part around the solar year




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part around the solar year. 
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Centuries before the Roman Empire, the Egyptians had formulated a municipal calendar having 12 months of 
30 days, with five days added to approximate the solar year. Each period of ten days was marked by the 
appearance of special groups of stars called decans. At the rise of the star Sirius just before sunrise, which 
occurred around the all-important annual flooding of the Nile, 12 decans could be seen spanning the heavens. 
The cosmic significance the Egyptians placed in the 12 decans led them to develop a system in 
which each interval of darkness (and later, each interval of daylight) was divided into a dozen equal parts. 
These periods became known as temporal hours because their duration varied according to the changing length 
of days and nights with the passing of the seasons. Summer hours were long, winter ones short; only at the 
spring and autumn equinoxes were the hours of daylight and darkness equal. Temporal hours, which were first 
adopted by the Greeks and then the Romans, who disseminated them through Europe, remained in use for 
more than 2,500 years. 
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In order to track temporal hours during the day, inventors created sundials, which indicate time by the length 
or direction of the sun's shadow. The sundial's counterpart, the water clock, was designed to measure temporal 
hours at night. One of the first water clocks was a basin with a small hole near the bottom through which the 
water dripped out. The falling water level denoted the passing hour as it dipped below hour lines inscribed on 
the inner surface. Although these devices performed satisfactorily around the Mediterranean, they could not 
always be depended on in the cloudy and often freezing weather of northern Europe. 
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The advent of the mechanical clock meant that although it could be adjusted to maintain temporal hours, it was 
naturally suited to keeping equal ones. With these, however, arose the question of when to begin counting, and 
so, in the early 14th century, a number of systems evolved. The schemes that divided the day into 24 equal 
parts varied according to the start of the count: Italian hours began at sunset, Babylonian hours at 
sunrise, astronomical hours at midday and 'great clock' hours, used for some large public clocks in Germany, at 
midnight. Eventually these were superseded by 'small clock', or French, hours, which split the day into two 12-
hour periods commencing at midnight. 
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The earliest recorded weight-driven mechanical clock was built in 1283 in Bedfordshire in England. 
The revolutionary aspect of this new timekeeper was neither the descending weight that provided 
its motive force nor the gear wheels (which had been around for at least 1,300 years) that transferred the 
power; It was the part called the escapement. In the early 1400s came the invention of the coiled spring or 
fusee which maintained constant force to the gear wheels of the timekeeper despite the changing tension of its 
mainspring. By the 16th century, a pendulum clock had been devised, but the pendulum swung in a large arc 
and thus was not very efficient. 

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To address this, a variation on the original escapement was invented in 1670, in England. It was called the 
anchor escapement, which was a lever-based device shaped like a ship's anchor. The motion of a pendulum 
rocks this device so that it catches and then releases each tooth of the escape wheel, in turn allowing it to turn 
a precise amount. Unlike the original form used in early pendulum clocks, the anchor escapement permitted 
the pendulum to travel in a very small arc. Moreover, this invention allowed the use of a long pendulum which 
could beat once a second and thus led to the development of a new floorstanding case design, which became 
known as the grandfather clock. 
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Today, highly accurate timekeeping instruments set the beat for most electronic devices. Nearly all computers 
contain a quartz-crystal clock to regulate their operation. Moreover, not only do time signals beamed down 
from Global Positioning System satellites calibrate the functions of precision navigation equipment, they do so 
as well for mobile phones, instant stock-trading systems and nationwide power-distribution grids. 
So integral have these time-based technologies become to day-to-day existence that our dependency on them is 
recognised only when they fail to work. 

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