Date sent: Tue, 17 Oct 2000 11:54:34 +0100 From: Chris Lusby Taylor <Chris.Lusby- [EMAIL PROTECTED]> Organization: Oracle Corporation To: "Frans W. MAES" <[EMAIL PROTECTED]> Copies to: Warren Thom <[EMAIL PROTECTED]>, sundial list <[email protected]> Subject: Re: Genk Sundial Park website
Dear Chris and others, > Chris Lusby Taylor wrote: > > I, too, would appreciate an English translation, as I have no idea why a > catenary has any special properties that commend its use in a bifilar > sundial. > > I have to admit to being rather a purist (read 'pedant') in that it seems > to me that the original bifilar sundial was invented to give the > remarkable property that the elliptical dial of a 'normal' horizontal can > be stretched into a perfect circle, with equally spaced hour lines and all > the benefits that gives. (The way I visualise the bifilar sundial is that > it handles the N-S and E-W scaling of the dial separately.) Many recent > so-called bifilar sundials merely use the intersection of the shadows of > two rather arbitrary lines/curves. These have no special properties other > than novelty. > > I hope we (English speakers) will learn that the catenary/polar gnomon > combination does indeed have some special property. In addition to the examples you mention, 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 Returning to the case of the catenary, I too could not see anything special in using the catenary, other than the mathematical recreation involved. (I too love mathematical recreations) The documentation I have does not say anything about the question of why Mr. Soler used a catenary here. Actually, the part of the catenary that is responsible for the intersecting shadows is not curved too much, so that the date lines are not too different from a 'normal' horizontal dial, such as Genk nr. 2. In fact, the date line for the equinoxes is so slightly curved, that it hardly shows up in the pictures I took. Having this instead of a straight E-W line seems to me not so much of an advantage. I am curious to hear other opinions, though. 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 The text is in Spanish, which I don't read. Another link for this dial (also Spanish, no picture): http://www.gnomonica.org/es/it1.htm I found a picture (without explanation) of another, unique bifilar dial by Mr. Soler at: http://sundials.org/conference/1999/confpix/reutte.htm (bottom of page). Has that design actually been realized somewhere? Does anyone know of other existing bifilar dials, whether gnomonically or mathematically interesting? Regards, Frans ===================================== Frans W. Maes Peize, The Netherlands 53.1 N, 6.5 E www.biol.rug.nl/maes/ =====================================
