Dear Bill, dear John, I realize that a shadow smeared over 2 minutes can be read to a fraction of that period (especially if it is symmetircal, as in John's dials), and that using images can give you a sundial with extreme accuracy. (What is the limit? Except with an azimuthal dial, I expect the first limit you hit would be the variation in atmospheric refraction.) The cost is comlexity (if focussing elements are used) or contrast/ease of reading (if pinholes are used). I did some experiments along the lines Bill suggests, although with pinholes, two years ago and convinced myself that I could determine a point in time under real-life conditions within 2 or 3 seconds. Making a complete sundial capable of this accuracy, however, looked like a difficult project.
I was just curious if caustics could possibly give you the accuracy of an image in a way that is intuitive to read. That is, if you use a simple image, you have to tell the user whether to use the leading or trailing edge of the image. Bill's idea of using a double image solves this problem neatly and is probably more accurate anyway, due to its symmetry. --Art -----Ursprungliche Nachricht----- Von: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Auftrag von [EMAIL PROTECTED] Gesendet: Tuesday, January 08, 2002 3:09 PM An: [EMAIL PROTECTED]; [email protected] Betreff: Re: Caustic and 2 minute limit. In a message dated 1/8/2002 4:19:42 AM Eastern Standard Time, [EMAIL PROTECTED] writes: > Since a caustic is a very different animal from an image, is there any > chance of getting around the 2 minute limit on sundial accuracy due to the > sun's angular diameter? Art, I can't address the issue of caustics, but the 2 minute barrier can be broken by using two focused images of the sun, side-by-side, separated by a tiny amount of space. This space could be, say, 15 seconds of time, and would serve as the time indicator. If you have any doubt that this is feasible, I have a close up photo of my dial which operates using a single focused image of the sun, and although the image is 2 minutes wide, it is readable to better than 1 minute. The edges of the image are razor sharp, and it is easy to see that a design with two of these images side-by-side is achievable. Someday I may make one, but it is not high on my list of things to do. This JPEG is available to any who request it. Bill Gottesman Burlington, VT 44.4674N, 73.2027W
