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    Re: Refraction question
    From: Fred Hebard
    Date: 2004 Nov 13, 21:38 -0500

    On Nov 13, 2004, at 6:32 PM, Alexandre Eremenko wrote:
    
    > I have a question about land observations
    > (NOT using natural horizon, say art horizon,
    > or distances).
    > My altitude over the sea level is 530 m.
    > This should somehow affect refraction (decrease the
    > magnitude of the correction?).
    > How much? Can I rely on the almanac refraction tables
    > in my observations? They are surely made for the sea
    > level.
    >
    > I have an unexplained error of the order of 0.3'
    > in many of my observations. Can my altitude be
    > responsible for this?
    >
    > Alex.
    >
    >
    
    Alex,
    
    Aside from Frank Reed's formulas, Table A4 of the Nautical Almanac
    allows you to correct your refraction for non-standard barometric
    pressures and temperatures.  It is useful up to elevations of about
    2000 feet above sea level.  (I might also add that the elevation of
    Purdue is probably about 530 feet not 530 meters).  You would need to
    know by how much your elevation decreases barometric pressure; at 2000
    feet our pressures are reduced about 2 inches of Mercury from the
    reported values, which are adjusted to sea level. Barometers also
    usually are adjusted for the local elevation to give sea-level values.
    Hopefully somebody here will chime in with the correction formula.
    
    If you study Table A4, you will find that changes in refraction for
    normal altitudes (above 20 degrees) only occur at temperatures above 50
    F or below 20F.  At temperatures of 60-70 F, the correction is only
    0.1' of arc for altitudes above 20 degrees.  Thus your unexplained
    errors of 0.3' of arc probably would not be due solely to non-standard
    conditions, if the observations were taken in the last month or so near
    Chicago and were altitude shots.
    
    A timing error of 1 second can easily lead to errors of 0.15' of arc.
    This would be another area to examine.  For altitude shots, I usually
    set the sextant and let the body drift into position, so as to time the
    contact accurately.
    
    Fred
    
    
    

       
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