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    Re: Ex-Meridian Exercise
    From: Ronald van Riet
    Date: 2011 Jan 7, 10:08 -0800

    this thread reminded me of a description I read a while ago (and which I have now found again) describing the use of the Blundell 504 slide rule for air and sea navigation (Podmore 1974).

    One of the chapters is dedicated to dead reckoning and celestial navigation with a section on Ex Meridian (page 69 in my copy).
    It presents the following equation:

    Reduction = .5235 . cos Lat cos Dec sec Alt x HA²

    This is exactly how the book gives the equation, no parentheses are used.

    This equation was deemed important enough to put a special gauge mark on the said slide rule at this value.

    I have been looking in my navigation books for this equation but without success.

    Is anyone familiar with the equation and how it is derived?


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