The Gnomoscope is a device I have invented in 1986, to draw simply and accurately the lines of a sundial with fixed axial style. No complicated computations are required, and the process is valid everywhere, whatever the surface of the sundial (flat or curved), whatever its orientation (exactely southward facing or not, horizontal, vertical or tilted).
This device is patented (86 13297), and published in Bulletin Officiel de la Propriété Industrielle, number 11, march 18, 198
Copie du brevet d'invention (PDF 1.1 Mo)
The way it works?:
A sundial with fixed axial style is an ancient tool to read the time. Usually, a sundial has a sun-lit surface with lines indicating hours, and a shadow-casting rod called "the style" which is, in most cases, fixed and parallel to the North-South axis of the Earth. The shadow of the style indicates the true local solar time. Sometimes, the rod is replaced by the straight and sharp edge of a plate.
Assuming the style is correctly oriented and firmly cemented, the problem of drawing the hour-lines still remains. For perfectly flat and well-oriented surfaces, there exist mathematical/geometrical methods to draw these lines. If the surface is not perfectly south-facing, these methods still work is suitably modified (and complicated!). For curved of simply not perfectly flat surfaces, such methods become really difficult to use, and require a lot of skill.
The device under consideration here has been invented to solve this problem. It is an electric lamp surrouned by a suitably designed mask. This tool has to be plugged on the tip of the style. When switched on, it casts some shadows onto th surface of the sundial. Those shawdos are the hour-lines of the sundial. It is then easy to paint or carve along these shadows, to get an accurate sundial. Of cours, this operation has to be done by night, for a better contrast of the shadows.
The shape of the shadow-casting mask is universal (up to a scale factor), so, one single mask is needed to operate everywhere in the world, on any kind of surface (a flat wall, a stone wall, a dome, a corrugated roof, or any architectural shape), even for a non perfectly south-facing wall, or an inclinated wall. The only constraint is to have a correctly oriented style. This can be done by various simple techniques.
A sample sundial made with this instrument:
Horizontal sundial constructed for a public scientific event at the University of Nice-Sophia Antipolis (UNS), lors de la Fête de la Science 2006 (du 11 au 14 october 2006).
