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    Re: Refraction correction
    From: Frank Reed
    Date: 2009 Apr 25, 20:13 -0700

    Peter Hakel, you wrote:
    "I am wondering, if the sun or the moon are observed through a cloud layer 
    that is thin enough so that the disk is still visible, what is the additional 
    atmospheric refraction effect on the sextant altitude?  Can the experts on 
    this list point me to any discussions on the subject or is this a non-issue? 
    If this does make a practical difference, one might have to consider the 
    effective thickness of the cloud, its composition (liquid droplets vs. ice 
    crystals), color dispersion, etc..."
    Most of those high thin clouds are composed of ice crystals. It's not a 
    problem for refraction in navigation because any refraction that does occur 
    as light passes through an ice crystal shifts the ray by a large angle. When 
    you see a big ring around the Moon, you can think of that as a series of 
    superimposed images of the Moon from those large angle refractions. There 
    aren't any gradual refractions so the main image of the Moon that you see in 
    the sky is not shifted.
    The same is largely true for water droplets though it might be a little harder 
    to see. Like a Moon halo, a rainbow is nothing more than light that has been 
    severely refracted away from the principal direction of the Sun. The 
    principal image of the Sun in the sky is not affected.
    By the way, there is a relatively easy observational test that should convince 
    you that the refraction is not significantly different. Look at the image of 
    the Sun through your sextant, with appropriate shades of course, as it passes 
    behind the edge of a layer of thin clouds. If there were a difference in 
    refraction, you would see a distortion in the Sun's circular outline (similar 
    to what you see when the Sun is very close to the horizon where the 
    refraction is actually quite variable). 
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