Hello again: Thank you all for confirming that the lines would be the same shape if drawn at any time of the year. As some of you pointed out, this technique for drawing hour lines will only work using a polar axis style. If you use a vertical obelisk-type gnomon this method would not work because the shadow would only mark a point and not a line. (However, if the face was flat and horizontal and not irregular, then you could draw a line from the point to the dial center and this would give you a straight hour line.)
If you insist on having a vertical gnomon on an irregular face and want to draw the hour lines using the shadow tracing technique, then you would first have to build a temporary polar axis gnomon to draw the hour lines, then remove it and replace with a vertical gnomon. Is this correct. Thanks John > >Yes it will work, provided the gnomon is of the usual sort pointing to the >celestial pole. > >At any given hour on a given date the Sun and the gnomon define a plane. >The intersection of that plane with the (irregular) ground surface defines >an (irregular) line on the ground. At the same hour (duly allowing for EoT) >on another date the sun has moved "sideways" in that plane if its >declination has changed but it's still the same plane, so it still >intersects the ground in the same way. All that has happened is that the >bit of shadow corresponding to a particular point on the ground is being >cast by a different point on the gnomon. This is why the polar gnomon >works. If you imagine the ground as made up of a small number of moderately >big flat sheets, you can have bits of non-horizontal dials on each of them - >inclining, reclining, declining, whatever - sharing the same gnomon. You >know each of those works perfectly well throughout the year by itself - so >all of them together will, too. Now increase their number and you have an >approximation to any curved surface such as your not flat ground. They all >still work! > >Regards >Andrew James > >------- re: >From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] >Sent: 09 February 2000 15:08 >To: [email protected] >Subject: drawing hour lines using gnomon > >
