Re: Perpendicular Gnomon Options
Hi Fritz (Sundial list for those that are interested) First, just want to echo what you said about how nice it was to see you again! Yes Fritz, there is an shadow offset from center and the shadow of the point would not always be symmetrical. This is true. I agree. But is this really a problem in practice? I can easily see how you and others think that this lack of symmetry would make reading the shadow difficult. But after observing the shadow from a gnomon like this, it wasn't that hard to tell where the tip of the shadow was. The tip is the tip, regardless of the symmetry of the shadow below the tip. Granted, it doesn't look quite as good and symmetrical as the perfect shadow cast by a round cone or round point, but it DOES work. So I must respectfully disagree with your statement: But you must agree that the obvious offset of the rest of the pointing shadow will severely skew the resultant impression of where the shadow is pointing. This to me makes a triangular (crossection) gnomon out of the question. The tip of the shadow still works and you can see it well, even if the shadow is skewed. I have seen it. So I don't think it is out of question, just not as perfect as a cone's shadow. Remember, you ONLY need the shadow to indicate a little point on the sundial face. Just a little point. Does this make sense? If not, do an experiment with a cardboard gnomon to prove it to yourself . Thanks though for your thoughts on this. It's worth discussing. I think it is an important topic. I would hate to see dial designers discard using multi-sided points on a pretty obelisk because they think they don't work. John - Original Message - From: Fritz Stumpges [EMAIL PROTECTED] To: John Carmichael [EMAIL PROTECTED]; Roger Bailey [EMAIL PROTECTED] Sent: Tuesday, September 05, 2006 7:09 AM Subject: RE: Perpendicular Gnomon Options Hi John and Roger, Teresa and I sure enjoyed our north west camping vacation with the children and especially the conference in Vancouver. Just a quick comment on this triangular gnomon, which I'm quite sure you both are aware of anyway. We know that the tip of the obelisk is all that is of concern. But you must agree that the obvious offset of the rest of the pointing shadow will severely skew the resultant impression of where the shadow is pointing. This to me makes a triangular gnomon out of the question. See this perspective view picture of a triangular obelisk. It's quite similar to the one you (John) showed on your email but I've added a central rod extending up to accentuate the huge offset of this view. Again, it was great seeing you both at this year's very enjoyable conference. Fritz Stumpges --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Perpendicular Gnomon Options
Hi Chris Roger: I guess I caused some confusion by not stressing that my perpendicular gnomons drawingshows the types of gnomons where you only use the shadow of the nodus (the tip, point or center of a ball). Everything below the tip on the shaft of the gnomon is irrelevant to reading the shadow of the tip. But, I agree with you that if you were to use a triangular, four sided, or any multisided gnomon shaft, they wouldn't work right if you needed to read the center or edge of the shaft's shadow instead of the tip. (example, a 3 sided post gnomon on an equatorial sundial). If my equatorial in Colorado Springs had a triangular post, instead of a round rod, it wouldn't work. I agree with you on that. Maybe I should retitle the drawing to: Best Nodi Designs on DifferentPerpendicular Gnomons or Best Perpendicular Gnomon Designsthat use Nodi thanks for clarifying things, John - Original Message - From: Chris Lusby Taylor To: Sundial List ; Roger W. Sinnott Sent: Friday, September 01, 2006 6:25 PM Subject: Re: Perpendicular Gnomon Options At 11:52 AM 9/1/2006 -0700, John Carmichael wrote: Hello Roger (Sinnott): Humm... I read your concerns that a three sided pyramid and a 3 sided post are problematic gnomons. And I tried to understand your reasoning, but for the life of me, I can't understand the problem you mention. I can't see any problems with either of these. ... The tip faces north and casts a beautiful shadow all day long. Do you still think there is a problem with these two types of perpendicular gnomon's. If so, could you maybe try to rephrase your explanation in a different way so that I might understand you? I really want to understand! thanks Roger, John At 23:47 9/1/2006, Roger W. Sinnott [EMAIL PROTECTED] replied, in part:All I meant to call attention to is that the centerline of the shadow of a post or pyramid does not necessarily pass through the shadow of the tip. I was thinking of vertical gnomons in which the tip is precisely centered over the cross section that tapers up to it, like the Washington Monument. If the post has a circular, elliptical, square, or rectangular cross section, then everything is fine. But if the cross section is triangular, the centerline of the post's shadow will not necessarily extend through that of the tip. (I'm about to leave on a trip for the weekend, but I can try to post a drawing of what I'm getting at next week!) snip I think the centerline of the shadow of a triangularpyramid does necessarily pass through the shadow of the tip. Butthis doesn't invalidate Roger's main point, which is that the triangular cross section further down, or the triangular section of a post of constant section, doesn't have a well-defined centre: it depends on which side or sides are being illuminated. At any one time,one of thegnomon's three faces will be creating the shadow. So we want the centreline of that face to go from the tip down to the earth in a lineparallel to the polar axis. But only one of the three faces' centrelines can do this. I agree with Roger that a symmetrical cross-section such as an ellipse or rectangle does not have this problem, as the centre of the shadow coincides with the shadow of the centre of the cross-section. John - your triangular-sectioned gnomon may appear to cast a beautiful shadow, but you can't rely on its centreline. Regards Chris ---https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Fw: Perpendicular Gnomon Options
- Original Message - From: John Carmichael To: Sundial List ; Roger W. Sinnott Sent: Friday, September 01, 2006 10:54 AM Subject: Re: Perpendicular Gnomon Options Hello Roger (Sinnott): Humm... I read your concerns that a three sided pyramid and a 3 sided post are problematic gnomons. And I tried to understand your reasoning, but for the life of me, I can't understand the problem you mention. I can't see any problems with either of these. Here is my reasoning: 1. A 3 sided pyramid, is similar to a cone, but has three sides. It's tip will cast a shadow that is visible throughout the day, irregardless of solar azimuth. 2. a 4 sided post with a point. My drawing doesn't show this very well, so I'm sending you two photos of such a gnomon designed by Woody Sullivan. Note that the post has three vertical sides and the cross section is an equilateral triangle. What my drawing doesn't show well is that the point at the top is formed by slanting or cutting off the top of the gnomon at an angle. This gives it a tip that's a point. The tip faces north and casts a beautiful shadow all day long. Do you still think there is a problem with these two types of perpendicular gnomon's. If so, could you maybe try to rephrase your explanation in a different way so that I might understand you? I really want to understand! thanks Roger, John - Original Message - From: Roger W. Sinnott To: Sundial List Sent: Thursday, August 31, 2006 3:18 PM Subject: Re: Perpendicular Gnomon Options John (and Larry), I think there may be a problem with two of the seven designs. Numbering them 1 through 7 from left to right in your illustration, the problematic ones are No. 3 (the three-sided pyramid) and No. 4 (the three-sided pointed post). All the others have the shadow axis of the pole, post, or pyramid passing directly through the shadow of the point at the top. But in these two, because the cross section is triangular, the shadow axis of the post or pyramid sometimes will NOT pass through the end point. (Whether it does or not depends on the Sun's azimuth.) So, someone reading the time might be biased or misled by the deviation of the shadow axis from that of the end point. -- Roger At 02:07 PM 8/31/2006 -0700, John Carmichael wrote: Hi Larry: Since you are interested in drawings that show the different possibilities for sundial design, I thought you might like to have this for your educational presentations. I made this for a client so he could see the many options for a perpendicular gnomon. These, I think, are the best perpendicular gnomons for face designs that require very long shadows produced from low solar angles. For that reason I have not included an aperture nodus. I'm not real thrilled with a long shadow cast by a ball on a rod, but I included it anyway just because it is so popular and traditional. Of course, these drawings can be modified as needed by a sundial designer to make the points sharper or fatter (the apex angles of the points, cone or pyramids) or the ball and rods bigger or smaller. These are just type samples. John Attachment Converted: c:\eudora\sky\attach\GNOMON OPTIONS (perpendicular).pdf --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial attachment: 3_sided_post_gnomon2.jpg --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Perpendicular Gnomon Options
Oops, I made a typo. I was talking about a 3 sided post not a four sided post in this sentence: 2. a3 sided post with a point. My drawing doesn't show this very well, so I'm sending you two photos of such a gnomon designed by Woody Sullivan. Note that the post has three vertical sides and the cross section is an equilateral triangle. What my drawing doesn't show well is that the point at the top is formed by slanting or cutting off the top of the gnomon at an angle. This gives it a tip that's a point. The tip faces north and casts a beautiful shadow all day long. - Original Message - From: John Carmichael To: Sundial List ; Roger W. Sinnott Sent: Friday, September 01, 2006 10:54 AM Subject: Re: Perpendicular Gnomon Options Hello Roger (Sinnott): Humm... I read your concerns that a three sided pyramid and a 3 sided post are problematic gnomons. And I tried to understand your reasoning, but for the life of me, Ican't understand the problem you mention. I can't see any problems with either of these. Here is my reasoning: 1.A 3 sided pyramid, is similar to a cone, but has three sides. It's tip will cast a shadow that is visible throughout the day, irregardless of solar azimuth. 2. a 4 sided post with a point. My drawing doesn't show this very well, so I'm sending you two photos of such a gnomon designed by Woody Sullivan. Note that the post has three vertical sides and the cross section is an equilateral triangle. What my drawing doesn't show well is that the point at the top is formed by slanting or cutting off the top of the gnomon at an angle. This gives it a tip that's a point. The tip faces north and casts a beautiful shadow all day long. Do you still think there is a problem with these two types of perpendicular gnomon's. If so, could you maybe try to rephrase your explanation in a different way so that I might understand you? I really want to understand! thanks Roger, John - Original Message - From: Roger W. Sinnott To: Sundial List Sent: Thursday, August 31, 2006 3:18 PM Subject: Re: Perpendicular Gnomon Options John (and Larry),I think there may be a problem with two of the seven designs. Numbering them 1 through 7 from left to right in your illustration, the problematic ones are No. 3 (the three-sided pyramid) and No. 4 (the three-sided pointed post).All the others have the shadow axis of the pole, post, or pyramid passing directly through the shadow of the point at the top. But in these two, because the cross section is triangular, the shadow axis of the post or pyramid sometimes will NOT pass through the end point. (Whether it does or not depends on the Sun's azimuth.) So, someone reading the time might be biased or misled by the deviation of the shadow axis from that of the end point.-- RogerAt 02:07 PM 8/31/2006 -0700, John Carmichael wrote: Hi Larry: Since you are interested in drawings that show the different possibilities for sundial design, I thought you might like to have this for your educational presentations. I made this for a client so he could see the many options for a perpendicular gnomon. These, I think, are the best perpendicular gnomons for face designs that require very long shadows produced from low solar angles. For that reason I have not included an aperture nodus. I'm not real thrilled with a long shadow cast by a ball on a rod, but I included it anyway just because it is so popular and traditional. Of course, these drawings can be modified as needed by a sundial designer to make the points sharper or fatter (the apex angles of the points, cone or pyramids) or the ball and rods bigger or smaller. These are just type samples. John Attachment Converted: "c:\eudora\sky\attach\GNOMON OPTIONS (perpendicular).pdf" --- https://lists.uni-koeln.de/mailman/listinfo/sundial ---https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Perpendicular Gnomon Options
At 11:52 AM 9/1/2006 -0700, John Carmichael wrote: Hello Roger (Sinnott): Humm... I read your concerns that a threesided pyramid and a 3 sided post are problematic gnomons. And I tried tounderstand your reasoning, but for the life of me, I can't understand theproblem you mention. I can't see any problems with either ofthese. ... The tip faces north and casts abeautiful shadow all day long. Do you still think there is a problem with thesetwo types of perpendicular gnomon's. If so, could you maybe try torephrase your explanation in a different way so that I might understandyou? I really want to understand! thanks Roger, John John, If you are *only* concerned with the very tip of the gnomon, then you can probably ignore my earlier comments! In many designs, the shadow of the pyramid or post will be irrelevant, since people are supposed to be looking at the shadow of the tip instead. In yours, as you say, the tip faces north and casts a beautiful shadow all day long. All I meant to call attention to is that the centerline of the shadow of a post or pyramid does not necessarily pass through the shadow of the tip. I was thinking of vertical gnomons in which the tip is precisely centered over the cross section that tapers up to it, like the Washington Monument. If the post has a circular, elliptical, square, or rectangular cross section, then everything is fine. But if the cross section is triangular, the centerline of the post's shadow will not necessarily extend through that of the tip. (I'm about to leave on a trip for the weekend, but I can try to post a drawing of what I'm getting at next week!) -- Roger PS: This problem came up in the gnomon design for the sundial at the entrance court of Texas Instruments' Forest Lane Facility in Dallas, for which I was the astronomical consultant in 1996. In that case, the gnomon is a 20-foot-long stainless-steel needle that points to the north celestial pole. They wanted the centerline of the gnomon's shadow to indicate the time (rather the shadow edge, which serves this purpose in the fat triangular gnomon of a garden sundial). The architectural firm originally proposed a 20-foot-long gnomon that had a sleek triangular cross section. But I persuaded them that this would not work. They could go with a tapered cylinder (like a turned aluminum flagpole, inclined) or a rectangular cross section, but not a triangular one. They thought the first option was not very elegant (and I totally agree!), so they chose the latter. --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Perpendicular Gnomon Options
At 11:52 AM 9/1/2006 -0700, John Carmichael wrote: Hello Roger (Sinnott): Humm... I read your concerns that a three sided pyramid and a 3 sided post are problematic gnomons. And I tried to understand your reasoning, but for the life of me, I can't understand the problem you mention. I can't see any problems with either of these. ... The tip faces north and casts a beautiful shadow all day long. Do you still think there is a problem with these two types of perpendicular gnomon's. If so, could you maybe try to rephrase your explanation in a different way so that I might understand you? I really want to understand! thanks Roger, John At 23:47 9/1/2006, Roger W. Sinnott [EMAIL PROTECTED] replied, in part:All I meant to call attention to is that the centerline of the shadow of a post or pyramid does not necessarily pass through the shadow of the tip. I was thinking of vertical gnomons in which the tip is precisely centered over the cross section that tapers up to it, like the Washington Monument. If the post has a circular, elliptical, square, or rectangular cross section, then everything is fine. But if the cross section is triangular, the centerline of the post's shadow will not necessarily extend through that of the tip. (I'm about to leave on a trip for the weekend, but I can try to post a drawing of what I'm getting at next week!) snip I think the centerline of the shadow of a triangularpyramid does necessarily pass through the shadow of the tip. Butthis doesn't invalidate Roger's main point, which is that the triangular cross section further down, or the triangular section of a post of constant section, doesn't have a well-defined centre: it depends on which side or sides are being illuminated. At any one time,one of thegnomon's three faces will be creating the shadow. So we want the centreline of that face to go from the tip down to the earth in a lineparallel to the polar axis. But only one of the three faces' centrelines can do this. I agree with Roger that a symmetrical cross-section such as an ellipse or rectangle does not have this problem, as the centre of the shadow coincides with the shadow of the centre of the cross-section. John - your triangular-sectioned gnomon may appear to cast a beautiful shadow, but you can't rely on its centreline. Regards Chris --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Perpendicular Gnomon Options
Hi Larry: Since you are interested in drawings that show the different possibilities for sundial design, I thought you might like to have this for your educational presentations. I made this for a client so he could see the many options for a perpendicular gnomon.These, I think, are the best perpendicular gnomons for face designs that require very long shadows produced from low solar angles. For that reason I have not included an aperture nodus. I'm not real thrilled with a long shadow cast by a ball on a rod, but I included it anyway just because it is so popular and traditional. Of course, these drawings can be modified as needed by a sundial designerto make the points sharper or fatter (the apex angles of the points, cone or pyramids) or theball and rods bigger or smaller. These are just type samples. John GNOMON OPTIONS (perpendicular).pdf Description: Adobe PDF document --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Perpendicular Gnomon Options
John (and Larry), I think there may be a problem with two of the seven designs. Numbering them 1 through 7 from left to right in your illustration, the problematic ones are No. 3 (the three-sided pyramid) and No. 4 (the three-sided pointed post). All the others have the shadow axis of the pole, post, or pyramid passing directly through the shadow of the point at the top. But in these two, because the cross section is triangular, the shadow axis of the post or pyramid sometimes will NOT pass through the end point. (Whether it does or not depends on the Sun's azimuth.) So, someone reading the time might be biased or misled by the deviation of the shadow axis from that of the end point. -- Roger At 02:07 PM 8/31/2006 -0700, John Carmichael wrote: Hi Larry: Since you are interested in drawings that show the different possibilities for sundial design, I thought you might like to have this for your educational presentations. I made this for a client so he could see the many options for a perpendicular gnomon. These, I think, are the best perpendicular gnomons for face designs that require very long shadows produced from low solar angles. For that reason I have not included an aperture nodus.I'm not real thrilled with a long shadow cast by a ball on a rod, but I included it anyway just because it is so popular and traditional. Of course, these drawings can be modified as needed by a sundial designer to make the points sharper or fatter (the apex angles of the points, cone or pyramids) or the ball and rods bigger or smaller. These are just type samples. John Attachment Converted: c:\eudora\sky\attach\GNOMON OPTIONS (perpendicular).pdf --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
RE: Perpendicular Gnomon Options
Hi John, Remember my "Timelines" presentation at NASS Vancouver.This outlined many interesting options for both time and date lines with perpendicular gnomons. Fer De Vries program uses computation methods based on perpendicular gnomons, even for sundials with polar gnomons. It is the tip of the gnomon, that point in space, at the that counts. The "Timelines.ppt" file can be downloaded from a temporary website http://www3.telus.net/public/ormerod/NASSVanRTB/Timelines.ppt It is a large 16.5 MB file so be prepared for a slow download. Regards, Roger Bailey Walking Shadow Designs N 48.6 W 123.4 -Original Message-From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]On Behalf Of John CarmichaelSent: August 31, 2006 2:08 PMTo: Larry McDavidCc: Sundial ListSubject: Perpendicular Gnomon Options Hi Larry: Since you are interested in drawings that show the different possibilities for sundial design, I thought you might like to have this for your educational presentations. I made this for a client so he could see the many options for a perpendicular gnomon.These, I think, are the best perpendicular gnomons for face designs that require very long shadows produced from low solar angles. For that reason I have not included an aperture nodus. I'm not real thrilled with a long shadow cast by a ball on a rod, but I included it anyway just because it is so popular and traditional. Of course, these drawings can be modified as needed by a sundial designerto make the points sharper or fatter (the apex angles of the points, cone or pyramids) or theball and rods bigger or smaller. These are just type samples. John --- https://lists.uni-koeln.de/mailman/listinfo/sundial
