A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sumner's Line (Navigation question)
From: George Huxtable
Date: 2006 Feb 2, 15:46 -0000
From: George Huxtable
Date: 2006 Feb 2, 15:46 -0000
Ken Gebhart wrote- I am looking for help in understanding something about Thomas Sumner's event during which he discovered his line of position. Space does not permit recounting the details of his situation, nor a drawing of the navigational plot of his attempt to find Small's light. However, this is covered in detail in Bowditch and many other navigational books. At the very end of the recount it says" The DR position was found to be in error by 8 min too far south, giving a longitude of 31 min, 30 sec too far west. The result to the ship might have been disasterous had this wrong position been adopted". My question is howso? Can anyone tell me from historical insight of navigational procedures what course Sumner would have set had he not questioned his first position? In other words would he have turned to go straight through St. George's Channel, or would he have turned to acquire Small's Light first? I can see that the latter choice might have been more dangerous because he would have gone south of Small's light, and his eta would have been upwards of 2 hours late, catching him off-guard when land (or rocks) appeared. But there were the Saltees Rocks light and the Tusker Rock light off the coast of Ireland which would have given him some protection had he opted to go straight for the Channel. This is not just a pedantic question I have. I talk about this situation in my seminars to show how his line of position was discovered, but thankfully no one has questioned me about exactly how a disaster was averted. Any comments are welcomed. ==================== Response from George. First, Ken should acquire a copy of "Line of position navigation" subtitled "Sumner and Saint-Hilaire, the two pillars of modern celestial navigation", by Michel Vanvaerenbergh and Peter Ifland, Unlimited Publishing, 2003, $13.99, ISBN 1-58832-068-5. Peter is a member of our list and occasional contributor, and also author of that wonderful book about navigational instruments, "Taking the stars". His book about the position-line contains, in good facsimile, the complete text and plates of Sumner's original article of 1843, with intelligent explanatory notes, and a host of stuff about St-Hilaire's improvements on Sumner. Please forgive a bit of pedantry to start with. The light Sumner needed to see (well, the lightHOUSE actually, as it was daytime) was not "Small's light", as both Sumner and Ken refer to it, but "The Smalls light", named after the lethal group of low rocks and reefs that it protects against. Sumner's passage was from Charleston (South Carolina) to Greenock (Western Scotland, on the Clyde), via St George's Channel (between Ireland and Wales). He was coming from the general direction of the Azores, and had no observations since about 1500 miles back, except for a single sounding, presumably South of Ireland. So, he was relying on his dead reckoning. Sumner had a nice beam wind, from a SouthEasterly direction, but that would have made the SouthEast corner of Ireland, with its Tuskar Rocks, a nasty lee shore. Presumably, then, he would wish to keep to the Welsh side of St George's Channel, to preserve his freedom to act in an emergency. Sumner describes the weather as boisterous, and very thick. This is important. The hidden Smalls Rocks lie on the Welsh side of the passage, but a ship can readily keep clear of them as long as she can see the lighthouse tower in time. However, if the weather is too thick, unless a vessel is heading straight toward that lighthouse, she might sail past without seeing it, and get into the middle of those rocks before realising it. There are also other rocks too, further East, off that coast. What Sumner's 10 am position line told him was the exact course to keep to reach the Smalls light. As long as he trusted that observation, he could sail straight toward it and be sure he would reach the light before he was in danger from the rocks around it. And so it turned out. The Smalls lighthouse was seen "close aboard" (so only just in time). Then, Sumner knew exactly where he was, and the rest was easy. Ken wrote- "At the very end of the recount it says" The DR position was found to be in error by 8 min too far south," giving a longitude of 31 min, 30 sec too far west. The result to the ship might have been disasterous had this wrong position been adopted". My question is howso?" It's strange, but I can't find those actual words in Sumner's account. Did he write it out elsewhere, I wonder? Or are those Bowditch's's words, perhaps? Sumner actually wrote- "The Latitude by dead reckoning, was erroneous 8 miles, and if the Longitude by Chronometer had been found by this Latitude, the ship's position would have been erroneous 31 1/2 minutes of Longitude, too far W, and 8 minutes, too far S. .." The next sentence, that Ken attributes to Sumner, (and may be somewhat over-dramatic) seems to be absent from Sumner's own text. Sumner was describing a common state of affairs round our coasts (as he points out), with bad visibility near the horizon but taking advantage of a momentary glimpse of the Sun. Ken asks what he would have done had he not been able to deduce that "Sumner Line". I think he would have tacked about, standing off and on, to mark time (as he had done the previous night), holding position until the weather cleared enough to allow him to tackle the St. George's passage with comfidence. I doubt if there was any great imminent danger. That's my own view, but others may put a different slant on it. It will be interesting to see what's offered. contact George Huxtable at firstname.lastname@example.org or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.George.