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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 initiated
clock-making, using primitive
yet accurate sundials. With
their bureaucracies and
formal religions, these
cultures found a need to
organize their time more
efficiently.
After
the Sumerian culture was
lost without passing on
its knowledge, the Egyptians
were the next to formally
divide their day into parts
something like our hours.
Obelisks (slender, tapering,
four-sided monuments acting
as a type of sundial) were
built as early as 3500 B.C.
Their moving shadows formed
a kind of sundial, enabling
citizens to partition the
day into two parts by indicating
noon. They also showed the
year's longest and shortest
days when the shadow at
noon was the shortest or
longest of the year. Later,
markers added around the
base of the monument would
indicate further time subdivisions,
thus these monuments then
became true sundials.
Another
Egyptian shadow clock or
sundial, possibly the first
portable timepiece, came
into use around 1500 B.C.
to measure the passage of
“hours.” 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 B.C. A pair of
merkhets were used to establish
a north-south line by lining
them up with the Pole Star.
They could then be used
to mark off night time hours
by determining when certain
other stars crossed the
meridian.
In
the quest for more year-round
accuracy, sundials evolved
from flat horizontal or
vertical plates to more
elaborate forms. One version
was the hemispherical sundial,
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
B.C., 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
B.C., Vitruvius could describe
13 different sundial styles
in use in Greece, Asia Minor,
and Italy.
Source:
National Institute of Standards
and Technology Physics Laboratory
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SUNDIALS
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