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    Re: Refraction near the Horizon ? Observation vs. Calculation
    From: Paul Hirose
    Date: 2013 Apr 07, 20:56 -0700

    Does the Almanac still have "methods and formulae for direct
    computation"? That chapter in the 1998 edition gives the dip formula
    0.0293° * √h, where h is meters. I tested that against the dip table
    (A2) and got perfect agreement at the extremes as well as several
    intermediate eye heights
    Refraction may be calculated with the formula 0.0167° / tan(H + 7.31 /
    (H + 4.4)) at standard conditions of 10C and 1010 mb, according to the
    same chapter. To correct for nonstandard conditions, multiply the result
    of the above formula by 0.28 * P / (T + 273).
    The refraction values in the Almanac are the total deflection of the ray
    from the observer to the top of the atmosphere. However, in the case of
    dip we are only interested in the portion of the ray from the observer
    to the point where it grazes the sea.
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