Re: 3-D gnomon for Polar Dial
Hi Mac et al, You are, of course, completely right - you can turn a polar dial around its gnomon to make allowance for the EoT - my humble apologies. It isn't intuitive (to me, at least) that this should work and I foolishly made my comment without experimenting or doing the maths. I have now done so and it is quickly clear that the distance of the shadow edge from the noon line is the height of the gnomon multiplied by the tan of (hour angle plus rotation angle of the dialplate). Question: does it become a declining polar dial once the dialplate is rotated?! The mechanics of making such a dial will be interesting if it is to be robust enough to survive the elements. Does anyone know of a real dial in a public place? Happy Hogmanay to all diallists, John Davis - Dr J R Davis Flowton Dials N52d 08m: E1d 05m -
Re: 3-D gnomon for Polar Dial
Hello Mac, I read this message with interest. I'm assuming Anna King has replied to you. Do you think this gnomon will work? Regards John (and Merry Christmas) - Original Message - From: "Mac Oglesby" <[EMAIL PROTECTED]> To: Sent: Sunday, December 29, 2002 9:06 PM Subject: Re: 3-D gnomon for Polar Dial > > Hi Dave, > > A while ago you offered to post sundial material on your website > (since we can no longer send attachments of any appreciable size to > Sundial List members. Does that offer still stand? > > I have received some information showing how a 3-D gnomon (though not > a solid) can be (and has been--20 years ago!) fashioned to give civil > time on a polar dial (without using any wavy hour lines). If I can > obtain permission to share this information, are you willing to place > it on your site? At a guess, I'd say the stuff totals about 300K in > size. > > Best wishes, > > Mac > - -
Re: 3-D gnomon for Polar Dial
Hi Dave, A while ago you offered to post sundial material on your website (since we can no longer send attachments of any appreciable size to Sundial List members. Does that offer still stand? I have received some information showing how a 3-D gnomon (though not a solid) can be (and has been--20 years ago!) fashioned to give civil time on a polar dial (without using any wavy hour lines). If I can obtain permission to share this information, are you willing to place it on your site? At a guess, I'd say the stuff totals about 300K in size. Best wishes, Mac -
Re: 3-D gnomon for Polar Dial
Hello John Davis and sundial list members, I think I should give a short comment since I worked on this problem (or at least on a closely related one) years back. I quote from the mail of John Davis. >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. A polar sundial (without EOT correction) is mentioned at the end of Section 3 of my old report http://www.math.tut.fi/~ruohonen/SundialRep.pdf (It can also be found in the NASS Repository CD (March 2001).) The gnomon consists of four identical parts shaped as astroidal cylinders (not cycloidal). And see also my article "Sundials and Mathematical Surfaces" in the Compendium (Vol. 8, March 2001). Using a shifted time (which can then be corrected for by rotating the dial face by a corresponding angle) it should be possible to get a someways simpler gnomon surface (see Section 6 of the report). >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!). Yes, this is a challenge! The construct I came up with has a gnomon surface divided into eight parts to achieve an EOT correction. These eight surfaces are quite complicated and certainly not of the "hourglass shape" any more, see Figure 46 in Section 7 of the report. Using a shifted time (countermanded by rotation of the dial face) one needs only two gnomon surfaces but they are truly complicated, and even difficult to plot, see Figure 47. One difficulty is that basically exact EOT correction, using "hourglass- shaped" gnomons and straight equispaced hour lines, is impossible already for equitorial sundials, as pointed out in Section 5 of the report, and only approximate solutions can be found. The problem is then much more difficult for polar and other "tilted" sundials. Regards, Keijo Ruohonen -
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 -
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
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
> 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
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" 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
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" 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
But wait a minute ...as soon as I'd sent my last message, I saw a problem with it. The plane of the sun's orbit doesn't really change with declination, so my way out of the problem of varying declination doesn't work - so I don't have a solution after all! Steve -
Re: 3-D gnomon for Polar Dial
Hi, I haven't seen any reply to the John Close / Mac question. Did I miss something? What's the verdict? For my part, I can see that it would be possible to construct a 2D shape (approximately oval), which when set in equatorial plane, would cast a shadow giving a certain EoT adjustment - you would read off the time from the leading edge of the shadow of the oval. Next day, the EoT is different so the oval is a different size. You would need to stack a parallel set of these ovals to form a "cylinder" along the polar axis, and have something to indicate which one to use for a given day. But on the second day, the declination is different as well. A sun ray trying to touch the edge of the smaller oval will be blocked by the larger one adjacent to it (or vice versa). So the thing wouldn't work. To get round the latter problem, you would have to set the plane of each day's oval according to the declination of the sun on that day. The ovals are not quite the same shape as before, but the principle is the same. You would end up with a gnomon which was a bit like a bull's horn - curved and changing in thickness along its length (according to EoT). You would have to swap the gnomon for another when the declination changes from decreasing to increasing. Your timescale would be an East-West line instead of a face with a set of hourlines, so it wouldn't handle morning and evening well (a bit like a polar dial) but I think it would work Steve > "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. > 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
> Mac Oglesby recently asked: > > > >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. > > As a Polar dial with an 'Oliver' type analemmical 'centrebody' is > effectively a Schmoyer 'Sunquest' with the hour arc flattened out into a > plate I don't seem to see any problem with this idea. Of course the > Oliver device is symmetrical so the meantime correction would be only a > close approximation. Of course it will suffer from the same problem all > polar dials demonstrate viz. a shadow of increasing fuzziness away from > Noon and problems with the early/late hours. > > Tony Moss > > P.S. Apologies for the delayed response but other problems have had my > undivided attention recently. > - > Pardon my ignorance, Tony, but could you please remind me/us what the Oliver device is. My initial thought was that the objective - a mean time polar dial without analemma-shaped hour lines - was impossible due to the varying distance from the gnomon to different parts of the dial. The Schmoyer type equatorial works because the same section of the gnomon is used all day long. Is the Oliver device immune to this problem? I cannot imagine any shape that would be. While it is true that a polar dial has increased fuzziness away from noon, this is exactly compensated by the faster rate at which the shadow moves across the dial. So the fuzziness is plus or minus two minutes at all times. Chris Lusby Taylor Newbury 51.4N 1.3W -
Re: 3-D gnomon for Polar Dial
Mac Oglesby recently asked: > >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. As a Polar dial with an 'Oliver' type analemmical 'centrebody' is effectively a Schmoyer 'Sunquest' with the hour arc flattened out into a plate I don't seem to see any problem with this idea. Of course the Oliver device is symmetrical so the meantime correction would be only a close approximation. Of course it will suffer from the same problem all polar dials demonstrate viz. a shadow of increasing fuzziness away from Noon and problems with the early/late hours. Tony Moss P.S. Apologies for the delayed response but other problems have had my undivided attention recently. -
RE: 3-D gnomon for Polar Dial
Title: Message Mac, Bill, John, Pete, et. al. Mac is to be commended for bringing the question John Close posted on the NASS message board to the attention of the mailing list. If John is not a member of this list, a discussion here may provide the information he needs. The purpose of the message board is to provide a dialog between the general public and sundial experts - experts like the international group who are reading this now. I go to the board every morning to weed out the spam and other garbage. I often see simple questions left unanswered. Frankly, the message board has not been very active recently, and I encourage the members of this list to bookmark the message board, visit often, and share your knowledge. http://sundials.org/message/ Thank you,Bob TerwilligerNASS WebmasterEmail: [EMAIL PROTECTED] > -Original Message-> From: [EMAIL PROTECTED]> [mailto:[EMAIL PROTECTED]] On Behalf Of Mac Oglesby> Sent: Monday, December 09, 2002 9:22 AM> To: Sundial Mail List> Cc: John Close> 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.> ->
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. -
