RE: Question about lunar phase and the sun's path in the sky
I juste have to make a comment on this: Warren wrote: 1. At northern latitudes, in the summer, a full moon rises in the southeast as the sun sets in the northwest. If you live far enough to the north, the full moon does not rise at all during summer (for about two months where I live).The summer is all sun if you discount the cloudy days. During midwinter (another two months) you can enjoy the full moon day and night. One of the autumn joys is to see the full moon for the first time after the summer, just above the horizon to the south in August/September. An other autumn joy is to see the first star in the midnight twilight (usually at the 12th of August if the weather permits). Best AnneB Tromsø, Norway 69.7N 18.9E http://nordnorsk.vitensenter.no/himmel/solursida/ --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: A Projection Dial Question
Hi All, Just to summarize the 'hollow' ball lens issue. A thin walled hollow ball lens with low refractive index ( similar to a soap bubble ) can act at long distances like a plate with a hole in it and actually appear to focus the sun's image like a pin hole, except that it is omnidirectional. This is due to the Brewster's and total internal reflection reducing the lens aperture. So, such a lens could possibly be used as a Gnomon for a large sundial. I leave it to younger and more active folk to see if this fits their needs. Hope this helps! Edley McKnight Hi John C. OK, right - that was the point about using a ball lens. A ball is symmetrical in all directions so it will focus equally well whatever the azimuth elevation of the sun. Here I'm envisioning a ball lens atop a slender rod, so there is no blocking of the sun's rays regardless of the sun's position. Now, if you put the ball lens in an opaque plate to cast a shadow for more spot contrast, as Fred did with his aperture/tower/cylindrical dial, the plate may occlude a few of the sun's rays depending on its orientation. However, the ball lens will still focus; it will just have fewer input rays. In a way, calling it a ball lens, as everyone does, is a contradiction in terms. If I recall correctly, the word lens derives from the shape of a lentil bean. This ain't no lentil bean; it's a pea! Best, John B. -Original Message- From: [EMAIL PROTECTED] To: [EMAIL PROTECTED] Cc: sundial@uni-koeln.de Sent: Tue, 29 Aug 2006 9:41 AM Subject: Re: A Projection Dial Question Hi John B: Ok, I think I understand the point you made about the benefit of using a lens in the aperture nodus: With a lens in the aperture nodus, the projected spot of light will always be brighter and smaller than the diameter of the lens, and therefore will give more precise sundial readings than a simple aperture without a lens. But please excuse me if I'm wrong, I thinkthis is only true if the sun's ray hit the lens straight on. I still don'tbelieveaperture lenseswill function at all times on a typical sundial that has low solar angles. What about the fact that the sun hits the lens at many different angles throughout the day and the seasons? Sometimes, the sun's rays will hit the lens at a perpendicular angle, straight on, and the focusing will work great. But at most other times (i.e.. late afternoon on a south wall dial) the sun's ray will hit the lens from a low angle. This affectsthe cone of light anddistorts it. Lenses don't work very well if they are not positioned perpendicularly to the sun's rays, do they? I just did a little experiment with a lens. I tilted the lens so that the sun hit it at low angles, and the spot of light grew to a big blob of dim unfocused light. What do you think about this? John C. - Original Message - From: [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Monday, August 28, 2006 7:46 PM Subject: Re: A Projection Dial Question Hi John C. Fascinating web page. Found a little typo: Shows both Standard (Dutch: Wintertijd) and Daylight Saving Time (Dutch: Wintertijd). OK, so how do you keep it focus for varying distances? Simple: you don't. What you do is settle for a long converging cone. This converging cone is smaller in diameter than the diverging cone from a pinhole at all places along its length except both are equal in diameter where the cones emerge from the apertures. So this special ball lens, if it works out, is not a magic bullet. It's just a lot better than a pinhole! (Brighter and smaller spot). I could have the ray tracer draw a light cone from a pinhole and from the special ball lens and put them on the same page if you like. John B. -Original Message- From: [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Mon, 28 Aug 2006 10:30 AM Subject: Re: A Projection Dial Question Hi John B. It's a very interesting topic that of using lenses for gnomons. Have you see the webpage of glass sundials? It has a photo collection of some amazing glass sphere dials. See: http://advanceassociates.com/Sundials/Stained_Glass/sundials_EGP.html Even one for the blind. But nobody yet has answered the main question and the biggest doubt I have: How do you solve the problem of keeping the sunspot in focus as the projection distance changes. For example, if the focal length of the sphere (or lens) is 3 inches, how do you keep the spot in focus if the projection distance is 30 inches?The projection distance on a dial would constantly be changing as the sun moves through the sky. HOW DO YOU KEEP THE SPOT IN FOCUS! John C. - Original Message - From: [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Saturday, August 26, 2006 10:30 AM Subject: Re: A Projection Dial Question Hi John C, I also contacted an optical engineer buddy (well, that's really his sideline - writing
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: sundial on a cylindrical wall
Hello Willy, I am surprised no one mentioned the article in the latest NASS Compendium (Sept 2006) "Towering Timepiece" by Howard Hebel, the architect forrenovation of Engleman Hall at Southern Connecticut State University. Fred Sawyer was the gnomonic consultant for the sundial inside a 50 foot cylindrical tower entrance. One of Fred's presentations at the Vancouver conference described the practical difficulties working with architects and contractors on such a large sundial.He described the field adjustments like a 15 degree rotation of the cylinder and~1 ft vertical offset. "All's well that ends well". Roger Bailey -Original Message-From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]On Behalf Of Willy LeendersSent: August 25, 2006 12:28 PMTo: Sundial sundiallistSubject: sundial on a cylindrical wallI made? the calculation for the hourlines and datelines for a sundial (6.50 m x 3.30 m) on a concave cylindrical wall in Brussels (Belgium). The sundial is situated 16 m high. (See low resolution picture) I am in search of other sundials on a concave cilindric wall. Willy LEENDERS Hasselt Flanders (Belgium) --- 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
