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    Lunar contrast
    From: Geoffrey Kolbe
    Date: 2016 Sep 11, 08:49 +0100
    A problem in taking a moon-sun lunar is that the background light from the sky renders the moon's edge difficult to determine. In consequence, it is not easy (even less easy) to determine the exact moment when the moon's limb touches the sun (as viewed through the sextant) as the background light between moon and sun is almost as bright as the moon itself.

    However, there are a couple of convenient facts which enable us to greatly improve the contrast between the moon and the background sky, and so improve accuracy when determining the moment of contact between moon and sun.

    The first fact is that lunars are more accurate at the half-moon phase when the distance from the sun is around 90 degrees and so lunars are usually taken at this time.

    The second is that the blue-sky background light comes from Rayleigh scattering of sunlight in the atmosphere, and that scattering is strongly polarised at a scattering angle of 90 degrees. This means that the background light around the moon  will be stongly polarised when the moon is 90 degrees from the sun.

    It follows that the use of a polarising element can almost completely eliminate the background light around the moon, but will only decrease the moon's light by half. It will thus be much easier to determine when the now dark gap between moon and sun (as viewed through the sextant) closes and the two bodies have 'made contact'.

    Geoffrey Kolbe
       
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