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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Coordinates on Cook's maps
From: George Huxtable
Date: 2007 Apr 18, 13:32 +0100
From: George Huxtable
Date: 2007 Apr 18, 13:32 +0100
Alex's latest posting contains much interesting stuff, including- | I used Cook's Astronomical observation logs. | Available on the web. | There they list all observations. | Most of them cannot be tied to certain places | whose position I can verify. Where did those come from, on the web? Please give us the address. I, for one, would like to get access. Were they derived from William Wales' "Astronomical Observations ...", that I suggested Alex might consult, I wonder? Many observations, of course, will have been made in the open ocean, and therefore useless for Alex's purpose. It's only those taken near a definable landmark that can be tied to Alex's Terraserver positions. Please, Alex, tell us a bit more about Terraserver, and give the address at which it can be found. | But some of these observations have very specific | places on the shore attached. Like "Point Venus". | I find this "point Venus" on a Cook map. | I suppose that this is the place whose position he | (they) determined with the most care | (Hundreds of observations are listed under "point Venus" | in the astronomical obs. log. | They had an observatory there. I suppose | that co-ordinates on the map were | all measured FROM this point | Venus, whose position they determined with such care. | Then I find this point Venus on the Terraserver. Yes, Point Venus was the main spot from which the Transit of Venus was observed, which was one main purpose of the whole expedition., It's exact location was very important. Taken together with reports from other observers in various parts of the World, the transit was used to determine the size of the Solar System. So Cook was surveying much more than just Earthly geography! | On other maps, I used suspicious capes, | anchor stations etc. I pointed out with reference to map 9, that in the | > Society Isles that the map shows, | > Cook's journal refers to only one | > determination of longitude, at Huaheine Island | | I was not using Cook's journal. | I was using the "Original astronomical Observations..." | by his associates like Bayly, and Wales and other officers | (available on the web) | to determine the points where | they made really carefull observations. Ah. Where, on map 9, had such observations been noted? | | Like 50-100 Lunars with 4 sextants and 4 | or 5 different people | from land. Yes, indeed. The Navy was by no means short-staffed on such expeditions, and there were also many young midshipmen under training. Green used all the talent he had, and took his training duties seriously. Many expert navigators emerged from their voyages with Cook; for example Bligh and Vancouver. | | Comparable with what I made from my balcony:-) Indeed, they were comparable with Alex's work. And like Alex, their measurements, when on land, extended over many weeks, in which case some of the monthly variation in the almanac predictions would cancel out. | | I used THESE points on THEIR maps. ===================== About possible errors in the Almanac predictions of lunar distance, I had written- | > That's a really important question, well worth asking, | > and answering. The | > best study of such matters that | >I know of was made by Nicholas Doe, "Captain | > Vancouver's longitudes - 1792", in Journal of Navigation, Alex asked- | American or British? The Journal of Navigation is the British publication; 3 per year, soon increasing to 4, from the RIN (Royal Institute of Navigation). The American complement, from the Institute of Navigation, is named "Navigation". ============Digression starts here- | I renently bought a CD from the American one, | and was very much disappointed. I mean I payed $50 | (including delivery) | for a piece of crap. I mean there are 4-5, maximum 10 | papers worth reading (from my point of view) on the whole | disc, and 3 or 4 of these | papers are available on the web for free | anyway. I agree with Alex here in one respect only. The CD cost $25, and "shipping" to the UK cost another $25! I thought that overseas buyers were being ripped-off over that delivery charge, when the stamp cost no more than $5, but am appalled to learn that the same charge was made to buyers within the US. That all leaves a sour taste. But I think that Alex's dismissal of its contents as "a piece of crap" is quite unjustified. It depends, I suppose, on the breadth of your navigational interests. Over recent years, Alex has concentrated, very hard, on a single (important) question, that of observational precision. I am delighted to note that he is now expanding his interests into navigational history, and I hope that will grip him as hard as it has gripped me. Having my own small boat, I've been tinkering into various aspects of navigation for forty-odd years, so I have found a MUCH greater fraction of the papers on that disc to be of interest, than Alex has. I suspect that many other listmembers are likely to find the same. | My general philosophical opinion is that | "REAL science is available for free", | on the web or otherwise, is strongly confirmed | by this CD. Well, though I agree that in a perfect World that might be the case, we have to take it like it is, make the best of it, and do what we can to change it. Things are changing, in the right direction. I wonder if any of Alex's mathematical publications have ever appeared in a journal which charges a cover price? I compare the IoS disc with a corresponding offer from RIN, in which they offer the digitised run of fifty-odd years of Journal of Navigation, on CD (the whole thing, not a distillation of celestial papers only) for about �70 (=$140) to members, a bit more to non-members. So far, that cost has put me off, though I know there's a wealth of stuff out there I would enjoy reading. Perhaps I'll drop some hints, nearer my birthday. =================end of digression. There's too much in that Doe paper to transmit to Alex over the ether, but if he sends me his postal address I will be pleased to post a scanned copy. | > It would be no surprise if | > corresponding errors occurred in the almanacs | > used by Cook, and it would be a valuable and | > simple exercise to discover if | > they did. | | I wolud like to do this. | (I am mostly interested in sextant performance, | so I would like to seratate the sextant errors from | the almanac errors). | The hardest thing (for me) is to discover | what almanacs precisely did they use, and to find | these almanacs on the web or elsewhere. | | The "true almanac" with exact positions is supposedly | available on the Frank web site. There's no problem about the Almanac that was used by navigators of those days, starting with Cook. It was "The Nautical Almanac and Astronomical Ephemeris", first published by order of the Commissioners of Longitude for the year 1767, and continuing, with some changes of title and publisher, every year to this day. Some voyagers, mostly French, would have the older "Connaissance du Temps" on board as well. Pirated versions emerged, which were generally copies of the original. I understand that early years of the Nautical Almanac are available in scanned form from various websites, for most years at least, perhaps by now for all years. Some reader, more familiar with the web than I am, will I hope point Alex to where they can be found. The early almanacs carried predictions for lunar distance, mostly to two celestial objects, one East and one West of the Moon, for most days. These were stated to an arc-second, though the errors in prediction were far greater than that, at three-hour intervals. The (human) computers deduced those figures by quadratic interpolation between Moon predictions, the ecliptic latitude and longitude of the Moon being given, on another page of the almanac, to the nearest arc-second for noon and midnight of each day. Longitudes were quoted in signs (there were 12 signs, each of 30 degrees), degrees (up to 30), minutes, and seconds, so degrees (to 360) are found by multiplying the sign-number by 30, then adding degrees. Discrepancies between the old almanac and modern calculations are best studied in terms of those twice-a-day Moon ecliptic lats and longs, rather than in the 3-hourly lunar distances themselves. This assumes that the computers did a precise job of interpolating, and then calculating the angular separation to the body; which indeed they did. Precise positions of those few reference-stars had by then been well enough established, by Halley and others. It's easy to convert between those ecliptic coordinates and the RA / dec of a modern prediction, for which you need an accurate value of the Earth's tilt of axis, at that date. And you need to be aware that the time scale of the old almanacs (up to 1834) was in terms of apparent time not GMT, so the Sun really was exactly on the Greenwich meridian at noon. For comparison, modern Moon predictions have to be taken, not for noon and midnight UT, but offset from that by the equation of time. Once the errors in the lunar positions have been uncovered for the dates of Cook's voyages, that shows the unavoidable errors that were imposed on Cook's longitudes, but also the same errors applying to all other navigators, using that almanac, anywhere in the World at those same dates. So it would be useful evidence for maritime history generally. A worthwhile bit of study. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---