Hello Peter and all, Yes, clouds are a significant, perhaps the overwhelming issue with sundials. Night knocks out half the time and clouds at least half of the remaining daylight hours. Our BSS colleagues know the problem. The most common sundial motto is "I count only the sunny hours". This is a truly defeatist attitude in these days of technology. Can we do better? The effects of clouds are subtle. A slight overcast can destroy the contrast required to read a sundial. Such sunlight, not enough to cast shadows can burn pale skin. Fleeting clouds cause the shadow to bounce back and forth giving indeterminate time readings depending on the side of the sun obscured. I don't think wavelengths are the solution. Wavelength effects giving us red sunsets but there seems to be no advantage through daytime clouds. But where there is light, there is hope. Polarization is detectable through light clouds. Take that old polarizing filter from your obsolescent SLR camera or an old pair of Polaroid sunglasses, hold towards the sun and turn to see the polarization of the sky. Direct views of the sun are not required. Polarized skylight can tell you where the sun is when it is obscured. The polarization effect is evident but not distinct. The phenomenon exist and is familiar to photographers with polarizing filters, a vanishing species. Has the effect been exploited by gnomonists? I don't think so. The opportunity remains, a chimera, like analemmatic moondials. I am working on the latter for the NASS conference in Victoria BC in June.
Regards, Roger Bailey From: Peter Mayer Sent: Sunday, March 29, 2015 3:13 PM To: [email protected] Subject: Clouding the issue Hi, In the "Last Word" section of a recent _New Scientist_ Stephen Parish raised the question of sundials that might work on cloudy days...Clearly, polarisation is possible, but I'm doubtful about shadow casting... Clouding the issue a.. 18 March 2015 b.. Magazine issue 3013. Subscribe and save c.. For similar stories, visit the Last Word Topic Guide Are there any wavelengths at which the sun still casts a shadow when the sky is full of clouds? Could I make a sundial that would work on a cloudy day? Stephen Parish, London, UK This article appeared in print under the headline "Clouding the issue" a.. From issue 3013 of New Scientist magazine, page 57. best wishes, Peter -- Peter Mayer Department of Politics & International Studies (POLIS) School of Social Sciences http://www.arts.adelaide.edu.au/polis/ The University of Adelaide, AUSTRALIA 5005 Ph : +61 8 8313 5609 Fax : +61 8 8313 3443 e-mail: [email protected] CRICOS Provider Number 00123M ----------------------------------------------------------- This email message is intended only for the addressee(s) and contains information that may be confidential and/or copyright. If you are not the intended recipient please notify the sender by reply email and immediately delete this email. Use, disclosure or reproduction of this email by anyone other than the intended recipient(s) is strictly prohibited. No representation is made that this email or any attachments are free of viruses. Virus scanning is recommended and is the responsibility of the recipient. -------------------------------------------------------------------------------- This email has been checked for viruses by Avast antivirus software. www.avast.com -------------------------------------------------------------------------------- --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial -------------------------------------------------------------------------------- No virus found in this message. Checked by AVG - www.avg.com Version: 2015.0.5856 / Virus Database: 4315/9410 - Release Date: 03/29/15
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