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    Re: Star-star distances for arc error
    From: Paul Hirose
    Date: 2009 Jul 04, 00:05 -0700

    Tonight I shot the angle between Spica and Arcturus six times with my
    Astra IIIB (3.5x scope and traditional split horizon glass). The extreme
    values were 1.0' apart, so I may have to take back my earlier remark
    that "I find star to star separation angles hard to measure consistently
    with a sextant." This was based on a few measurements attempted soon
    after I got the Astra. My results were so disappointing, I dismissed the
    technique as bunk, at least with my eyesight.
    
    Based on tonight's results, I suspect my inexperience was to blame.
    Though I'd owned bubble sextants for years, the Astra was my first
    marine sextant.
    
    Now I'm more skillful at positioning both bodies in the split image
    horizon glass and swinging the arc. That last is important, I've found.
    The pair of stars should appear to collide instead of passing each other
    as the sextant is rocked. Contrary to what I expected, attempting to
    keep the stars in contact makes it harder to judge coincidence.
    
    When viewing a single star to check index error, that last technique is
    all you have, since swinging the arc has no effect.
    
    My six observations tonight don't prove anything. It's easy to get lucky
    a few times in a row. However, I think the results are pretty good for
    someone who has done maybe a dozen of these in his life. With more
    practice I may be able to judge star coincidences as well as I can judge
    anything else through a sextant.
    
    Conditions tonight were hardly ideal. The wind was enough to make the
    stars dance maddeningly most of the time. My arms soon became tired of
    holding the sextant for long periods, waiting for the image to settle
    down. I have several tripods, but to make the experiment more
    challenging I did not use anything to steady myself.
    
    My opinion of this method for checking sextants has improved enough that
    I intend to try it some more.
    
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