A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Peter Ifland's new book on Sumner and Saint-Hilaire.
From: George Huxtable
Date: 2003 Jun 29, 16:38 +0100
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---.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. ================================================================