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Old 06-11-2019, 09:15 AM
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Chronos is online now
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Join Date: Jan 2000
Location: The Land of Cleves
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Quoth Chessic Sense:

You don't need to estimate the length of a shadow to find noon. You plant an object with an obvious tip in the sand, standing vertically. A water bottle will do. At some point in the day, at any time, you mark the shadow's tip with any object. Another water bottle will work, especially if laid on its side, but I'd prefer a seashell or pebble.

Then you wait at least 15 minutes, but preferably several hours. You then mark the tip again. Connect the two marks with a line. Wait a few hours more and verify with a third point.

This line is a direct east/west line. It's not an arc.
The reason nobody suggested that is because it only works at the equinox. At any other time of year, that line will indeed curve. If I were to do it today, for instance, at noon the shadow would be north of the gnomon, and at sunrise and sunset, the shadow would be south of the gnomon. It's tough to make a straight line do that.