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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Celestaire Bubble Horizon
From: Bill B
Date: 2005 Feb 28, 00:37 -0500
From: Bill B
Date: 2005 Feb 28, 00:37 -0500
Apologies Screwed the pooch. Not meant for the list. Bill ---------- > From: Bill> Date: Mon, 28 Feb 2005 00:32:26 -0500 > To: Navigation Mailing List > Subject: Celestaire Bubble Horizon > >> on 2/11/05 7:22 AM, Jim Thompson at jim2@JIMTHOMPSON.NET wrote: >> >> -----Original Message----- >> From: Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On >> Behalf Of Frank Reed >> Sent: Friday, February 11, 2005 12:20 AM >> To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM >> Subject: Re: More on Thomas Hubbard Sumner >> George H wrote: >> "What surprises me, is that it took until 1837 for navigators to realise >> that a useful position line could be drawn from a single observation of the >> altitude of a body, even if it wasn't at meridian passage." >> >> ... Plotting a celestial line of position would have seemed alien and >> inexact when first proposed. >> >> And yet Sumner's pamphlet was ordered onto all US Navy ships the year it >> came out, although by then Sumner had spent 6 years verifying, refining and >> writing up his manuscript. His pamphlet is still very convincing to read, >> partly because it was written so logically, and partly because it was >> verified with so many real examples by a practising ship's captain who used >> the method at sea himself. At publication it had the endorsement of a >> highly regarded Harvard mathematics Professor, who had written to Bowdtich, >> so those clear-thinking experts, might have promptly focussed the navigation >> community on Sumner's discovery. From Vanvaerenbergh and Ifland, we know >> that he had been talking about his ideas prior to publication. It was >> rapidlly adopted in the UK, but only slowly in France, which explains >> perhaps why it was still there for Saint Hilaire, 40 years later. >> >> He opens his Introduction with this glorious single-sentence paragraph: >> >> "It is not so much the object of this work to present the navigator with a >> new method of 'Double Latitudes', as to afford him an accurate method of >> finding, by one Altitude of the Sun taken at at any hour of the day, wiht >> the Chronometer time, the True Bearing of the Land, the Latitude, &c., >> being, from any cause, uncertain; and to place him on his guard, when near a >> dangerous coast (and all coasts are dangerous when the Latitude is not >> accurately known) against those errors of Longitude by Chronometer, which >> arise from an erroneous Latitude used in finding the apparent time at the >> ship; directing, particularly, his attention to the fact, as shown in these >> pages, that when the Latitude is uncertain, a single altitude of the sun, at >> any time of day, whne not less than say 7 degrees high, is, with a good >> Chronometer, as useful as a Meridian Observation for Latitude ; and the >> errors above alluded to are rendered apparent." >> >> In the Introduction, he also hints at extant navigators' thinking: >> >> "...the fact, that ship-masters universally understand, and daily practice >> the numerical calculation, namely, that of finding the apparent time at the >> ship, which is the only one used." >> >> "Many navigators, having taken morning sights for the Chronometer, supposing >> the observation useless without 'the Latitude', wait for the meridian >> observation, in order to deduce the Longitude by Chronometer; or, if the sun >> be obscured till afternoon, think a single altitude under such circumstances >> is of small value ; and, by the common methods, with good reason ; for then >> the Latitude by dead reckoning form the preceding noon, must, in general, be >> used to find the apparent time of the ship ; and here is the source of error >> ; because, 26 to 30 hours having probably elsapsed, in such time the ship >> may have sailed 250 to 300 miles ... cause an error in the Latitude by dead >> reckoning, and consequently in the Longitude by Chronometer.' {Jim: we >> always talk about the "quest for longitude", but forget that in the early >> 1800's there was a less advertised and perhaps less understood "quest for >> accurate latitude".] >> >> "None of the works on Navigation, within the writer's knowledge, exemplify, >> or even hint at this important source of error" [Jim: he was referring to >> error in estimating longitude owing to error in dead reckoning the latitude, >> which was a necessary ingredient for calculating longitude] "but merely >> direct the observations be taken when the sun bears nearly 'East or West as >> possible', but it is impossible, for nearly 7 months in the year, to observe >> the sun in the East or West points." >> >> "It is hoped, that the 'Method by Projection', which explains these errors, >> and renders a single altitude, taken at any bearing of the sun, available, >> in a similar manner as a Meridian Observation, will supply a want which >> every practical navigator must have frequently experienced." [Jim: he was >> referring to accurate latitude.] >> >> I think his most convincing evidence is the last plate, which he mentions in >> the last sentence of his Introduction, almost as an afterthought. It shows >> their trip from the Mississippi, through the Gulf of Mexico, around Florida >> and then north up the coast. He was out of sight of land the whole time, >> and in waters plaqued with strong currents, which made dead reckoning >> treacherous. His plate shows 3 tracks: (1) if done by dead reckoning alone >> (they would have thought that they had sailed past the tip of Florida and >> could turn north safely, if they had not grounded on Cuban beaches on the >> way, but in fact they were still in the Gulf and would have grounded at >> Tallahassee), (2) if done by dead reckoning modified by noon sights alone >> (miles out compared to #3), and (3) their true track made good, determined >> by using his procedures (tacked precisely between the Florida keys and >> Cuba). Studying that chart, I imagine that any navigator of the day could >> see the advantages of studying the rest of the book. I can imagine rushing >> into a tavern to show that plate to buddies, if only for the obviously >> interesting sea tale it told. >> >> Sumner was pleased with this plate because it proved how his method made it >> possible to more accurately calculate the various currents in the Gulf. His >> mind really did soar -- he had found a better way not only to navigate, but >> also do to oceanography. >> >> On re-reading his pamphlet this morning, I was stunned to finally understand >> that he also clearly discovered the Celestial Fix!! Page 11: "And likewise >> if two altitudes be observed, the times being noted by Chronometer, and the >> two lines, corresponding to the two altitudes, be projected as before, then >> both the true Latitude and the true Longitude is found at the intersection >> of the two projected lines." Wow -- I had not heard that before about >> Sumner, but it only seems logical. (I should have read Vanvaerenbergh and >> Ifland more carefully). The term "line of position" did not surface until >> about 1866. I don't know about the term "fix". >> >> He took brilliant advantage of special cases during his voyages. In another >> example, his ship was becalmed at 25W, 44N. Because he was basically not >> moving (about 1 knot, he wrote), his two sights varied primarily in time, >> about one hour. The mid-morning sun's altitude climbed from 14 to 19 >> degrees. He worked out the lat/longs of the two sights each twice, using >> latitude 43d and 44d. That produced two LOPs subtended by a small angle. >> "It is seen that these two lines intersect each other in Latitude ... which >> is the true Latitude, and the true Longitude is ...". He had plotted a >> celestial fix, in January 1939. >> >> It seems to me as though the world then was ripe for the idea, hot tinder >> waiting for the match, as if everyone had it on the tips of their minds, but >> could not put it into practice. Richardson suggests that another feature >> distinguishing Sumner was that, for some reason, he took the time out to >> nail his ideas down and finish a 25,000 word manuscript. Perhaps other >> navigators had the same idea over years prior, but procrastinated, perhaps >> attending to ship's business, a snooze after lunch or a good novel, rather >> than hauling out old notes and going over them yet again. So they lost the >> opportunity to beat Sumner to his well-deserved place in history. >> > Ken > > For optimal results with Celestaire's Johnson practice-bubble horizon for Sun > and Moon observations (and acknowledging corrections to Hs/Ha differ in each > case), would you recommend: > > 1. Centering the Sun/Moon on the hairline? > > 2. Observing the body's limb tangent to the hairline? > > Thank you > > Bill