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Sundials are one of the oldest tools for measuring time using the shadow of the Sun. The Egyptians used a shadow stick or shadow clock as early as 1500 BC. The vertical stick or "gnomon" marked the time of day by the length and position of the stick's shadow. Gnomon in Greek means "the one that knows." Sundials are often mounted on a base while some are designed to be hung vertically on a building, wall or tree.