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    Re: Star-star distances for arc error
    From: Paul Hirose
    Date: 2009 Jun 20, 11:54 -0700

    The effect of refraction on the separation angle between stars has been
    discussed from time to time. Back in September 2000 I said this in
    another forum:
    http://www.i-DEADLINK-com/lists/navigation/0009/0065.html
    
    My tables are slightly mis-formatted on the web page. The column labels
    are altitudes, and the row labels are separation angles.
    
    The first table shows that refraction affects separation angle in a way
    nearly independent of altitude, if both stars are at the same altitude.
    For example, if two stars are the same altitude and 45° apart,
    refraction decreases their separation angle by .80' at 15° altitude and
    .86' at 75° altitude. Throughout the table, refraction has slightly
    increasing effect as altitude increases.
    
    Bowditch (1984) recommends selecting stars at the same altitude "to
    minimize refraction errors". But that's wrong. Instead of using stars at
    the same altitude and ignoring refraction, I believe it's better to use
    stars at the same *azimuth* and apply refraction corrections. For
    example, if the lower star is refracted 1.5' and the upper star .5', the
    refracted separation angle is 1.0' less than the unrefracted angle.
    
    I find star to star separation angles hard to measure consistently with
    a sextant. It's easier to test my accuracy with lunar distances.
    
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