See "Wheatstone's Polarizing Sundial" by Jim Mahaffey in The Compendium 8(2):1-3, Jun 2001. This is an expanded version of his article that first appeared in Optics and Photonic News, 11(7):14-15, Jul 2000.
Fred On Mon, Mar 30, 2015 at 12:24 AM, Roger Bailey <rtbai...@telus.net> wrote: > 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 <peter.ma...@adelaide.edu.au> > *Sent:* Sunday, March 29, 2015 3:13 PM > *To:* sundial@uni-koeln.de > *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 > > - 18 March 2015 > - Magazine issue 3013 <http://www.newscientist.com/issue/3013>. *Subscribe > and save* > > <http://subscription.newscientist.com/bundles/bundles.php?promCode=8014&packageCodes=PTA&offerCode=Q&cmpid=nsarticletop&intcmp=SUBS-nsarttop> > - For similar stories, visit the *Last Word* > <http://www.newscientist.com/topic/lastword> 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"* > [image: Issue 3013 of New Scientist magazine] > <http://www.newscientist.com/issue/3013> > > - From issue 3013 <http://www.newscientist.com/issue/3013> of New > Scientist magazine, page 57. > > best wishes, > > Peter > > -- > Peter Mayer > Department of Politics & International Studies (POLIS) > School of Social Scienceshttp://www.arts.adelaide.edu.au/polis/ > The University of Adelaide, AUSTRALIA 5005 > Ph : +61 8 8313 5609 > Fax : +61 8 8313 3443 > e-mail: peter.ma...@adelaide.edu.au > 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. > > > > ------------------------------ > [image: Avast logo] <http://www.avast.com/> > > 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 > > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > >
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