Re: 3-D gnomon for Polar Dial
Hi Mac and all, I am afraid I don't quite understand the question. John would not like analemma shaped hour lines, you wrote. How might figure-8 shaped hour lines be used here?? One can simply incorporate the EoT into the hour lines of a common polar dial as a function of date, running from January to December. No figures-8, just somewhat wavy lines. And a date scale, of course. If one would manage to make a wavy gnomon, one gets a wavy shadow falling on straight hour lines. Not much of a difference, I would say. And one would also need a date scale to read the time. So what is the question actually? Regards, Frans 53.1N. 6.5E - Original Message - From: Mac Oglesby [EMAIL PROTECTED] To: Sundial Mail List sundial@rrz.uni-koeln.de Cc: John Close [EMAIL PROTECTED] Sent: Monday, December 09, 2002 3:21 PM Subject: 3-D gnomon for Polar Dial Hello All, John Close has posted this message on the NASS Message Board: Does anyone have any ideas for a Polar Dial which allows for an Equation of Time adjustment. I would not like analemma shaped hour lines as I think this would detract from the simplicity and minimalistic quality of a Polar Dial. I am told an analemma shaped gnomon for a polar dial would be hideously complicated and probably not work . Any ideas anyone? John Close John has discussed this problem with several members of this list, including me, Bill Gottesman, John Davis, and Pete Swanstrom. Some of us think that a 3-D gnomon could be designed for use with straight hour lines to correct for EoT, and some are skeptical. Please share your comments and suggestions. Best wishes, Mac Oglesby P.S. Since I do not know if John Close is a member of this List, please cc to him. - -
Re: 3-D gnomon for Polar Dial
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? So that's the question--can a 3-D gnomon be designed for a normal polar dial to give civil time where the gnomon's shadow intersects a straight line of hour points? If yes, what does the shape look like? If no, why not? Best wishes, Mac Oglesby P.S. Bill Gottesman has sketched a possible solution and has given permission to share. Contact me OFF LIST if you want a copy of his drawing (16K gif). P.P.S. Looking a a current list (today's) of members of the sundial list, it appears that John Close still needs to be cc'd at John Close [EMAIL PROTECTED] [period] Hi Mac and all, I am afraid I don't quite understand the question. John would not like analemma shaped hour lines, you wrote. How might figure-8 shaped hour lines be used here?? One can simply incorporate the EoT into the hour lines of a common polar dial as a function of date, running from January to December. No figures-8, just somewhat wavy lines. And a date scale, of course. If one would manage to make a wavy gnomon, one gets a wavy shadow falling on straight hour lines. Not much of a difference, I would say. And one would also need a date scale to read the time. So what is the question actually? Regards, Frans 53.1N. 6.5E - Original Message - From: Mac Oglesby [EMAIL PROTECTED] To: Sundial Mail List sundial@rrz.uni-koeln.de Cc: John Close [EMAIL PROTECTED] Sent: Monday, December 09, 2002 3:21 PM Subject: 3-D gnomon for Polar Dial Hello All, John Close has posted this message on the NASS Message Board: Does anyone have any ideas for a Polar Dial which allows for an Equation of Time adjustment. I would not like analemma shaped hour lines as I think this would detract from the simplicity and minimalistic quality of a Polar Dial. I am told an analemma shaped gnomon for a polar dial would be hideously complicated and probably not work . Any ideas anyone? John Close John has discussed this problem with several members of this list, including me, Bill Gottesman, John Davis, and Pete Swanstrom. Some of us think that a 3-D gnomon could be designed for use with straight hour lines to correct for EoT, and some are skeptical. Please share your comments and suggestions. Best wishes, Mac Oglesby P.S. Since I do not know if John Close is a member of this List, please cc to him. - - -
Re: 3-D gnomon for Polar Dial
P.S. Bill Gottesman has sketched a possible solution and has given permission to share. Contact me OFF LIST if you want a copy of his drawing (16K gif). I would like to see that drawing, Mac! I can imagine how such a gnomon would work and might look, but believe it would need to be split into two seasonal units. It's tempting to realize that the polar dial is itself inherently split, but the dial separates naturally at the Equinoxes, and the gnomons would be swapped out at the Solstices, so I don't see an automatic way offhand... Dave 37.28N 121.97W -
Re: 3-D gnomon for Polar Dial
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 -
Re: 3-D gnomon for Polar Dial
Hello Friends, With respect, I have to disagree with John Davis. If you rotate a polar dial around its polar axis (or any axis parallel to the Earth's--an edge of the dial plate, for instance) by 15°, each hour line will be one hour off--ahead or behind depending upon which way you turned the dial. Should you want an 8 minute change, twist the dial 2 degrees. Etc. At least that's how my polar dials seem to work. Best wishes, Mac Oglesby 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 - -
Re: 3-D gnomon for Polar Dial
On Mon, 23 Dec 2002, Mac Oglesby wrote: With respect, I have to disagree with John Davis. If you rotate a polar dial around its polar axis (or any axis parallel to the Earth's--an edge of the dial plate, for instance) (Which of course, IS rotation about the gnomon axis, PLUS translation in the plane normal to that axis) by 15?, each hour line will be one hour off--ahead or behind depending upon which way you turned the dial. Should you want an 8 minute change, twist the dial 2 degrees. Etc. At least that's how my polar dials seem to work. That's the way I thought it worked, also... Something about it does bother me, particularly when I consider a dial already incorporating EOT and longitude correction (with a date/time grid and an index on the gnomon), but I can't quite put a finger on it! Dave -
