SUNDIAL

earliest of the three types of timekeeping devices, which indicates the time of day by the position of the shadow of some object on which the sun's rays fall. As the day progresses, the sun moves across the sky, causing the shadow of the object to move. The first device for indicating the time of day was probably the gnomon, dating from about 3500 BC. It consisted of a vertical stick or pillar; the length of the shadow that it cast gave an indication of the time of day. By the 8th century BC, more precise devices were in use; the earliest known sundial still preserved is an Egyptian shadow clock of green schist dating at least from this period. It consists of a straight base with a raised crosspiece at one end. The base, on which is inscribed a scale of six time divisions, is placed in an east-west direction with the crosspiece at the east end in the morning and the west end in the afternoon. The shadow of the crosspiece on this base indicates the time. Clocks of this kind are still in use in primitive parts of Egypt. Another early device was the hemispherical sundial or hemicycle, attributed to the Babylonian astronomer Berosus, about 300 BC. Made of stone or wood, the instrument consisted of a cubical block into which was cut a hemispherical opening; to this was fixed a pointer or style, the end of the style lying at the centre of the hemispherical space. The path travelled by the shadow of the style was, approximately, a circular arc. The length and position of the arc varied according to the seasons, so an appropriate number of arcs was inscribed on the internal surface of the hemisphere. Each arc was divided into 12 equal divisions, and each ray, reckoned from sunrise to sunset, had, therefore, 12 equal intervals or hours. Because the length of the day varied according to the season, these hours, likewise, varied in length from season to season and even from day to day and were consequently known as temporary hours. The dial of Berosus was widely used for many centuries, and, according to the Arab astronomer al-Battani (Albategnius, c. AD 858929), was still in use in Muslim countries during the 10th century. The Greeks, with their geometrical prowess, developed and constructed sundials of considerable complexity. Apollonius of Perga (c. 250 BC) developed the hemicyclium by using a surface of conic section upon which the hour lines were inscribed; this arrangement gave greater accuracy. Ptolemy used the analemma, a device that enabled shadows to be projected geometrically onto flat surfaces inclined at various angles to the horizontal. In general, it appears that the Greeks constructed instruments with either vertical, horizontal, or inclined dials, indicating time in temporary hours. The Tower of the Winds in Athens, octagonal in shape, and dating from about 100 BC, contains eight sundials. It is, therefore, clear that dials facing various cardinal points were in use for a long time. The Romans also used sundials with temporary hours. The first dial set up in Rome in 290 BC had been captured from the Samnites, but it was not until almost 164 BC that a sundial was actually constructed for the city. In his great work De architectura, the Roman architect and engineer Marcus Vitruvius Pollio (1st century BC) mentions many types of sundials, some of which were portable. The Arabs attached much importance to the study of sundials, the principles and design of which they derived from the Greeks. They increased the variety of designs available and, at the same time, simplified the processes of design and construction by using the principles of trigonometry. Abu al-Hasan, at the beginning of the 13th century AD, wrote on the construction of hour lines on cylindrical, conical, and other surfaces and is credited also with introducing equal hours, at least for astronomical purposes. With the advent of mechanical clocks in the early 14th century, sundials with equal hours gradually came into general use.