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A sundial is a device that 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, which is a thin rod or a sharp, straight edge), onto a flat surface marked with lines indicating the hours of the day. The style is the time telling edge of the gnomon. As the sun moves across the sky, the shadow-edge progressively aligns with different hour-lines on the plate. Such designs rely on the style being aligned with the axis of the Earth's rotation. Hence, if such a sundial is to tell the correct time, the style must point towards the true north (not the north magnetic pole or south magnetic pole) and the style's angle with horizontal must equal the sundial's geographical latitude.


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