On Tue, 10 Feb 2004, John Carmichael wrote: > 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. One significant problem we have in reverse engineering many of these dials is that we cannot be sure the original photo was taken straight-on, normal to the dial face. Many of the photos submitted to John have been shot from the ground or other accessible vantage point. In orthogonlizing the images, errors are introduced, because the gnomon and dial face are not in the same plane. I don't think these can easily be corrected. > 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 Geoffrey is correct in noting that the early morning shadow moves downward on the dial face. Note that the pre-6 AM lines only work when the sun is North of the ecliptic. This places it "below" the style at sunrise, and as it rises, it crosses over the plane of the style... Dave 37.277N 121.966W -
