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    Refraction formula
    From: Paul Hirose
    Date: 2000 Sep 25, 10:25 PM

    The tables I posted showing the effect of refraction on angular
    separation used the refraction values from the table in the Nautical
    Almanac. For some reason it never occurred to me that the Almanac
    would have a formula I could program into my calculator, but it does:
    R = 1 / tan (H + 7.31 / (H + 4.4))
    H is the non-refracted altitude. R is the number of minutes to add to
    H to obtain apparent (refracted) altitude. The formula is for standard
    conditions of 10 degrees C and 1010 millibars. Be sure to observe
    correct order of arithmetic operations: divide 7.31 by (H + 4.4), then
    add H and take the tangent.
    Of course the formula is an approximation; note that refraction is not
    exactly zero at H = 90!
    I rewrote my calculator program to use this formula and recomputed my
    tables. The differences were trifling.

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