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    Re: Basics of computing sunrise/sunset
    From: Thomas Kleemann
    Date: 2009 Jun 19, 23:17 +0200

    Brad Morris schrieb:
    > I use a sextant from about 1920.
    > Should I be worried about the filtering qualities of my shade glasses?
    Probably not, in the twenties standards about eye protection were well
    established (e.g. for iron workers)
    > Would there be a way to check to see if my shade glasses are up to the mark?
    A first qualitative guess (not quantitative tough):
    If you own some self tinting sunglasses (heliomatic, solarchrome or
    whatever trademark), you could direct a filtered sunbeam through your
    sextant at them sitting in the shadow.
    Compare the grade of tinting against an unfiltered beam.
    Be aware this method covers mainly UV and natural light, since that is
    what phototrophic glass reacts on. Exposure to IR can cause eye damage too.
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