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: 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
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: peter.ma...@adelaide.edu.au
CRICOS Provider Number 00123M
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