High Quality Content by WIKIPEDIA articles! 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 onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a sharp, straight edge. As the sun moves across the sky, the shadow-edge aligns with different hour-lines. All sundials must be aligned with the axis of the Earth`s rotation to tell the correct time. In most designs, the style must point towards true celestial north (not the north magnetic pole or south magnetic pole). That is, the style`s horizontal angle must equal the sundial`s geographical latitude.