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    Re: Problem with a sextant
    From: Bill B
    Date: 2006 Apr 26, 15:26 -0500

    > From: Bill
    > Bonus question.  "Within two miles" seems to be the standard for cel nav.
    > Most observations appear to be running fixes from the sun.  What is the
    > level of expectation from nearly simultaneous star/planet observations?
    
    Peter wrote:
    
    But yachts make a terrible platform for astronomical observations. Just
    awful. Move in all directions at once while drenching the wretched observer
    and his precious instrument, itself at great risk from physical damage. Plus
    they are much too low, sometimes almost at sea level. Can be hard to get a
    good horizon, then the one you get can be false. So the rule of thumb is
    that if you achieve a fix within 10 nm of the real position that's good, and
    if within 5 nm that's excellent.
    
    I have been able to do consistently do far better than that on on mid-30
    medium displacement sailboats in8-10 foot seas and with hazy horizons.  With
    my cardboard unit 5-7 mile intercepts in rough seas with sand dunes as a
    backdrop behind the horizon.  Within 1.2' with my Astra with a very hazy
    horizon.  Better in good conditions.
    
    Point being this is just one intercept.  If taking a running fix from the
    sun, even if both intercepts are spot one, if I misjudge my course and speed
    over ground when advancing my first LOP, that would throw my running fix
    off.
    
    I guess the answer to my question would be in two parts:
    
    1.  How close were the intercepts to true position for each observation?
    2.  How much error in COG and SOG between observations?
    
    Let's limit it to question #2.  How close do you think one can come in a
    30-40 ft sailing craft over a 4-5 hour period of time?  Given points of
    sail, varying leeway, sea conditions, tides etc.?
    
    If I recall, Sumner's DR was off 1 mile in 75 for latitude, but 1 in 20 for
    longitude.
    
    As a guest or bareboat renter, I do not get the time on any given craft
    under a range of conditions to get a real feel for her.  I understand this
    might be more of a range of error, but I am interested in the
    opinions/experience of the list.
    
    
    Thanks
    
    Bill
    
    
    

       
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