Hi Tom, Do you mean the large 50th Anniversary dial? From photos on the Internet I don't see an analemma. For instance, see http://commons.wikimedia.org/wiki/File:ClarkCollegeSundial.jpg
However, a 3D analemma is certainly possible on an armillary dial, although it is far from easy to make as it is in two tear-drop-shaped parts which meet at their points. Theoretically, you should have two analemmas - one for each half of the year. But in practice you might get away with one, since the two are very similar, to within a minute or two. The formula for the gnomon is very complex, but approximately its longitudinal cross-section is the same as any other analemma in a polar plane. At a distance R.tan(dec) from the plane of the dial it needs to have a circular cross-section of radius R.sin(EoT) where R is the dial radius, dec is the sun's declination and EoT is the mean absolute value of the Equation of Time for the date(s) with that declination, expressed as an angle. For instance in May when dec=15 degrees, EoT=2m54s which is 0.725 degrees (at 15 degrees/hour), so, the radius should be R.sin(0.725) at a distance R.tan(15) above the dial plane. For a dial radius of 1 metre, that's only 13mm, so pretty narrow. For any date you do need to know which edge of the shadow you are reading, but the analemma does not need moving periodically, except, as I said, that the two analemmas need to be swapped every six months. With a single analemma it might be possible to make it such that its section isn't circular, but two 'D's back-to-back. This would allow a better approximation, especially in winter. Much easier to make, but requiring a moving part to be turned whenever the dial is read, is a gnomon in the form of a metal plate with the analemma cut out of it. The plate must be rotated to be at right angles to the sun. A similar but (unnecessarily, I believe,) complex solution is the Schmoyer dial which has a 3D casting with S-shaped slits. If you have a photo of the analemma gnomon at Clark College, perhaps you could post a link to it, please, or email it privately. Chris Lusby Taylor 51.4N 1.3W ----- Original Message ----- From: Tom Laidlaw To: sundial@uni-koeln.de Sent: Friday, July 24, 2009 8:10 AM Subject: analemma on armillary sundial Hi there, At Clark College in Vancouver, WA there is an armillary sundial with a 3-D analemma on the gnomon. Reading the appropriate edge of the analemma on the time band rather than the center gives the correct clock time. I have some questions. 1. Does it have to be moved periodically? 2. What is the formula for this analemma?. I suspect it must be related to the distance between hour lines or the radius. Tom Laidlaw ------------------------------------------------------------------------------ --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
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