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    Peter Ifland's new book on Sumner and Saint-Hilaire.
    From: George Huxtable
    Date: 2003 Jun 29, 16:38 +0100

    Peter Ifland has written recently, bringing his new book to the attention
    of this list. The full title is "Line of Position Navigation, Sumner and
    Saint-Hilaire, The Two Pillars of Modern Celestial Navigation.", by Michel
    Vanvaerenbergh and Peter Ifland.
    I wish to add a few words of my own.
    To those that are familiar with Peter's "Taking the Stars", with
    photographs and text about navigators' cherished instruments of their
    craft: well, this book couldn't be more different. It's about the early
    texts, from the mid-1800s, that transformed the way that navigation was
    done, and introduced the concept of "position-line navigation" that
    celestial observers still use to the present day.
    Until Sumner's time, finding position required a knowledge of latitude from
    a meridian altitude of the Sun or some other body. This knowledge of
    latitude could be combined with an "observation for time", the altitude of
    a body which lay as far from his North-South line (and as near his
    East-West line) as possible. Taken with a knowledge of Greenwich Time (from
    a chronometer or from a lunar distance) this allowed the longitude to be
    determined. Without that known latitude, determination of longitude just
    couldn't be done.
    The American merchant Captain, Thomas H Sumner, hit on the idea that even
    if a navigator didn't know his latitude, he could assume a plausible value,
    and calculate a longitude on that basis. Then he could assume another value
    for latitude, and calculate a new longitude. Then, joining those two points
    by a straight line on his chart (and extending it as necessary) he would
    know that his position must lie somewhere along that oblique line (the
    "position line"), whatever his latitude happened to be. From then on,
    mariners could derive a position-line from just a single altitude of any
    body in the sky, as long as its coordinates could be identified in the
    almanac. If two position lines could be obtained, where they crossed became
    a fix.
    Sumner proposed his method in a booklet published in 1843, which is now
    very rare, and readable only at specialist libraries. I've never seen a
    copy myself, and have had to rely on summaries by others (such as Cotter).
    What Peter Ifland has done is to obtain a crisply clear copy of the
    original paper, and then scan all 88 pages into his book to make every
    detail available to us all, for the first time ever. Sumner himself has a
    clear and direct way of explaining, and a further explanation has been
    added to help the modern reader, who may have difficulty in grasping how
    the concepts were understood before Sumner came on scene.
    A second, distinct part of the book deals with the contributions made by
    the French Naval Captain, Marcq Saint-Hilaire, about 30 years later. This
    has presented Ifland and his co-author, Michel Vanvaerenbergh, with a more
    tricky problem, as Saint-Hilaire wrote only in French, so for an anglophone
    readership a translation was required: because labelling in French was
    deeply embedded in the many diagrams, they all had to be drawn anew.
    Saint-Hilaire's main contribution was to show how the navigator could take
    any "assumed position", somewhere in the vicinity of where he must be. The
    azimuth of an observed body from the assumed position could be calculated,
    and a position line drawn in, at right-angles to that azimuth, displaced by
    the "intercept", the difference between the measured altitude and that
    calculated from the assumed position.
    Saint-Hilaire contributed some deep study to the geometry of the problem:
    some of it, it should be said, highly mathematical. The present book omits
    some of this more detailed mathematical analysis.
    All this work was published in the "Revue Maritime et Coloniale", as "Note
    sur la determination du Point", pages 41-58, Mars-Octobre 1873, and "Calcul
    du Point Observ?", pages 341-376, Mar-A?ut 1875".
    A modern introduction and a very full series of modern "Technical notes"
    has been added to help the present-day reader to understand what
    Saint-Hilaire was getting at with his analysis.
    The modern text goes on to bring the reader up to date with celestial
    navigation, to the beginning of the 21st century.
    I think the authors should be congratulated on bringing these deep roots of
    our craft to the attention of us all, and what's more at a US dollar cost
    of only $14. It's a real bargain!
    If you have an interest in celestial navigation and its development, buy
    this book.
    It can be ordered from-
    http://www.unlimitedpublishing.com/authors   (Unlimited Publishing,
    Bloomington, Indiana)
    ISBN 1-58832-068-5
    George Huxtable.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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