Dear Fer: Thank you, and you are indeed correct. The formula I should have used for calculating the shape of the analemmic cutout is yi = r * cos(ai) * tan(bi) , rather than just yi = r * tan(bi) . I should attempt to deceive everyone by stating that as an Engineer, I anticipated that the error would be negligible, so I deleted the term to simplify the equation. ;-) But in truth, I chose Engineering rather than Mathematics because even then I knew I would be a poor excuse for a Mathematician!
I did find one other error I made on my web page and in the Compendium article... somehow in my discussion of averaging the sun's position data, I added 365 days plus 365 days and came up with "760 days". Fortunately this was only a "right-brain" error and not a "left-brain" error, as it only appeared in my narrative but not in my actual calculations. Thanks again very much for your comments! Pete S. fer j. de vries wrote: > Dear Pete Swanstrom, > > With much interest I read your article in the Compendium vol.5 number 4, > dec. 1998 concerning your analemmic-equatorial dial. > Few months ago I also have read your Internet site and I already was > familiar with your design. > At that time I had a question about your formulae but I didn't ask it. I > will do now. > > To my opinion the formula for the y coordinate for the point on the > analemma should read : yi = R . cos(ai) . tan(bi) > In your figure Top View the distance from 12:00 on the equatorial ring > to the point (xi,-) is R . cos(ai). > This means that the distance to the y axis through (xi,-) is r cos(ai) > and on that axis we need to calculte the y coordinate. > In the Side View then we need this value and not R so the y coordinate > will be R . cos(ai) . tan(bi). > > In practice this only will give small differences in the y coordinates. > The max. value for ai is about 4 degrees so the value of cos(ai) will be > between 0,99756 and 1 and the affect for yi is very small. > > Do I think in a right way or do I make an error in my mind? > > Fer de Vries.
