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    T B Corrections
    From: Chuck Taylor
    Date: 2000 Nov 09, 18:01 EST

    The T & B (Temperature & Barometric Pressure) corrections in the Nautical
    Almanac are provided for occasions when temperature and barometric pressure
    differ from the "standard" temperature of 10 degrees Celsius (50 degrees
    Fahrenheit) and 1010 millibars (29.83 inches of mercury).  They are applicable
    mostly to low-altitude sights and allow for the fact that refraction can vary
    with T & B.
    An aeronautical engineer friend of mine tells me than "Standard Temperature and
    Pressure" in his world are 15 degrees Celsius (59 degrees Fahrenheit) and 1013
    millibars (29.92 inches of mercury).
    Does anyone know the logic for selecting 10 degrees Celsius and 1010 millibars
    as the basis for the refraction tables?  There must be some historical reason,
    but I can't find it in Bowditch.
    Chuck Taylor
    Everett, WA, USA

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