|A sundial is an instrument specifically designed for determining the hour of the day by projecting the sun's shadow or pinhole image on a set of hour lines. These hour lines can be marked on a flat surface (horizontal, vertical or arbitrarily inclined) or a curved surface (spherical, cylindrical or more complex). The shadow is usually cast by an upright or inclined gnomon (Greek: 'pole') but in some cases a small image of the sun is produced by a suitably located pinhole aperture (which is also termed the gnomon). In this respect a sundial has to be distinguished from the simple gnomon or the meridian line. The latter instruments only determine the exact hour of noon, or, if the meridian line is graduated with a calendar scale, the day of the year.|
The history of the sundial reaches far back into antiquity and the earliest descriptions and examples date from the Egyptian Period (around 1500 B.C.). Also from ancient Greece and Rome numerous examples, large as well as pocket-sized, are preserved. Especially in the Islamic world the need for observing the daily prayer times, a number of which are mathematically defined by proscribed altitudes of the sun above the horizon, further stimulated the development of instruments for observing the time from the Sun's altitude.
In medieval and early modern Europe the sundial was by far the most commonly used instrument for determining the time. From the 16th to the 18th century the steady flow of books and manuals on sundials and their use produced by mathematicians, astronomers and instrument makers attests to the popularity of this instrument and the great variety in its design and construction. Even the development of the mechanical clock from the 14th century onwards by no means made the sundial obsolete, despite improvements in the second half of the 17th century with the introduction of the pendulum and the balance-spring. Until far into the 19th century, an accurate sundial was essential for regularly checking and adjusting the rate of mechanical timekeepers.
Sundials can commonly be divided into two groups:
1) Altitude dials: here the time is determined from the sun's altitude (i.e. angle above the horizon). In some cases the dial has to be properly oriented to the compass directions, in other cases the dial has to be aligned to the sun.
2) Direction dials: here the time is determined from the sun's azimuth (compass direction) or hour angle (the angle along its daily arc before or after the meridian passage). In most cases the dial has to be correctly oriented and for that purpose a magnetic compass is often incorporated in the instrument.
Altitude dials can be further subdivided into:
i) Plane dials: the sun's shadow or pinhole image is cast on a set of hour lines inscribed on a horizontal, vertical or arbitrarily inclined flat surface.
ii) Cylinder dials: the surfaces bearing the hour lines can be either convex or concave. A common example of this type is the shepherd's dial. Another term for instruments in this category is pillar dial.
iii) Scaphe dials: the hour lines are inscribed on the inner surface of a spherical or a conical cup.
iv) Ring dials: the sun's pinhole image is projected on a set of hour lines inscribed on the inner surface of a thin cylindrical ring.
Direction dials can be further subdivided into:
i) Horizontal dials: the hour lines are laid out on a horizontal plane surface and all converge to the foot of the gnomon that is directed to the celestial pole.
ii) Vertical dials: similar to above, with the hour lines laid out on a vertical plane surface that usually faces south.
iii) Polar dials: dials with a gnomon directed to the celestial pole with hour lines laid out on a plane surface with an arbitrary inclination (i.e. neither horizontal nor vertical).
iv) Equinoctial dials: here the hour lines are equally spaced and are inscribed either on a plane surface parallel to the celestial equator or on a spherical or cylindrical surface with a symmetry axis perpendicular to the same. Dials under this class include the globe dial, the universal dial, the self-setting dial, the mechanical dial, the astronomical ring-dial, the universal ring-dial, the crescent dial and the crucifix dial.
v) Azimuthal dials: in this type the dial is oriented in such a way that the sun's shadow falls along a specified line, the hour is then indicated by the needle of an in-built compass.
vi) Multiple dials: these include dials with two or more sets of hour lines laid out on multiple plane surfaces, such as diptych dials, polyhedral dials and dials in compendia.
In additional to conventional sundials, it is also possible to have moon or lunar dials, usually in the form of a sun and moon dial. In principle, a sundial can also be used during the night, provided that the moon is sufficiently bright and that the lunar age is known. The 'solar time' can then be obtained from the 'lunar time' (both expressed in equal hours) by adding four-fifths of an hour for each day of the lunar cycle. In a moondial the correction is effected automatically by adjusting the hour scale for the lunar age.
J. Drecker, Gnomone und Sonnenuhren (Aachen, 1909); J. Drecker, Zeitmessung und Sterndeutung in geschichtlicher Darstellung (Berlin, 1925).
S. L. Gibbs, Greek and Roman Sundials (New Haven and London, 1976); P. Gouk, The Ivory Sundials of Nuremberg 1500-1700 (Cambridge, 1988); K. Higgins,"The Classification of Sun-dials", Annals of Science, 9 (1953), 342-58; D. A. King,"Mizwala", in The Encyclopaedia of Islam (2nd edition, Leiden, 1991), vol. 7, pp. 210-1; reprinted in D. A. King, Astronomy in the Service of Islam (Aldershot and Brookfield, 1995), paper no. VIII; S. A. Lloyd, Ivory Diptych Sundials 1570-1750 (Harvard, 1992). R. R. J. Rohr, Sundials: History, Theory, and Practice (Toronto, 1970); A. J. Turner,"Sun-dials: History and Classification", History of Science, 27 (1989), 303-18; reprinted in A. J. Turner, Of Time and Measurement: Studies in the History of Horology and Fine Technology (Aldershot and Brookfield, 1993), paper no. II; A. J. Turner, 'Sun-dial', in: R. Bud & D. J. Warner (eds.), Instruments of Science: An Historical Encyclopedia (New York and London, 1998), pp. 588-9.
Robert van Gent