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    Re: Reducing back sights
    From: Dave Weilacher
    Date: 2004 Aug 10, 12:35 -0400

    What you have is pretty good.
    You are trying to come up with the acute angle you would have got had you 
    taken the sight against the normal horizon.
    The only issue you might want to consider (although I wouldn't go for that 
    degree of accuracy myself), is caused by refraction.  If you look at the 
    correction for semi-diameter and refraction, there is a small difference 
    between lower and upper limb corrections.  That difference is all about 
    refraction at the top of the sun vs the bottom of the sun.
    So, even though you are doing a lower limb reduction (I assume) and the 
    arithmetic works the same (add semi-diameter), you may consider using the 
    upper limb number for the correction.
    This level of exactitude and complication should appeal to many on this list.
    -----Original Message-----
    From: Jim Thompson 
    Sent: Aug 10, 2004 6:50 AM
    Subject: Reducing back sights
    Neither Dutton's nor Bowditch explain back sight reduction in sufficient
    detail for me to be confident about the process.  My understanding is this:
    1. Raw altitude = back sight.
    2. Corrected raw altitude: apply Index Correction and dip.
    3. Corrected hs = 180d minus corrected raw altitude.
    4. Then reduce the corrected hs as usual.
    True, or bungled?
    Jim Thompson
    Outgoing mail scanned by Norton Antivirus
    Dave Weilacher
    .US Coast Guard licensed captain
    .    #889968
    .ASA instructor evaluator and celestial
    .    navigation instructor #990800
    .IBM AS400 RPG contract programmer

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