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    Re: Suitable Sextants - Mirrors
    From: Mike Hannibal
    Date: 2005 Oct 15, 08:22 +1000

    For a rank beginner "rocking the sextant" is the least
    well described technique in any of the literature I've
    read. And, respectfully, the members of this list
    haven't improved on the situation.
    
    We have rocking happening about handles; about lines
    through various mirrors; and in one of the pictures
    profered around the sun itself as a fulcrum.
    
    Please bear with my naive descriptions here: it seems
    to me that we are seeking to achieve one simple
    outcome and that is to ensure that the sextant is
    perpendicular at the moment that we believe that we
    have brought the body to the horizon.
    
    It also seems to me that in doing that the "pivot
    point" or axis for any rocking must be the line of
    sight to the body. In other words we are rotating the
    sextant, through a small arc, with the pivot point
    being our line to the body. In so doing it is the
    horizon which indeed sweeps across the field of view.
    Put another way we could have the sextant at any
    angle, indeed upside down and still have the body in
    our line of sight. What changes is the chunk of the
    horizon (or the presence of any horizon at all) that
    appears directly "below" the body.
    
    The question of perpendicularity therefore can be
    posed another way "Which chunk of the horizon should
    appear under this body when I'm holding this sextant
    perpendicular?". We arrive at our answer to that by
    gently rotating the instrument around an axis that is
    from my eye to the body. What then happens is that the
    body should only move off centre because we introduce
    instrument movement in other planes because we are
    humans not machines with defined freedoms of movement.
    
    I'm sorry if this doesn't meet the technical needs of
    the list. However I would be pleased to be corrected
    if in practical terms I am wrong. I am concerned that
    for the beginner this area is unnecessarily obfuscated
    by less than satisfactory descriptions of what is
    trying to be achieved.
    
    I'm now heading to my boat!
    
    Regards from the antipodes,
    
    Mike
    
    
    
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