Reverse Engineering

[John Carmichael]

Hello Geoff  (cc. Sundial List): Topic 1: Geoffry Lane writes: Sorry to trouble you, but one or two things are puzzling me that I hope you might just be able to help with.  I know a certain amount about glass-painters but I am no mathematician, and know very little so far about the technical side of sundials.    First concerns the Weavers' Company (stained glass) dial. Have you ever tried to back-calculate its latitude?  I don't know how to do this.  Behrendt did it for quite a lot of English dials, usually coming up with a fairly convincing latitude for England, i.e. somewhere between 51 and 55N.  But when he tried the Weavers' dial he came up with a figure of 47.5N which, as he said, is closer to northern Switzerland than England.  He was working from a photograph, and admitted that the "error" might be caused by foreshortening.  Two other dials that seem to have given him false readings were Lullingstone and Gray's Inn Hall (both 48.7 - just south of Paris!)   I would be very interested in your comments on this.   John's answer: Reading your letter lit a spark.  When accumulating data on all the dials, I've been constantly frustrated by all the unknown original locations of the stained glass dials.  You're right, in theory, you can back-calculate the dial's design latitude by looking at the position of the hour lines.  This is fairly simple to do with a computer if you know the dial's declination on the wall (the direction it faces).  If you don't know the direction it faces, then the determination of its latitude is much harder.   There are probably at least 50 glass dials on the website that we could and should reverse-engineer, but can you imagine how much work that would be?!  I could do the Weaver's, Lullingstone, and Grey's Inn Hall Dials for you, but I am so busy that I can't do it now.  I am forwarding your letter to the sundial list in case there's somebody out there who would could help you now.   Topic 2: Geoffrey writes: Another thing I don't understand is how morning dials like Berkeley and Goldfinch dials work. From the hour-lines it looks as if the shadow of the gnomon is thrown upwards, but I don't see how it can be.  Also, is it possible to have evening dials that work the same way, and how would you tell them apart (if the hours were restricted or not legible)?   John's answer: If you look at the east wall morning Berkeley Dial the number IIII (4am) is in the upper right corner and the X (10am) is at the bottom. This dial faces almost due east so the gnomon is almost parallel to the face and points to the north star. When day dawns the shadow falls correctly on the top part of the dial where the gnomon is located.  As the sun rises, the shadow moves down (not up as you suggest).  Perhaps you misinterpreted 4am as 4pm?   An afternoon dial is the same only in reverse.  The face of an afternoon (west wall) would have the early afternoon hour lines at the bottom of the dial and the late afternoon lines at the top. So, as the sun sets, the shadow would move up the dial face instead of down.  Does this make sense?   John  Stained Glass Sundials Website: http://advanceassociates.com/Sundials/Stained_Glass ----- Original Message ----- From: Geoffrey Lane To: John Carmichael Sent: Tuesday, February 10, 2004 3:49 AM Subject: Glass dials Dear John,   Sorry to trouble you, but one or two things are puzzling me that I hope you might just be able to help with.  I know a certain amount about glass-painters but I am no mathematician, and know very little so far about the technical side of sundials.    First concerns the Weavers' Company dial. Have you ever tried to back-calculate its latitude?  I don't know how to do this.  Behrendt did it for quite a lot of English dials, usually coming up with a fairly convincing latitude for England, i.e. somewhere between 51 and 55N.  But when he tried the Weavers' dial he came up with a figure of 47.5N which, as he said, is closer to northern Switzerland than England.  He was working from a photograph, and admitted that the "error" might be caused by foreshortening.  Two other dials that seem to have given him false readings were Lullingstone and Gray's Inn Hall (both 48.7 - just south of Paris!)   I would be very interested in your comments on this.   Another thing I don't understand is how morning dials like Berkeley and Goldfinch dials work. From the hour-lines it looks as if the shadow of the gnomon is thrown upwards, but I don't see how it can be.  Also, is it possible to have evening dials that work the same way, and how would you tell them apart (if the hours were restricted or not legible)?   All the best,   Geoff Lane