sundial -
A simple timekeeping device in which the shadow cast by a vertical pole or plate the gnomon is used to indicate the time of day.
Wikipedia
A sundial is a device that uses sun to measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style (also called its Gnomon, a thin rod or a sharp, straight edge) onto a flat surface marked with lines indicating the hours of the day. As the sun moves across the sky, the shadow-edge progressively aligns with different hour-lines on the plate.
OmegaWiki Dictionary
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sundial A device used to tell the time with the help of the sun.
