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 it sounds.

The shadow 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 

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