A sundial is a device that tells time using the position of the sun. Most sundials have two primary components: a style, also called a gnomon, and a numbered dial. The sundial uses the sun to cast a shadow from the style over the numbered dial.
The numbered dial has multiple lines to indicate the hour of day. The style or gnomon is a pointer that resembles a straight edge or a thin rod. When the sun moves during the day, the shadow-edge of the style aligns with a corresponding hour line. To tell the correct time, the style on the sundial must be parallel to the axis of the Earth's rotation. Sundial designs include the traditional horizontal design and vertical designs that can be placed on the sides of buildings or other structures.