The clearest demo' of a moving 'shadow' I've seen was Bill Gottesman's big helical dial with double strip mirrors casting two light lines with black 'hairline' of 'shadow' in between them. I recall his real-time video of this at a BSS Conference?? where movement was easily seen.
Tony Moss -----Original Message----- From: Roger Bailey <rtbai...@telus.net> To: Dave Bell <db...@thebells.net>; 'Kevin Nute' <kn...@uoregon.edu>; 'SundialMailingList' <sund...@rrz.uni-koeln.de> Sent: Thu, 8 Aug 2013 5:30 Subject: Re: Visibly Moving Gnomon Shadows I am reminded of Galileo's supposed recant, the mutter translated as "and yet it moves". Yes it does and it can be observed in specific circumstances. When we setting the Ottoman sundial in St Louis I sensed I could see the shadow move as we tracked the shadow against the scale. At the NASS conference in Tucson we observed with John Carmichael the rapid movement of the shadow using shadow sharpeners determining the timelines under the McMath Solar Observatory. Here we could clearly see the shadow move. A huge sundial gnomon and favourable geometry accelerated the motion. I expect observers of the "Sun in the Church" phenomenon on meridians in various churches in Europe can see the sun spot move. Math? Phaff. Believe what you see. Regards, Roger Bailey ps. The word for the day, "pfaff" is a slang term for wasting time, doing nothing very productive. From: Dave Bell Sent: Wednesday, August 07, 2013 6:47 AM To: 'Kevin Nute' ; 'SundialMailingList' Subject: RE: Visibly Moving Gnomon Shadows I’m a little surprised at the hair-splitting responses regarding extreme precision (Kevin was specifying 0.05 in/sec) and surface characteristics, all of which are true, but missed the simple point of how large would the dial have to be. For a very rough first approximation, we know the shadow (or the apparent Sun) moves through 360° in 86,400 seconds. This converts to about 7 x 10^-5 radians per second, and the tangent o that angle is the same, as far as matters. Dividing 0.05 inch by 7 x 10^-5 gives a radius of a hair under 720 inches, or 60 feet from the gnomon to the shadow surface. Dave (any maths errors can be attributed to responding before my second cup of coffee!) From: Kevin Nute <kn...@uoregon.edu> To: sundial@uni-koeln.de; Peter Ransom <pran...@btinternet.com>; JOHN DAVIS <john.davi...@btopenworld.com> Sent: Tuesday, 6 August 2013, 21:38 Subject: Visibly Moving Gnomon Shadows The movement of the gnomon shadow at the famous Samrat Yantra equitorial sundial in Jaipur is reputed to be clearly visible to someone standing near the projection surface. I've read it moves as fast as 1 mm/s, though obviously not all the time. At a given latitude, say 40º N, can anyone suggest a simple formula for estimating how far a projection surface would need to be from a vertical or horizontal gnomon for the shadow to move at 1.27 mm/s (the practical lower threshold of perceptible movement) I wonder? Or in other words, what's the smallest sundial you could build to see real-time movement of the gnomon shadow with the naked eye? Kevin Nute Professor of Architecture University of Oregon School of Architecture and Allied Arts Eugene, OR 97403 USA kn...@uoregon.edu --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial No virus found in this message. Checked by AVG - www.avg.com Version: 2013.0.3392 / Virus Database: 3209/6556 - Release Date: 08/06/13 No virus found in this message. Checked by AVG - www.avg.com Version: 2013.0.3392 / Virus Database: 3209/6558 - Release Date: 08/07/13 --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
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