A sundial is a silent and motionless instrument that indicates solar time by moving the shadow of an object of variable shape, the gnomon or the style, on a surface, the table of the dial, associated with a set of graduations plotted on this surface. The table is generally flat but can also be concave, convex, spherical, cylindrical, etc.
The gnomon usually indicates the time by the length or direction of its shadow. On the current dials, the shadow-bearing element is generally an axis (or the edge of a plane) inclined parallel to the axis of rotation of the Earth or axis of the world. It then takes the name of "polar style". This inclination, the angle of which depends on the latitude of the place, makes it possible to read the time all year round directly on the same set of graduations: the range of hour lines. The sundials have several forms: round, rectangle, square, etc. The sundial is considered, because of its simplicity, as one of the first objects used by man to measure the flow of time. The oldest known solar indicators were found in Egypt, but they indicated only indefinite moments and not hours in the sense we hear them today. The first real sundials are probably polos and scaphé, based on the sphere, supposed to be introduced by Berose in ancient Greece in the fourth century BC. J.-C .; other models derived from it (hemispherical, conic, plan ...), inventoried in the ancient dials. © Photo of Emmanuel Buchot
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