This has been an excellent discussion. As several of my designs purport to be readable to a couple of minutes, I, too, had been exploring ways to correct for the EoT.
The Poncet platform rotates the entire sundial about a polar axis, but has to be made for a specific latitude, so cannot be mass-produced. A latitude-independent version is described in US patent 09874026 filed just last year (see http://www.patentstorm.us/patents/7035005-description.html) Patrick is correct in saying that if the gnomon has two edges you must rotate it, not just the dial, as the orientation of the edges would remain correct, but their position in space would not. An alternative that I've investigated is to use two wedges, similar to those Bill Gottesman uses for the latitude adjustment of his sundials (also patented: see http://www.precisionsundials.com/equant%20dial.htm). The wedge angle can be vey small - just 2 degrees. But having to set three alignments seems an unacceptable burden. My latest approach is to take the "Housewife's Trick" from AP Herbert: just turn the whole horizontal sundial about a vertical axis, so the dial plate remains horizontal but the gnomon and the 12 o'clock line are no longer North-South. This is very much easier to do than any of the other suggestions. But, is it accurate? Of course not - an article in The Compendium a few years ago analysed it and rejected it. But The Compendium is written with a USA perspective. AP Herbert was English. The housewives to whom he referred were at 51 or more degrees North. Does this make a difference? Yes, it does. By rotating the sundial appropriately, the maximum time error can be made to be less than one minute except in July when it may be as much as 90 seconds. Well, for my money that's a pretty good result. I'm sorry it doesn't work as well in the USA because you're too near the equator. For us in northern Europe I suggest it is quite good enough. The further north you are, the less the error. Should I patent it, or at least the calculation of the "appropriate" angle? I think I'm too late. It appears that it's common knowledge amongst housewives in this country. Chris Lusby Taylor 51.4N 1.3W ----- Original Message ----- From: Edley McKnight To: Roger Sinnott ; sundial@uni-koeln.de Sent: Tuesday, June 05, 2007 6:59 AM Subject: RE: Equation of Time Hi Folks, just a short note on hinging in latitude/EOT corrections. I tend to think of the hinge as one side of a parallelogram with the gnomon as the opposite side. The parallelogram extending down through the dial plate and base. When any set of parallel lines is rotated around one of them, they still stay parallel, further, if only a subset of these lines is rotated, all of them are still parallel. So, really, the hinge could be just below an edge of the dial plate if the dial plate is allowed to swing to positive and negative angles. So long as the hinge is parallel to the gnomon. A half circle with a screw clamp or any number of other ways could be used to set the angle/time. I hope this makes sense. It is all equivalent to translating the dial in Longitude( rotating around the earth's axis as a hinge). Enjoy the Light! Edley. ------------------------------------------------------------------------------ --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
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