Hi John and other dialists Perhaps a precise solution would be to calculate the intersection of the hour line with the enclosing frame of your sundial. It must be done by a computer but its easy to give a very good precision.
The result would be given as a length and a direction (north, east, south, west side of the sundial), the origin could be one of the two opposite corners. The only problem then is to precisely draw your frame, with parallel sides and a good perpendicularity. With one intersection point you can draw the line by joining it to the gnomon foot. I plan to include such kind of data in my Shadows program in a futur version. François Blateyron [EMAIL PROTECTED] Home page: http://web.fc-net.fr/frb/ Cadrans solaires / Sundials: http://web.fc-net.fr/frb/sundials/ -----Message d'origine----- De : John Carmichael <[EMAIL PROTECTED]> À : [email protected] <[email protected]> Date : lundi 24 mai 1999 03:21 Objet : plotting timelines for giant sundials >Hello dialists: > >I have been giving more thought to the practical aspects of designing and >constructing a very large sundial, particularly the problem of accurately >laying out the time lines. > >THE PROBLEM: The plotting techniques which use tabulated angles or >computation produce timeline plotting angles in degrees which the dialist >must mark onto the dial plate using a protractor. These angles will be as >precise as the number of decimal places used in the calculations. However, >even though one takes great care to obtain precise timeline angles, this >amount of precision is useless if one's protractor isn't equally precise. >The graphical plotting method also requires an accurate protractor, of course. > >SOLUTION A: By definition, large protractors are more precise than small >ones. So physically laying out the hour lines for a giant sundial would >require a giant protractor. Even Robert Terwilliger's laser trigon wouldn't >help because it's degree markings are too small. Computer drawn lines don't >help either, because you can't easily enlarge a small paper drawing by a >hundred fold. I'm thinking that during the construction phase of a very >large sundial, I could make a temporary giant protractor located just >outside the hourline radius. This would be a fairly simple thing to do using >plane geometry. > >SOLUTION B: If the unit square method is used (Waugh, pg. 40-43), then no >protractor is needed. One only needs a good long measuring tape for laying >out the lines. ( The limits of precision would again depend on the number of >decimal places used.) > >SOLUTION C: What if I built the gnomon first and use its shadow to tell me >the position of the time lines???? With this method, no calculations, >plotting, protractors or tape measures are needed. Using a shadow >sharpener, the exact position of each timeline could be marked onto the dial >face. Of course, using this method would require the proper EOT, DST, and >longitude corrections. This method would also work well on an irregular >surface. (I think) Marking the time lines would be easier and faster on >those days when EOT=0, right? > >Do any of you have any thoughts on this problem and which would be the best >solution? > >Thanks, > >John Carmichael >
