Hello John, Please treat this as a "letter to the editor" of the BSS Bulletin.
I was pleased to see the article by M. K. Head in the June 2007 BSS Bulletin acquainting BSS members with "Sunrise and Sunset Hours on a Garden Analemmatic Sundial". Seasonal Markers allow you to see when and where the sun rises and sets. They are a useful addition to any analemmatic sundial. Nothing in this world is perfect, except perhaps Lambert's Circles serving a similar function. Mr. Head's article replicates the error analysis in the original paper but I recommend that your readers have a look at the original article. "Seasonal Markers for Analemmatic Sundials" was published in the NASS Compendium, in Sept 2003 and in the NASS "Sourcebook for Analemmatic Sundials" " Anyone is free to download the article at my personal website www.walkingshadow.info. The picture on the home page shows an analemmatic sundial in Calgary, Alberta with these seasonal markers. Follow the links to Walking Shadows (Sundials) and click on #10 "Seasonal Markers for Analemmatic Sundials" a 205 kb pdf file. Helmut Sonderegger's program "Alemma" available at http://web.utanet.at/sondereh/sun.htm is the best program I know of to calculate these sundials and seasonal markers. But there is a difference of opinion on minimizing error from Head, Sonderegger and Bailey. Head suggest calculating the seasonal marker using the solstices. Unfortunately this is an extreme point. Sonderegger suggests using declination 20.2ยบ. This minimizes error assuming a Gaussian normal distribution for that error. But this error is not normal but harmonic. My recommendation is to calculate the seasonal marker position for both extreme positions, the solstice and the equinox* to define the range and selecting the average value in between as the seasonal marker point. A disc with a diameter equal to the range centered on the mid position makes a great seasonal marker. If instrumental accuracy is required, use the correction epicycle outlined in the original paper. This would allow you to set up a theodolite on the marker and use the analemmatic sundial to show you exactly** when and where the sun rises and sets. * Not exactly the equinox due to zero and infinity problems. Use a declination of 0.01. This is close enough for sundials. ** The usual cautions on refraction, semi-diameter and horizon apply. Roger Bailey Walking Shadow Designs N 48.6 W 123.4 --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
