Hi Folks, I feel that I should comment on this as it was me that initially told John Close that he couldn't achieve what he wanted in any simple way, when he came for a day's tutorial (he's a relative novice at dialling).
The problem is that a polar dial is not equiangular or have equally spaced hour lines. Thus if one attempts to rotate the dial about the polar axis, the amount of twist has to be altered on an hour-to-hour basis, not just on a day-to-day one. The solution, I think is the suggestion of Fer de Vries to use the polar dial designed originaly by his namesake. This has a polar dialplate and a gnomon that is shaped on its UNDERSIDE, I think (from memory) that the shape required is a cycloid. This was shown in the NASS Compendium a few years back. The gnomon only touches the dialplate on the split noon lines with a gap increasing under it. Thus the distance of the part of the gnomon that casts the indicating shadow moves further from the dialplate, and in the E'W directions, for increasing times from noon. The hourlines are straight and equi-spaced so the whole dial can be rotated to account for EoT, BST and longitude. For some reason, Fer's reply didn't seem to come to all the list members, though John forwarded it to me. I will be interested to see if anyone can come up with a more conventional "hourglass" gnomon solution - I believe it is impossible (there's a challenge!). Regards, John -------------------------------------- > Hello Frans, and sundial list members -- > > Thanks for your interest. > > I apologize for any confusion caused by my use of the phrase "for use > with straight hour lines." The words were mine, not John Close's, and > I guess I was trying to reinforce his desire to have a polar dial > which didn't use wavy hour lines, whether the lines looked like 8's > (full analemmas), S's (half analemmas), or stacked S's (unfolded > analemmas). > > I suggested to John that if for civil time he wouldn't use wavy > lines, he might pivot his dial around an axis parallel to the > Earth's, changing the dial's facing direction slightly every day (or > every few days) to account for EoT. Some dialists feel that a pivoted > polar is no longer a polar, but that's a different problem, for > "polar dial" seems to have some different meanings, depending on what > source is consulted. > > A 3-D gnomon on a polar dial would cast a wavy shadow on the flat > dial plate, and civil time could be read where that shadow crossed a > straight line of hour points. That is, if a suitable 3-D shape is > possible. Some think yes, some are skeptical, some say no. At the > moment, I'm in the skeptical group. What do you think? > Dr J R Davis Flowton Dials N52d 08m: E1d 05m -
