Sundial is a device that uses the position of the sun to measure time. It is mainly composed of an index that casts the shadow of the sun, a projection surface that bears the projection of the index (i.e. sundial surface) and a scale line on the sundial surface.

The sundial can be designed on the surface of any object to make the fixed pointer produce shadow to measure time. Therefore, the sundial has many different forms, which can be basically divided into horizontal sundial, equatorial sundial, meridian sundial, Mao unitary sundial and so on.

Source: @chingonarte96