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    Re: Refraction and Artificial Horizons
    From: Frank Reed CT
    Date: 2006 May 23, 14:50 -0500

    You don't need to apply a dip correction when you use an artificial
    horizon, but in fact you do need to apply refraction.

    For a specific case: suppose I measure the angle between a star and its
    image reflected in a pan of mercury and find that it's 90 degrees
    exactly. What's the true altitude? First, of course, I divide by two.
    At 45 degrees altitude, the refraction is just about 1.0 minutes of
    arc. Refraction always lifts the stars, so if I could remove the
    atmosphere, the star is lower: 44d 59' in altitude. That's the altitude
    I would use in celestial navigation calculations. If you measure the
    same altitude from the sea horizon, you would find approximately 45d 4'
    from an altitude of 16 feet, or 45d 5' from an altitude of 25 feet, and
    so on (dip is nearly 1' * sqrt(ht in feet)).


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