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    Slide rule Bygrave sight reduction
    From: Paul Hirose
    Date: 2017 Aug 20, 22:14 -0700

    On 2017-08-20 17:33, Greg Rudzinski wrote:
     > Using tangents rather than cotangents seems to work better when
    performing Bygrave sight reduction with the slide rule or trig calculator.
    My "360 degree Bygrave slide rule sight reduction" message in February
    described a variant suitable for a conventional straight slide rule.
    It's written entirely in cosines, tangents, and arc tangents. But in
    practice, cotangents must be employed in some cases, such as the
    penultimate line of the example:
    Hc = arc tan(cos 128.5 * tan 116.20)
    In my terminology, 116.20 is a "red angle" on the T scale. Therefore,
    instead of multiplying by tan 116.20, divide by cot 116.20.
    A double T scale eliminates most such inversions, but not always. For
    instance, 85° is beyond its range. That's no problem if you know
    multiplication by tan 85 is equivalent to division by cot 85, aka tan 5,
    the latter being found on scale ST.

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