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Subj: Early Clocks
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Early Clocks
Not until somewhat recently (that is, in terms of human history) did people
find a need for knowing the time of day. As best we know, 5000 to 6000 years
ago great civilizations in the Middle East and North Africa began to make
clocks to augment their calendars. With their attendant bureaucracies, formal
religions, and other burgeoning societal activities, these cultures apparently
found a need to organize their time more efficiently.
Egyptian Shadow Clock Sun Clocks
The Sumerian culture was lost without passing on its knowledge, but the
Egyptians were apparently the next to formally divide their day into parts
something like our hours. Obelisks (slender, tapering, four-sided monuments)
were built as early as 3500 BCE. Their moving shadows formed a kind of
sundial, enabling people to partition the day into morning and afternoon.
Obelisks also showed the year's longest and shortest days when the shadow at
noon was the shortest or longest of the year. Later, additional markers around
the base of the monument would indicate further subdivisions of time.
Another Egyptian shadow clock or sundial, possibly the first portable
timepiece, came into use around 1500 BCE. This device divided a sunlit day
into 10 parts plus two "twilight hours" in the morning and evening. When the
long stem with 5 variably spaced marks was oriented east and west in the
morning, an elevated crossbar on the east end cast a moving shadow over the
marks. At noon, the device was turned in the opposite direction to measure the
afternoon "hours."
The merkhet, the oldest known astronomical tool, was an Egyptian development
of around 600 BCE. A pair of merkhets was used to establish a north-south
line (or meridian) by aligning them with the Pole Star. They could then be
used to mark off nighttime hours by determining when certain other stars
crossed the meridian.
In the quest for better year-round accuracy, sundials evolved from flat
horizontal or vertical plates to more elaborate forms. One version was the
hemispherical dial, a bowl-shaped depression cut into a block of stone,
carrying a central vertical gnomon (pointer) and scribed with sets of hour
lines for different seasons. The hemicycle, said to have been invented about
300 BCE, removed the useless half of the hemisphere to give an appearance of a
half-bowl cut into the edge of a squared block. By 30 BCE, Vitruvius could
describe 13 different sundial styles in use in Greece, Asia Minor, and Italy.
Elements of a Clock
Before we continue describing the evolution of ways to mark the passage of
time, perhaps we should broadly define what constitutes a clock. All clocks
must have two basic components: a regular, constant or repetitive process or
action to mark off equal increments of time. Early examples of such processes
included the movement of the sun across the sky, candles marked in increments,
oil lamps with marked reservoirs, sand glasses (hourglasses), and in the
Orient, knotted cords and small stone or metal mazes filled with incense that
would burn at a certain pace. Modern clocks use a balance wheel, pendulum,
vibrating crystal, or electromagnetic waves associated with the internal
workings of atoms as their regulators.
The history of timekeeping is the story of the search for ever more consistent
actions or processes to regulate the rate of a clock.
Water Clocks
Water clocks were among the earliest timekeepers that didn't depend on the
observation of celestial bodies. One of the oldest was found in the tomb of
the Egyptian pharaoh Amenhotep I, buried around 1500 BCE. Later named
clepsydras ("water thieves") by the Greeks, who began using them about 325
BCE, these were stone vessels with sloping sides that allowed water to drip at
a nearly constant rate from a small hole near the bottom. Other clepsydras
were cylindrical or bowl-shaped containers designed to slowly fill with water
coming in at a constant rate. Markings on the inside surfaces measured the
passage of "hours" as the water level reached them. These clocks were used to
determine hours at night, but may have been used in daylight as well. Another
version consisted of a metal bowl with a hole in the bottom; when placed in a
container of water the bowl would fill and sink in a certain time. These were
still in use in North Africa in the 20th century.
More elaborate and impressive mechanized water clocks were developed between
100 BCE and 500 CE by Greek and Roman horologists and astronomers.
The added complexity was aimed at making the flow more constant by regulating
the pressure, and at providing fancier displays of the passage of time. Some
water clocks rang bells and gongs; others opened doors and windows to show
little figures of people, or moved pointers, dials, and astrological models of
the universe.
A Macedonian astronomer, Andronikos, supervised the construction of his
Horologion, known today as the Tower of the Winds, in the Athens marketplace
in the first half of the first century BCE. This octagonal structure showed
scholars and shoppers both sundials and mechanical hour indicators. It
featured a 24 hour mechanized clepsydra and indicators for the eight winds
from which the tower got its name, and it displayed the seasons of the
year and astrological dates and periods. The Romans also developed mechanized
clepsydras, though their complexity accomplished little improvement over
simpler methods for determining the passage of time.
In the Far East, mechanized astronomical/astrological clock making developed
from 200 to 1300 CE. Third-century Chinese clepsydras drove various mechanisms
that illustrated astronomical phenomena. One of the most elaborate clock
towers was built by Su Sung and his associates in 1088 CE. Su Sung's mechanism
incorporated a water-driven escapement invented about 725 CE. The Su Sung
clock tower, over 30 feet tall, possessed a bronze power-driven armillary
sphere for observations, an automatically rotating celestial globe, and
five front panels with doors that permitted the viewing of changing manikins
which rang bells or gongs, and held tablets indicating the hour or other
special times of the day.
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