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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).
an instrument for telling time by the sun. A sundial consists of a gnomon, which may be a rod or triangular plate, and a graduated plate on which the gnomon casts a shadow. The place on the dial plate where the shadow falls indicates the apparent solar time. Three types of sundials are distinguished according to the position of the dial plate: equatorial, horizontal, and vertical. In all three types, the rod or the edge of the gnomon is made parallel to the earth’s axis and intersects the dial plate at the dial’s “center,” which is the point from which the hour lines radiate. The line corresponding to noon is located in the plane of the meridian passing through this center.
In equatorial sundials, the plane of the dial plate is parallel to the plane of the celestial equator. The dial plate is calibrated with equally spaced graduations on the basis of 360° being equal to 24 hours.
In horizontal sundials, the dial plate is horizontal and is calibrated in accordance with the equation
tan x = (tan t)(sin φ)
where x is the angle formed at the center of the dial by the given hour line and the noon line, t is the hour angle of the sun (the apparent solar time), and φ is the geographic latitude of the place where the dial is located. The hour lines for6 A.M. and 6 P.M. are always perpendicular to the noon line.
Vertical sundials are usually found on the walls of various structures. Consequently, the plane of the dial plate may have any azimuth. In such sundials, the hour lines are symmetric with respect to the noon line only when the dial plate is perpendicular to the meridian. In this case, the equation for the calibration of the dial has the form
tan x = (tan t)(cos φ)
Various types of portable sundials are also known.
The position of the shadow on the dial plate indicates the apparent solar time. To obtain the mean solar time, the equation of time must be added to the apparent solar time. Standard, or zone, time can be obtained by taking into account an additional correction that depends on the time zone and the geographic longitude of the place where the sundial is located. The accuracy of sundial readings is generally not better than several minutes.