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    Re: Refraction at the horizon. was: Re: Celestial Navigation without a sextant.
    From: Marcel Tschudin
    Date: 2008 Mar 14, 23:47 +0200

    I missed to reply on one important item:
    
    George wrote:
    >  refraction depends on temperature gradients at FAR greater heights.[than the observer near sea level]
    
    The word far (even written in upper case) may lead here to a
    misunderstanding. The air layers near the observer (below his eyes and
    above them) contribute most to the total refraction. Air layers high
    above him contribute only an epsilon to the total. In case of
    inversions, only those in layers close to the observer may cause
    ducting (infinite refraction); an inversion at a layer having a great
    distance (in height) to the observer can't produce ducting because the
    angle of incidence is too large.
    
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