Re: Gnomon for Vertical Decliner
Bill, I would certainly like to have a look at it, but don't normally have access to the Compendium. Can I find a picture of it somewhere on the net? Thank you very much, Rudolf Hooijenga [EMAIL PROTECTED] - Original Message - From: [EMAIL PROTECTED] (...) On page 30 of the NASS Compendium for September 2000 there is a diagram of a self similar rosette pattern. If this pattern is reduced to an appropriate size (say 1 inch diameter) onto transparent plastic and mounted, with its plane parallel to the sundial surface, at the point where the nodus would ordinarily be, it will serve the purpose of a nodus. Surprisingly, the shadow it makes will be equally sharp and well defined regardless of the distance to the dial plate. (...)
Re: Gnomon for Vertical Decliner
Mac Oglesby [EMAIL PROTECTED] writes: It does leave one surprised that apertures are quite commonly installed at an angle to the plane receiving the shadow. Is this irrational or are they just optimizing to some other feature? I mean, what's really so great about circular spots? What you really want is readability, which is a compromise between brightness and blurriness for any pinhole. Assuming you are interested in an accurate reading of the declination as well as the time, the best pinhole may be the usual choice of a circle in a plane perpendicular to the sun's rays (which is itself also a compromise). Extending this logic, a vertically elongated pinhole in a vertical plane might have some advantages over either of the other arrangements. Hmmm. Art
RE: Gnomon for Vertical Decliner
Arthur Carlson [EMAIL PROTECTED] writes: ... What you really want is readability, which is a compromise between brightness and blurriness for any pinhole. ... ... Extending this logic, a vertically elongated pinhole in a vertical plane might have some advantages over either of the other arrangements. Hmmm. Surely the problem with this is that the slit (which it will logically become if elongated enough) will not cast a line of light onto a mark on the dial at the same time throughout the year as the Sun's declination varies, except on the vertical (noon) line? (Of course this is absolutely fine if you just want a noon mark!) Moreover the elongation of the slit will decrease the accuracy of reading the declination. The former objection is of course overcome by making the slit polar instead of vertical so that you have an ordinary polar gnomon but which casts light instead of darkness ... I suspect some compromise is needed in the end - Tony's .gif showed how even for a very shallow countersink there comes a time when its bevel intersects the light. In any case the vertical hole may work better at higher latitudes with lower solar altitude, as it will become nearer to the normal hole anyway. But it's not obvious to me that the vertical hole will generally produce a much less readable spot than the normal one. Can anyone comment on that? Regards Andrew James
Re: Gnomon for Vertical Decliner
Hi Sarah (and Ken): you wrote:, Does anyone have to hand the approximate distance over which a pinhole gnomon is useful, please? My pinhole experiments showed that this distance should not excede 24 inches (70 cm.), roughly. so one must consider whether the thickness of the material that the pinhole is contained in will obstruct the rays of light intended to pass through it(especially when the sun is at a small angle from the wall), requiring material to be removed from both faces around the hole. Yes, Tony's drawing shows this well. If the material is thick, then beveling the sunside edges of the hole would be indicated. It is probably easier to use a thin piece of metal sheeting instead to avoid this problem. As other have pointed out, when the sun is at low angles to the pinhole, the light passing through the hole produces an elipse. At very low angles, the light could completely disappear! I think pinholes work best for sundials like the equatorial heliochronometer described in Mayall that have a pinhole situated on a moveable alidade. This arrangement keeps the pinhole parallel to the light receiving face at all times. One option for Ken's dial might be to make his pinhole sheet rotateable on it's support rod. This would make the dial interactive. The user would have to rotate manually the pinhole sheet when taking a reading in order to keep the sheet perpendicular to the sun's rays. Personally, for the type of dial Ken is considering, I think that a more traditional spherical nodus would be more appropriate. Best John Carmichael Tucson Arizona
Re: Gnomon for Vertical Decliner
Ken Clark and All, The large amount of interest in this topic has stimulated me to add one more contribution. On page 30 of the NASS Compendium for September 2000 there is a diagram of a self similar rosette pattern. If this pattern is reduced to an appropriate size (say 1 inch diameter) onto transparent plastic and mounted, with its plane parallel to the sundial surface, at the point where the nodus would ordinarily be, it will serve the purpose of a nodus. Surprisingly, the shadow it makes will be equally sharp and well defined regardless of the distance to the dial plate. As far as I know this is a new type of nodus (invented by Dale Walton) and a new use for a self similar, or fractal, pattern. It may present a clumsy appearance at first, but the shadow it produces is beautiful and might be worth incorporating into an artistic dial. May your shadows always be sharp. Bill Walton 41d 58m N, 70d 41m W
Re: Gnomon for Vertical Decliner
[EMAIL PROTECTED] writes: Oy vey! Maybe this will restart the Shadow Sharpener thread going again! Sounds like quite a project-good luck. I would like to suggest that if you use a pin-hole, that the aperture be parallel to the dial face. This may seem obvious, but it wasn't obvious to me 2 years ago. This way a round hole will always cast a round image, and will not spread into an ellipse as the angle of the sun changes. It is true that parallel rays shining through a circular hole will produce a circular image on a flat surface parallel to the hole. But an extended source shining through a pinhole will produce an elliptical image (unless the optical axis is normal to the surface). Consequently, the edges will be fuzzier in one direction than the other, whether or not you want to consider that to be an elliptical image. I suspect that a circular hole parallel to the surface is still the best compromise. Regards, Art Carlson
Re: Gnomon for Vertical Decliner
Hello Ken, The problem of water running down the wall from the footpoint of a pin gnomon can be partly avoided by mounting in the wall just beneath that point a piece of a sloping pipe, cut in the length. Most of the water drops via that half pipe. I have seen this on some old dials with iron styles. More important however is the material of the gnomon. Nowadays iron won't be used anymore. Stainless steel perhaps? If only the straight hourlines are painted I would prefer a polestyle. The dial can be read over a longer period of time. The shadow of the end of a pin gnomon disappears from the dial at an earlier time. Fer. Fer J. de Vries [EMAIL PROTECTED] http://www.iae.nl/users/ferdv/ Eindhoven, Netherlands lat. 51:30 N long. 5:30 E - Original Message - From: [EMAIL PROTECTED] To: sundial@rrz.uni-koeln.de Sent: Thursday, November 30, 2000 1:46 AM Subject: Gnomon for Vertical Decliner Gnomon for Vertical Decliner I am debating what type of gnomon to put on a 28d SE decliner at 40.15d latitude, 76.60d W longitude. The perpendicular dial height of 8 should be enough and would be part of a large painted mural. The artwork will be the focus, not the gnomon. I do not want to obstruct the view of the dial center, so I was thinking of just a tapered perpendicular pin and just read the tip of the shadow. I am also considering a pin hole gnomon and just read the spot of light. I was thinking of attaching the gnomon vertically though, and then coming out, tilting it to the style angle. I do not believe I've seen any pictures of sundials done this way. Are there any? I've only seen it at the end of the style from the dial center. I would have to determine disc and hole size. I originally proposed a sundial that included declination lines and analemma and would work with either type of gnomon. Would either of these gnomons be appropriate, just for hour lines? The artist I am working with prefers a more simple design with hour lines only and no other furniture. So would a more traditional gnomon be better and just read the edge of the shadow if no other furniture is included? Many of the dials I've seen pictured in Europe, are of the style with bracing on each side. I was wondering how accurately they are installed without the ability to check with a digital level years ago? I feel the gnomon needs to be installed first to get the right distance to the dial center to allow for any distortion when the gnomon is attached so the tip of the gnomon is in the right location compared to the dial center. I realized the larger the dial the more fuzzy the shadow becomes. I am just trying to avoid any added errors. I am also concerned about the effects of the attachment points of the gnomon discoloring the paint from the water running down the wall. I am considering some type of brass but I am not sure what is the best way to anchor the gnomon to a masonry wall. I have not really checked out what's available in hardware or anchors that will not rust or corrode over time. I do like vertical dials because they are somewhat vandal proof but is there any way to attach a gnomon that would also be easily repairable if broken off? I was just thinking of having a small permanent hole for the dial center to aid in laying out the lines now and in the future repaints. I do not know if this sundial will come to fruition, but I just was wondering if others had experiences in attaching a gnomon to a wall or suggestions or opinions on laying out an accurate sundial on a wall so I can be assured that gnomon is correct and the dial center is truly at the center so my measurements will be correct when I lay out my lines. Thanks in advance, Ken Clark
Re: Gnomon for Vertical Decliner
In a message dated 9:11 PM 11/29/00, Bill Gottesman wrote: (snip) I would like to suggest that if you use a pin-hole, that the aperture be parallel to the dial face. This may seem obvious, but it wasn't obvious to me 2 years ago. This way a round hole will always cast a round image, and will not spread into an ellipse as the angle of the sun changes. (snip) Bill, I need some help understanding this. It seems to me that, using a round hole aperture in a plate parallel to a vertical dial plate, the sun's cast image will never be circular unless the sun is at the horizon. What am I missing? Best wishes, Mac Oglesby 43 N 72.5 W
Re: Gnomon for Vertical Decliner
Hello All, My thanks to Bill Gottesman and to Andrew James who have explained to me what should have been obvious. (And, I'm hoping that I'm not the only one who was confused about this parallel aperture arrangement.) It does leave one surprised that apertures are quite commonly installed at an angle to the plane receiving the shadow. Regards, Mac Oglesby
Re: Gnomon for Vertical Decliner
Hello All, Does anyone have to hand the approximate distance over which a pinhole gnomon is useful, please? I'm sure it is probably out there along with all the shadow sharpener discussion - which I must re-read; but a quick refresher from those of you who have experimented with this design would be most useful in the meantime, (or any other time that pleases you). Also one must consider whether the thickness of the material that the pinhole is contained in will obstruct the rays of light intended to pass through it(especially when the sun is at a small angle from the wall), requiring material to be removed from both faces around the hole. Best wishes Sarah Edmondson-Jones
Re: Gnomon for Vertical Decliner
Ken Clark wrote ... I do not want to obstruct the view of the dial center, so I was thinking of just a tapered perpendicular pin and just read the tip of the shadow. I recall seeing an Austrian?? dial where an aperture plate was suspended in front of the wall by taught stainless steel wires in the vertical plane although how well this would survive in a vandal-prone area raises some doubt. SNIP I am also concerned about the effects of the attachment points of the gnomon discoloring the paint from the water running down the wall. I am considering some type of brass but I am not sure what is the best way to anchor the gnomon to a masonry wall. I would suggest stainless steel for its increased stiffness and lack of corrosion products if you are concerned about weather stains. Gold plating to the equivalent of UK Service Grade 4 (severe exterior atmospheric conditions) is not as expensive as you might imagine. Alternatively it could just be painted or left self-colour. I have not really checked out what's available in hardware or anchors that will not rust or corrode over time. I do like vertical dials because they are somewhat vandal proof but is there any way to attach a gnomon that would also be easily repairable if broken off? A sympathetically-designed wallplate above the dial with horizontal slots would allow lateral movement. Ornamental caps could be made to cover the bolt heads while concealing the slots. From the centre of the wall plate a horizontal arm could support a vertically-adjustable rod supporting the aperture plate. Or this could be assembled as a temporary 'lash-up' and then used as a jig for a final integrated design as it might be difficult to arrive at a concept which is both adjustable AND elegant. Apart from this, that which is adjustable can go out of adjustment! Good Luck with your project. Tony Moss
Re: Gnomon for Vertical Decliner
Fellow Shadow Watchers Recent contributions on the shape of a circular hole projected onto a parallel plane sent me rushing to my drawing software to try out/confirm what has been suggested. The results are depicted in the tiny ProjHole.gif attached. Tony Moss Attachment converted: Macintosh HD:ProjHole.gif (GIFf/GKON) (0001F17A)
Re: Gnomon for Vertical Decliner
Sarah Edmondson-Jones contributed: Also one must consider whether the thickness of the material that the pinhole is contained in will obstruct the rays of light intended to pass through A shallow 'countersink' from one side of the plate will do much to solve this problem. Tony Moss
Re: Gnomon for Vertical Decliner
In a message dated 11/29/00 7:50:14 PM Eastern Standard Time, [EMAIL PROTECTED] writes: I am also considering a pin hole gnomon and just read the spot of light. I was thinking of attaching the gnomon vertically though, and then coming out, tilting it to the style angle. I do not believe I've seen any pictures of sundials done this way. Are there any? I've only seen it at the end of the style from the dial center. I would have to determine disc and hole size. Oy vey! Maybe this will restart the Shadow Sharpener thread going again! Sounds like quite a project-good luck. I would like to suggest that if you use a pin-hole, that the aperture be parallel to the dial face. This may seem obvious, but it wasn't obvious to me 2 years ago. This way a round hole will always cast a round image, and will not spread into an ellipse as the angle of the sun changes. I support your thoughts about mounting the gnomon in a manner that permits adjustment (or replacement). If, after installing the dial, it does not read precisely right, it is a good thing to be able to tweak it into perfection. Bill Gottesman Burlington, VT 44.4674 N, 73.2027 W
