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    Re: Sextant precision
    From: Robert Gainer
    Date: 2004 Sep 30, 14:27 +0000

    This is a sharp group of people. I see Alexandre has had something to say
    about the ice before I finished composing my note in word. If I just sit
    back and read everthing coming by I can become very much smarter, maybe.
    
    The only time that I have heard of using an artificial horizon other then
    the type you can buy from Plath is on a Ship in an ice field. There would be
    no movement of the ship under those circumstances. I have been on ships and
    yachts and even in a dead calm the yachts roll too much to even consider a
    pan of anything. The ships will also roll and I can�t imagine any one having
    success with an oil filled pan. The Plath artificial horizon is built into a
    scope and would be useful under the right conditions.
    
    One of my sextants was made by Brandis & Sons and has an arc with a radius
    of 5-1/2 inches to the graduations on the limb. The limb has a silver insert
    graduated in 20-minute increments from �5 degrees to 185 degrees. The
    vernier is graduated in 30 second divisions from �30 seconds to 20 minuets
    30 seconds. The largest reading you can make is 162 degrees before you hit
    the index arm stop. I can read it to a precision of 15 seconds of arc by
    estimating the vernier. I have not used this instrument in 25 years and did
    not know how to measure the accuracy at the time. I pulled it off the self
    yesterday and if time permits I will test it this Sunday.
    
    My Cassens & Plath has a 6-inch arc that is graduated from �5 degrees to 125
    degrees by 1-degree increments. The micrometer drum is graduated in 1-degree
    increments with a vernier that reads to 1/10 minute. I think I can read it
    to a precision of 6 seconds of arc. The instrument came with a certificate
    stating that it is accurate to better then  + - 9 seconds of arc.
    
    Now that this question of precision and accuracy has come up I am wondering
    what the significance of the statement on the Plath�s eccentricity
    certificate is when it says the sextant has no errors in practical use. Is
    it conceivable that the accuracy is such that you can measure an angle in a
    meaningful way to the nearest 20 seconds of arc, the + - 9 seconds on the
    certificate.
    
    In use on the boat I am happy if my position is within 2 miles or less
    compared to the chart and to be less then 1 mile off I think is doing very
    well indeed.
    All the best,
    Robert Gainer
    
    _________________________________________________________________
    Get ready for school! Find articles, homework help and more in the Back to
    School Guide! http://special.msn.com/network/04backtoschool.armx
    
    
    

       
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