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    Re: Navigation in Fog
    From: Frank Reed CT
    Date: 2007 Feb 19, 17:01 -0800

    Nicolas, you wrote:
    "I assume you will need to calibrate the whole setup so that when the
    polarising filter 'disappears' against the background the orientation
    is truly towards the sun. Is there an easy way to do this calibration?
    Would you be able to do this experiment while still observing the sun
    directly, or should the sun already be below the horizon?"
    Yes, the polarization is also visible when the sky is clear and the
    Sun is in the sky. You can observe this with polarizing filters, but
    you can also observe it with an ordinary piece of glass, painted black
    on the back, functioning as a front-surface mirror. Light reflected
    from glass is polarized, most strongly when the angle of incidence is
    around 60 degrees, if I remember correctly (just experiment to find
    it). The light of the sky is maximally polarized in a wide band
    centered along a great circle 90 degrees from the Sun, no matter what
    time of day it is. This polarization is strongest when the sky is very
    A smooth pool of water will also serve as a polarizing filter so if
    you walk around a bucket of water looking at the reflecting light from
    the sky at the right angle, you will see a darker blue reflection from
    the sky in certain directions.
    One final example: car windows near sunset (or a similar event, which
    I understand is called "sunrise", for you morning people). I find that
    this is especially obvious with large, nearly horizontal windows like
    on car "hatchbacks". Near sunset, these windows seem to display a
    mottled pattern of indistinct blue and purple patches, each patch
    about an inch across. These are apparently polarizing effects in the
    glass or maybe a coating on the glass. They show best when the sky
    overhead is strongly polarized.
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