Sundial is one of the oldest known devices for the measurement of time. It is based on the fact that the shadow of an object will move from one side of the object to the other as the sun moves from east to west during the day. The sundial is believed to have been used in Babylon at least as early as 2000 B.C.
The earliest description of a sundial comes from Berossus, a Babylonian priest and author of the 200’s B.C. His sundial was a hollow half-sphere, or dome, set with its edge flat and with a small bead fixed at the center. During the day the shadow of the bead moved in a circular arc, divided into 12 equal parts. These were called temporary hours because they changed with the seasons. Equal hours were decided upon about A.D. 1400, when clocks were invented.
A sundial consists of the plane (dial face) and the gnomon (style). The dial face is divided into hours and sometimes half and quarter hours. The gnomon is a flat piece of metal set in the center of the dial. It points toward the North Pole in the Northern Hemisphere and toward the South Pole in the Southern Hemisphere. The upper edge of the gnomon must slant upward from the dial face at an angle equal to the latitude of the location of the sundial.
