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    Re: finding your latitude through double altitudes (and elapsed time).
    From: Frank Reed CT
    Date: 2007 Apr 01, 02:07 -0700

    Hi Joel.
    You may want to get a copy of "Peirce's Trigonometry". Benjamin Peirce
    (yes, that's how he spelled it) was the mathematician who did most of
    the calculational work launching the American Ephemeris & Nautical
    Almanac back in the 1850s. He was a student, colleague, and great
    admirer of Nathaniel Bowditch.
    Like most textbooks from that era, the full title of "Peirce's
    Trigonometry" runs for half a page. The key part of the subtitle for
    this discussion is: "particularly adapted to explaining the
    construction of Bowditch's Navigator". And it lives up to that
    promise. Peirce explains the derivation of the vast majority of the
    trigonometric rules in Bowditch. Naturally, the language is that of
    19th century math, so it takes some getting used to, but it's a
    helluva lot easier than trying to reverse-engineer the sparse rules as
    laid out in Bowditch and other navigation manuals from that era.
    Remember when finding old books was an adventure in dusty old
    bookstores? And then three years ago, it was an adventure in watching
    online auction sites? Well, the adventure is over. :-) There are 19th
    century editions of Bowditch online, and there are at least two
    different scans of Peirce's book. There's one on google books here:
    I believe the method you were asking about is outlined starting on
    page 198.
    42.0N 87.7W, or 41.4N 72.1W.
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