The Challenge: Build accurate clocks
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How does an accurate clock work?
As the earth turns on its axis, the sun appears to move across our sky and the shadows cast
by objects North of the tropics move in a clockwise direction around the sun.
If you watch a shadow
during the day cast by a rock, a tree, or even your own body, you will
notice that it is quite long in the morning and decreases in length until
noon when it is at its shortest.
The shadow will then grow longer again as night approaches. You can tell
what time of day it is by observing a shadow, but it's not as simple as
stick was the earliest form of sundial. People judged the time of
day by the length and position of the stick's shadow. If the sun rose
and set at the same spot on the horizon every day, a shadow stick would
be pretty accurate. However, the sun's path through the sky changes each
day because the earth's axis is tilted. On the earth's yearly trip around
the sun, the North Pole is tilted towards the sun half of the time and
away from the sun for the other half.
In addition, because
the sun doesn't often pass directly overhead at noon and the earth's surface
is curved, the shadow cast by a shadow stick doesn't move at a uniform rate.
So even if we mark the shadows at sunrise, noon and sunset we cannot evenly
divide the space up to mark the individual hours.
More problems with sundials