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    Re: Star - Star Observations
    From: George Huxtable
    Date: 2010 Mar 13, 22:30 -0000

    Douglas Denny wrote-
    "I have re-drawn the Rational Horizon Diagram as there were some mistakes on
    the previous one.  I hope this diagram can make the rigorous method
    clearer."
    
    Comment from George- Yes, that looks perfectly good, to me. The red arc,
    between the true positions of the two stars, is what has been calculated
    from Almanac coordinates. Then the (smaller) apparent arc between the stars,
    the angle that is actually seen in the sky and compared with the sextant
    reading, is shown by the black arc between the apparent star positions. Just
    as it should be.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ----- Original Message -----
    From: "Douglas Denny" 
    To: 
    Sent: Saturday, March 13, 2010 9:43 PM
    Subject: [NavList] Re: Star - Star Observations
    
    
    Initial Post
    George Huxtable:
    
    Copy:
    The altitudes of the two stars have to be calculated. Then the refractions
    have to be ADDED, as I have been insisting all along, and you have
    contradicted. To get from a calculated altitude to what would be an observed
    altitude (if anyone had observed it) requires applying refraction in the
    OPPOSITE DIRECTION to what's needed when converting an observed altitude to
    a true altitude, as in reducing an altitude observation. The effect will be
    to produce an apparent angle between the stars that is always LESS than the
    theoretical value deduced from the almanac. George."
    Ends.
    =================
    
    George,
    I am sorry I missed this posting of yours for some days somehow as I did not
    notice it. I wondered what had happened. I seem to be missing postings
    occasionally as they change so frequently.
    
    Agreed: from calculated true altitude to a calculated apparent refraction,
    refraction is to be added, as in the zenith diagram in fact. I should have
    seen this immediately.
    I was convincing myself of standard subtraction to get from apparent to true
    as in Lunar clearing of distance.
    
    I have re-drawn the Rational Horizon Diagram as there were some mistakes on
    the previous one.  I hope this diagram can make the rigorous method clearer.
    
    Douglas Denny.
    Chichester. England.
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