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sundial

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 BC, 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. AD) 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).

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sundial

sun·di·al / ˈsənˌdīl/ • n. 1. an instrument showing the time by the shadow of a pointer cast by the sun onto a plate marked with the hours of the day. 2. (also sundial shell) a mollusk (family Architectonicidae) with a flattened spiral shell that is typically patterned in shades of brown, living in tropical and subtropical seas.

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sundial

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Sundial

SUNDIAL

Although sundials can be traced back to earlier periods in Egypt and Mesopotamia, the earliest textual reference to a sundial appears to be ii Kings 20:8–11 (cf. Isa. 38:7–8). There a shadow-tracking device, ascribed to Ahaz, is used for a sign that Hezekiah would be healed. The Masoretic Text speaks of ma'alot ahaz, literally "the steps of Ahaz." Various ancient versions and commentators disagree over whether the device is an actual sundial or a set of stairs attached to Ahaz's palace. Unfortunately, 1QIsaa, which contains the variant ma'alot 'lyt, does not resolve this question.

A fragment of a portable, disk-shaped sundial excavated at Tel Gezer has been dated to the reign of Merneptah (1225–1215 b.c.e.) whose cartouches were inscribed on its back. Earlier representations of this type were found on the ceilings of the early 15th century tombs of Amenhotep I and Serenmuth. These appear as a circle subdivided by radiating lines into 24 equiangular sections.

A later development, the sundial found at Qumran, was shaped like a shallow bowl with three circular dials and a small vertical gnomon in its center. The upper dial was divided into approximately 90 sections. The middle dial resembled those known from i Enoch 72, with 18 equiangular 20° "parts." This appears to be a shallow form of the hemisphaericum of Aristarchus described by Vitruvius (end first cent. b.c.e.).

For the preceding two dials, since the increments were represented by equally spaced "steps" on each dial, and since the movement of the shadow of the gnomon travels faster at midday and slower at the day's beginning or end, the actual time that the shadow spent within each step varied accordingly. Also, the number of steps through which the shadow passed each day either increased or decreased depending on the season. The solstices, equinoxes, and months (or "gates") were tracked by noting where the first shadow of the gnomon became visible on the dial or by the rising of certain constellations at night.

The latest and most common sundials were the typical Greco-Roman, quarter-spherical hemicyclium, and the "conical" conicum. Twelve equiangular sections on these dials measured hours which in real time varied both according to the time of the day and season. Three concentric circles, running perpendicular to the hour lines, marked the full extent of the shadow at the four cardinal points of the year.

bibliography:

S. Adam, "Ancient Sundials of Israel," in: BSS Bulletin 14 (2002), 52–57, 109–114; Y. Yadin, "Ma'alot Ahaz," in: Eretz Israel 5 (1959), 91–96; pl. 10 (Heb.).

[Stephen Pfann (2nd ed.]

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