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.
