Polarization
Sundials
Montgomery College Planetarium
by Dr. Harold Williams
http://montgomerycollege.edu/Departments/planet/
http://montgomerycollege.edu/Departments/planet/planet/polarsun.ht
ml
All Sundials
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All sundials tell time by the position of the sun in
the sky. Mostly they tell local apparent sun time.
At sunrise the sun is just coming up on the
horizon.
At midday, noon, the sun is crossing the
meridian, the line that runs from north to south
through the zeinth, (the point over head).
At sunset the sun is just going down on the
horizon.
Most Sundials
 Most
sundials work by casting
shadows with a “gnome,”
shadow caster.
 The shadow is projected on a
surface that is a dial face
telling you the time.
Polarization Sundials
Determine the position of the sun by
measuring the polarization of the sky.
 The polarization of scattered sunlight in
our atmosphere tells us where the sun is
even when the sun has no shadows
because it is covered by a cloud or you
are shadowed by a building.
 But people can not see polarization so
how do you measure it in the sky!
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What is Polarization and how can
you see it to measure it?
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Most light sources produce unpolarized light, or
light with both polarization states mixed equally.
So you must have an analyzer to see
polarization, unless you are an bee or some
other insects or octopus, squid, cuttlefish, or
mantis shrimp, but evidently not pigeons then
you have an analyzer in your eye already
evolved.
Haidinger's brush, some people can be
trained to see polarization weekly!
Visible Light
Visible light is electromagnetic radiation of
a limited frequency and wavelength.
 All electromagnetic radiation: radio waves,
micro waves, infrared, visible light,
ultraviolet, x-ray, through gamma rays, has
two polarization states!
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http://en.wikipedia.org/wiki/Polarization
Linear
Circular
Elliptical
Polarization is perpendicular to the plane
containing the sky point, the sun, and the observer.
Maximum polarization in the sky occur at a point
90 degrees from the sun in the plane containing
the sun observer and zenith.
The plane containing the sun, observer, and the celestial
pole rotate counter clockwise uniformly about the celestial
pole making one turn in a local apparent solar day. The
major polarization vector is always 90 degrees to the plane.
So it should be possible to make a gnomeless, no
shadow caster,sundial using the polarization
pattern in the sky. This should be easiest if you
use the polarization pattern around the celestial
pole since it rotates uniformly. Sort of like an
equatorial gnomned sundial has uniform dial
with hours 15 degrees apart since
15 degrees/hour x 24 hours/revolution=360 degrees/revolution.
Equatorial Sundial with Gnome
pointing toward the pole
Equatorial Sundials across the
planet
Equatorial Sundial Summer
Equatorial Sundial Winter
History of Polarization
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Etienne-Louis Malus (1775-1812)
Discovered polarization in 1808
François Arago (1786-1853) discovered
polarization of the sky in 1809
Babinet, Brewster, and others extended
sky polarization
Charles Wheatstone (1802-1875) actual
built a polarization sundial in 1848
Eventually John William Strutt alias Lord
Rayleigh (1842-1919) in 1870s Rayleigh
scattering in the sky
Wheatstone’s polarizing sundial
at the Science Museum, London
Folding-pattern polarization
sundial designed for demo by
Charles Wheatstone
So how come polarization
sundials are not common
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Improvement in technology has made polarization
sundials much easier and cheaper to make now than
in Wheatstone's time when Nicol prism (calcite prism
where one polarization encountered black paint) were
used.
Edwin Land discovers how to embed iodosulphate of
quinine (herapathite) microscopic crystals, which are
dichroic crystals which are both birefringent and
colored, aligning them by stretching them in polyvinyl
alcohol films between sheets of cellulose acetatebutyrate (CAB)HN32 linear polarizer, Polaroid.
All references in the Literature are in obscure
English Journals.
Of course why build sundial at
all when we have ephemeral
watches and clocks
To teach physical principles about the
movement of the sun and the earth, the
day and the seasons, in the case of
shadow sundials.
 Polarization sundials besides teaching
about daily motion can be used to
explain spooky facts about polarization.
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Spooky Polarization
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Regeneration of polarization by putting the third
Polaroid between the two crossed Polaroids.
Quantum mechanics in action 2. The
polarization of photons pages 4-7 in the 4th
edition of Quantum Mechanics by P.A.M. Dirac.
This is in the first chapter on The Principle of
Superpositon after 1. The need for a quantum
theory section. Polarization is a true quantum
mechanical effect! In the limit of h goes zero
there is no polarization.
Novelty factor of making a
simple instrument to do science
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This sundial works when it is in the shadows
since it does not use shadows. So the sun
behind a cloud is no problem. Works in
courtyards, too, with a patch of blue sky
available.
The sun being down is also no problem for an
hour before sunrise and an hour after sunset.
When won’t it work? White clouds depolarized
sunlight. So you do need a patch of blue in the
north for the simplest type of polarization sundial
to work.
An astro-compass dating from
World War II
Half-Shade Analyzer
Holder for Polaroid and Scale
Assembly
Putting it together
Reference List
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David Colchester, “A Polarized Skylight
Sundial,” Bulletin of the British Sundial
Society, 96.3, pp. 1315 (1996)
Allan A. Mills, “The Sellotape Sundial,”
Bulletin of the British Sundial Society, 98.1,
pp. 3-9 (1998)
Allan A. Mills, “Polarization of Light from the
Sky, and its application to timetelling and
Navigation,” Bulletin of the Scientific
Instrument Society, No. 33, pp. 8-14 (1992)
A.H. Pfund, “A Polaroid Half-Shade Analyzer,”
Journal of the Optical Society of America, 26,
p. 453 (1936)