Quick Answer
To calculate approximate time using shadow lengths, one must know their latitude and the angle of the sun at their location, typically between 10 and 20 degrees from the vertical. By measuring the length of a shadow and comparing it to an object's height, you can estimate the time. This method, called natural navigation, relies on the sun's predictable shadow patterns.
Understanding Shadow Patterns
Natural navigation relies on the sun’s shadow patterns to estimate time. As the sun moves across the sky, its shadow shortens and lengthens at predictable rates. At the equator, the sun’s shadow is longest at 6:00 AM and 6:00 PM, and shortest at noon. At higher latitudes, the shadow is longest in winter and shortest in summer.
Measuring Shadow Lengths
To estimate time, measure the length of your shadow and compare it to an object’s height. Choose an object with a known height, such as a stick or a rock. Hold the object vertically and measure the shadow’s length. Use the sun’s angle, your latitude, and the object’s height to estimate the time. For example, if you’re at 30°N latitude and the sun’s angle is 15°, the shadow will be approximately 1/2 the object’s height at 12:00 PM. In the morning, the shadow will be longer, and in the afternoon, it will be shorter.
Using the Sundial Technique
To further refine your estimate, use the sundial technique. Draw a vertical line on a flat surface, representing the sun’s shadow. Mark the end of the shadow with a small stick or rock. As the sun moves, mark the new end of the shadow. By counting the number of marks, you can estimate the passage of time. This technique is most accurate near the equator, where the sun’s shadow moves more slowly.
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