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    Re: Peter Ifland's new book on Sumner and Saint-Hilaire.
    From: Jim Thompson
    Date: 2003 Jul 22, 19:39 -0300

    I received my copy a few days ago from Peter, and read it on the boat (a
    fitting setting!).  Wonderful.  It answered all of my technical questions
    about the nature of navigation in Sumner's time.  It provided me with more
    insight to his Dec 17 1837 discovery of the celestial LOP, which in turn
    helps me up the learning curve with modern CN.  I have updated my
    description of Sumner's discover accordingly:
    I am still immensely curious about the human aspects of that morning.  What
    was his ship like?  What was he like?  What was the cabin like where he
    actually reduced his sight and worked his simulations?  What books did he
    have with him?  What sextant did he use?  How was it lit?  His academic
    description of how he made the discovery is quite humble and matter of fact.
    We might never know these details.
    Jim Thompson
    Outgoing mail scanned by Norton Antivirus
    > -----Original Message-----
    > From: Navigation Mailing List
    > [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of George Huxtable
    > Sent: Sunday, June 29, 2003 12:38 PM
    > Subject: Peter Ifland's new book on Sumner and Saint-Hilaire.
    > 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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