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    Re: Problem with a sextant
    From: Peter Fogg
    Date: 2006 Apr 25, 17:22 +1000

    > 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?
    
    Hey Bill; how long is that bit of string? I've had zero intercepts with
    sights made on land, meaning that the error was less than the resolving
    power of my method - one tenth of a minute of arc; less than 200 metres. But
    could these have been flukes, coincidences? Maybe.
    
    On land getting within one nautical mile with sextant should be achievable.
    A surveyor using a theodolite, the same celestial bodies, and assistance
    from a variety of methods statistical and graphical, plus being very careful
    about common sense stuff like the timing of sights, apparently hits the wall
    at one second of arc - about 30 metres.
    
    On sea its a different story. Ships have two great advantages; their
    elevated and more stable platform. I guess this is how navigators could
    pride themselves in fixing positions to within a mile or so of actual. Even
    with less than nice calm weather moderate movement could yield to
    experience.
    
    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.
    
    Incidentally, like Alekandre my sextant seems to have a small systematic
    error. Sextants aren't perfect either.
    
    
    

       
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