Thanks very very much to all of you who responded to my request for input on the analemmatic dial instructions! Each and every one of you offered thoughtful and useful advice, most of which I will take. Adaptation to correct to standard time is way at the bottom of my priority list but I certainly erred in neglecting to mention local apparent time, etc.
I agree that graphical methods would be more suited for many people, maybe even most people. I think an explanation of how to construct the dial graphically would be vastly more successful if it included plenty of illustrations. Of course the same thing is true of plotting the points in a coordinate system but it seems to be more tolerant of a lack of illustrations. Willy, thanks very much for your effort in checking my calculations. You were completely correct all down the line. The declinations I used are from the table in Waugh. Since nobody has disagreed with you I will assume that your values are better and change things accordingly. Thanks again! John Mon, 4 Dec 2000, Willy Leenders wrote: > Hello John, > > In your analemmatic dial instructions in the table for de coordinates of the > hour markers I found a > type error: in the column of the Y-values 0.996 must be 0.966. > > I calculated the declination values you use in the tangens function for the > coordinates of the data > scale (first values below). Compared to the average values (I found them, I > do no more know where, > on the world wide web) for the years 2000-2099 (second values below) there > are significant > differences. > > Jan. 1 -23,13 -22,97 > Feb. 1 -17,30 -17,00 > March 1 -8,00 -7,36 > April 1 4,25 4,77 > May 1 15,00 15,26 > June 1 22,00 22,13 > June 21 23,44 23,43 > July 1 23,00 23,06 > Aug. 1 18,00 17,85 > Sept. 1 8,50 8,05 > Oct. 1 -2,90 -3,42 > Nov. 1 -14,00 -14,61 > Dec. 1 -21,67 -21,88 > Dec. 21 -23,44 -23,43 > > On a sundial in Cairo (Lat. 30) with a width of 6 meter for instance the > difference on March 1. and > Nov. 1 results in a difference in distance of 3 cm on the data scale and a > difference in read time > of several minutes. > That is to much for "the oldest an most accurate scientific instrument". > > Who gives us precise values for the sun declination? > > Willy > > > > john hoy wrote: > > > I've gotten a fair amount of email lately from people who want to make an > > analemmatic dial but are not comfortable with technical or theoretical > > explanations -- they just want a sundial. > > > > I've tried to prepare some simple, hopefully not confusing, instructions, > > which I would like to invite any of you who are interested to review. I > > will welcome any criticism or corrections. > > > > It's at > > http://www.cyberspace.org/~jh/dial/analemmatic.html > > > > Thanks, > > > > John Hoy >
