Armillaries And Sundials
A sundial is a device that measures time by the position of the Sun. In designs such as the horizontal sundial, the sun casts a shadow from its style onto a plane 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).
Alternatively, the surface receiving the shadow need not be a plane, but can have any shape, provided that the sundial maker is willing to mark the hour-lines. If the style is aligned with the Earth's rotational axis, a spherical shape is convenient since the hour-lines are equally spaced - the sundial is equiangular. This is the principle behind the armillary sphere.