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From: "::darkshadows::" <bat@cave.org>
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Subject: How does a sundial tell time?
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Date: Thu, 07 May 2009 05:54:43 -0500
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How does a sundial tell time?
A sundial is a simple, yet accurate, instrument, which tells time by
the movement of the shadow a pointer casts upon a dial, which marks
the hours of the day. The shadow moves as the sun changes position in
the sky throughout the day, and the dial tracks this movement.
For a sundial to be accurate, the pointer must be slanted at an angle
equal to the latitude of its location. Vertical pointers are useful at
showing the correct time only at one latitude and during one season,
and flat dials must have unequally spaced hour marks upon them for
accuracy.
Prior to the invention of the sundial, man guessed at the time of day
by observing the sun's movement from morning until night. Sunrise and
sunset required no calculation, but mid day, or noon, when the sun's
position in the sky was at its peak, proved to be more difficult to
predict. The times in between these three reference points left them
baffled.
After scratching their heads for quite some time, it finally dawned on
ancient man that shadows cast changed in length, and moved throughout
the day. They concluded that, instead of craning their necks to watch
the sun change position throughout the day, they could simply look
down at the ground and note the passing of time by observing the
change in shadow lengths. Additionally, this shadow clock method
proved to be more accurate.
Prototypes of the sundial most likely consisted of poles stuck in the
ground surrounded by rocks. The pole served as a pointer, and the
stones as markers. The use of poles as pointers gave way to immense
stone columns, one of which is Cleopatra's Needle in New York's
Central Park. Sundials of smaller proportions were used, such as a "L"
shaped Egyptian model dating back 3,000 years, which lays flat on the
longer of its two legs, which bears marks indicating periods of time.
Around 300 BC, a Chaldean astronomer invented a bowl-shaped sundial
with a pointer that cast a shadow onto the dial, which marked 12 hours
of the day as the shadow inched along. Because this sundial proved to
be highly accurate, its use continued for centuries.
More accurate, and portable, watches and clocks replaced sundials, as
time telling instruments, but sundials still exist as ornamental
additions to gardens. Oddly enough, some crude vertical sundials used
for telling time, may be found on the walls and windowsills of old
houses, and are configured so that a nail or the edge of the window
casing casts a shadow. Perhaps, they served as the poor man's answer
to the Swiss watch!
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