"Frans W. MAES" wrote: > > I know one more case of > an interesting bifilar dial. Using a pole style and a specially shaped > curve in the equatorial plane, one may obtain a polar dial with > straight, parallel E-W date lines, perpendicular to the hour lines. > This principle was described in the Bulletin of the Dutch Sundial > Society in 1979 by Th.J. de Vries. > > An example is found in Appingedam, a cute little city in the very > north of the Netherlands. It was constructed in 1988. A picture is in > the sundial pages of my website. > > As the shape of the second, curved line could not be seen well, > I just added some pictures. The deep-linking URL is: > http://www.biol.rug.nl/maes/zonwyzer/en/zwappi-e.htm >
This is an exciting sundial. Who would have guessed that you could achieve straight, parallel, date lines? Brilliant. Is the formula for the curve available, please? (Don't tell me - it's a catenary, right?) > I have searched the web for additional information on Mr. Soler's > motivation. I found a large pole-style bifilar dial, including picture, in > Barcelona, where the catenary is a parabola, at: > http://www.ub.es/geohum/inventari/fitxes/invt106.htm > > > I found a picture (without explanation) of another, unique bifilar dial > by Mr. Soler at: > http://sundials.org/conference/1999/confpix/reutte.htm I have seen an explanation of this somewhere. The gnomon consists of a disc on top of a thin vertical rod. The time is read at the point where the shadows of the edge of the disc and the rod intersect. I am unaware of any theoretical merit in this arrangement, other than novelty, although I suppose it is safer than a thin rod on its own. Safety is an important consideration in public places. It seems Mr Soler likes making sundials that use the intersection of two shadows. Perhaps just for the amusement, but perhaps to gain some other advantage. Chris Lusby Taylor Newbury, England 51.4N 1.3W
