Gnomon

A gnomon is the name of simplest the Sundial: a stick planted vertically in the ground, or even even simpler the man himself. He is known since antiquity. The hour can be determined either according to the length of the shade, or according to its orientation.

By extension, the gnomon indicates the part of a Sundial which forms the shade, but it is to better use “style”.

Generally, the style is parallel to the axis of rotation of the Earth, it acts of a polar Cadran. On certain dials, however, the style is perpendicular to the dial and only its end is used for its reading, in this case the usual name is gnomon, or right style. On more worked out dials, the form of the style makes it possible to directly take into account the correction of the equation of time.

The gnomon gave its name to the art of the construction of the sundials: the gnomonic .

On the photograph on the right the gnomon perpendicular to the plate of the dial projects the end of its shade at solar midday on a curve into eight which represents the equation of time throughout the year; the detail opposite makes it possible to see the indication of the date, here mid-March.

The two curves in eight partial on the left for the time being make the corrections of summer and the hour of winter.

One will also note the almost horizontal curves of the advance of the end of the shade, throughout one day, each curve corresponds to a specific date (birthday) drawn by his author the gnomoniste.



Geometry

In Geometry, a gnomon is a plane figure formed by removing a parallelogram, of a corner of a larger parallelogram. It is a particular case of the gnomon pythagorician, a geometrical figure which must be added to a given figure, so that the new figure is similar the latter.

Being given a continuation of illustrated numbers, the gnomon is a provision of points in a plan, representing a number, and forming a model which makes it possible to obtain by juxtaposition with the figure corresponding to an illustrated number of the continuation, the figure of the number of following row. This process is described in the article on the numbers of illustrated, in which square numbers are built starting from the gnomon of a odd Nombre.

The gnomonic projection (or central ) is a projection of circles of a sphere, whose center (eye from which the rays leave) is the center of the sphere, on a tangent sighting plane to the sphere. It is used in cartography, and especially for the aerial navigation: its advantage is to represent the large circles of the sphere by lines.

See too

  • Gnomoniste

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