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    Re: Altitude correction using Artificial Horizon?
    From: Dan Allen
    Date: 2000 Sep 05, 10:27 PM

    There are several corrections for a sextant reading.
    Dip, refraction, parallax, and semi-diameter are the main correction 
    variables.  (Index error, or the innate sextant error, is more of an 
    individual constant that varies sextant to sextant.)
    Dip and semi-diameter are not needed when measuring an angle with an
    artifical horizon.  Why?
    Dip is a function of height above the horizon, but since the horizon is not 
    being used (an artificial one is instead), dip does not apply.
    Semi-diameter is often not used when measuring the sun and moon when using an 
    artificial horizon, as you can center the body and dispense with this 
    correction, but if you measure to the upper or lower limb, then you still 
    need it.
    Parallax and refraction still need to be applied.  Both of these depend upon 
    the angle observed, while refraction is also a function of temperature and 
    barometric pressure.  These are combined in most sight reduction schemes are 
    are often referred to together as an "altitute correction".
    Dan Allen
    -----Original Message-----
    >> Is it necessary to apply any correction for altitude when reducing a sun
    >> sight taken with an artificial horizon?
    Kirk Benson
    34d 10.8' N
    84d 12.7' W

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