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    Re: Impressed with the Raft Book
    From: Hewitt Schlereth
    Date: 2010 Feb 1, 11:58 -0400

    FWIW
    
    I just got back from 5 days at sea in the Caribbean and we got two
    LOP's less than 2NM from the GPS position using 249. This was one of
    my favorite sights at sea in the pre-GPS era and they always squared
    up with the DR satisfactorily - i.e. within 5-10% of the distance run
    since the last fix.
    
    Henry Shufeldt in his book Slide Rule for the Mariner cites similar
    results to Dutton and my experience sure bears them both out.
    
    Also, I didn't understand the reference to latitude in connection with
    horizon sights. They give you an LOP, just like any usual sextant
    sight, only here Hs is 0�.
    
    Hewitt
    
    On 2/1/10, Apache Runner  wrote:
    >
    > I tried to estimate the uncertainty from this method.   Refraction is
    > definitely one of  them, although there are ways of guesstimating refraction
    > effects that will probably be good to maybe 5 arc-minutes.   If you  don't
    > correct for refraction, you can be systematically off by 30 arc minutes or
    > more.
    >
    > The rising and setting angle of the sun can also be a nuisance.
    >
    > Certainly low lying clouds, fog banks etc will be a big issue.
    >
    > My estimate was that under ideal conditions one could expect an accuracy of
    > about 20 arc-minutes, but you have to put in a refraction correction - ie.
    > know when to start the clock and when to stop the clock.
    >
    > Doesn't a watch count as an instrument?
    >
    >
    > On Sun, Jan 31, 2010 at 10:31 PM, Mike Boersma 
    > wrote:
    >
    > >
    > >
    > >
    > > Frank Reed wrote: "But in the real world, the observed times of sunrise
    > and sunset are variable for a number of reasons and those variations will
    > significantly diminish the accuracy of this approach to finding latitude."
    > >
    > > What might these reasons be? Refraction? Clouds?
    > >
    > >
    > > "It's certainly not a worthless method, but its accuracy is low."
    > >
    > > This method does not appear to be more or less accurate than other no
    > instrument or crude instrument methods of determining latitude.
    > >
    > > I would be surprised if results by this method were better than 1/2d when
    > attempted under real world conditions.
    > >
    > >
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