth and seventeenth century), and the introduction of the swing-wheel as an oscillating element. After that, until the introduction of electrical clocks and then of clocks based on quartz and atomic oscillators, mechanical clocks did not go through substantial changes, at least in the operation principle, until today. Some milestones in the progress of timekeeping are mentioned by Allan et al. in their outstanding review paper [6.1]:

3500 BC	Egyptian obelisks and sundials
2000–1500 BC	Mayan calendar
1900–1600 BC	Stonehenge site
1094	Sung Su’s Chinese water clock perfected
1583	Galileo discovers pendulum period constancy
1656	Huygens’ pendulum clock
1736	Harrison’s maritime chronometer tested at sea
1918	Quartz-crystal oscillator developed
1948–1949	Lyons develops ...