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    Re: Making an artificial horizon, and leveling thereof
    From: Geoffrey Kolbe
    Date: 2011 Jan 26, 06:10 +0000

    At 04:56 26/01/2011, you wrote:
    
    >Gary:
    >
    >In answer to your question, the usual, so far as I know, way. Zero
    >the micrometer, I'm using an Astra 111B sextant. Then zero the index arm.
    >
    >Re the AH shots listed, I focoused the sextant scope on the roof
    >lines of small buildings, private residences, within 1/4 mile of
    >where I stood, beyond that proceeding as sextant instructions I've
    >seen say, adjusting the micrometer till I got an "unbroken horizon
    >line", that being the aforementioned roof line. Read the error off
    >the micrometer, subtracting for "on the arc", adding for "off the arc".
    
    
    The index mirror and the horizon mirror on the sextant are separated
    in height by about two inches. 1/4 mile is 440 yards, so there will
    be a parallax error of around half a minute in your IE determination,
    which is quite significant. Sextant altitudes of celestial bodies
    will end up being around half a minute higher than they should be.
    
    
    Geoffrey Kolbe
    
    
    
    
    

       
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