sundial, instrument that indicates the time of day by the shadow, cast on a surface marked to show hours or fractions of hours, of an object on which the sun's rays fall. Although any object whose shadow is used to determine time is called a gnomon, the term is usually applied to a style, pin, metal plate, or other shadow-casting object that is an integral part of a sundial. Forerunners of the sundial include poles or upright stones used as gnomons; pyramids and obelisks were so used in Egypt. Both stationary and portable sundials were probably developed in Egypt or in Mesopotamia. The earliest extant sundial, an Egyptian instrument of c.1500 B.C., is a flat stone on which is fixed an L-shaped bar whose short vertical limb casts a shadow measured by markings on the longer horizontal limb. The sundial was greatly improved (c.1st cent. A.D.) by setting the gnomon parallel to the earth's axis of rotation so that the apparent east-to-west motion of the sun governs the swing of the shadow. The development of trigonometry permitted precise calculations for the marking of dials and stimulated the advance of gnomonics (dial marking). Although watches and clocks came into popular use in the 18th cent., sundials were long employed for setting and checking them. The heliochronometer, a highly accurate instrument in which the shadow is cast by a fine wire, was used until c.1900 to set the watches of French railwaymen. Solar (or apparent) time indicated by sundials and clock (or mean) time are different and must be correlated by the use of tables showing daily variations in sun time. A correction must also be made for the difference in longitude between the position of a sundial and the standard time meridian of a given locality. Although sundials are still used in many areas, including Japan and China, they are regarded today chiefly as adornments. The largest sundial in the world, constructed c.1724 in Jaipur, India, covers almost one acre (.4 hectare) and has a gnomon over 100 ft (30 m) high surmounted by an observatory. Notable collections of sundials are at the Adler Planetarium, the Metropolitan Museum of Art, and the Harvard College Observatory.
See F. W. Cousins, Sundials (1969); R. R. J. Rohr, Sundials (tr. 1970).