Sundials

A sundial is a device that determines the time of day 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. 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.
Some Archeological examples









 From Beijing