A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: How did Sumner navigate in 1837?
From: Jan Kalivoda
Date: 2003 May 23, 21:48 +0200
From: Jan Kalivoda
Date: 2003 May 23, 21:48 +0200
I cannot agree with Herbert Prinz completely, if he says: "Before the chronometer, a navigator who had a recent lunar observation necessarily had gone through the whole ritual of latitude, apparent time, LD, GMT, longitude. He would did not need a line of position. He had a position." If the altitudes of the Moon and the distance star were OBSERVED together with the lunar distance, not calculated, a sailor didn't need the latitude for obtaining the GMT from this LD measurement (of course, for some minor corrections, he had to use his latitude accurate within one-two degrees). With this GMT in hands, he could proceed as if he had had the chronometer value at his disposal - e.g. he could use both observed altitudes of the Moon and the distance star for constructing two Saint-Hilaire LOP's after 1875, supposed that the azimuth difference was sufficient, which probably was an exception in lower latitudes. But other bodies would have helped. But in the whole, Herbert is fully right in my opinion. The accuracy of the time obtained by LD's was absolutely disparate with the accuracy of the observer's latitude (found by any astronomical method common at sea). This latitude was known within 5 arc-minutes at least (DR errors for the distance run from the last observation left aside). On the other hand, the maximal, only theoretical accuracy of LD's yielded the longitude within 20 arc-minutes, but in the vast majority of cases the error was much greater - some handbooks assess it as great as 60 arc-minutes of longitude and more. This was not unrealistic for inexpensive sextants and unreliable lunar tables in, say, 1800 -1850. But the applicability of LOP depends on roughly the same accuracy of both coordinates of its pivoting point(s). There is no use of LOP in the azimuth of some 45 degrees from meridian, if one ought to suppose its shift in W-E direction for 45 nautical miles or so in view of the error of the "lunar" GMT used. Therefore sailors had to work with their observed latitude and longitude (by LD) separately on sailing ships without chronometers (and with them too, as during long voyages chronometers weren't reliable after 1-2 months at sea before cca 1880, according to their unknown or unprecise temperature corrections). They used the observed latitude for the latitude sailing and LD's (if used) for determining the day when they could expect the land on the E/W horizon while running the parallel. It was useful (although not necessary) to know the day of landfall thanks to LD and not only the week or fortnight thanks to DR. (But on the other hand, one can use those both altitudes from LD measurement for gaining the latitude by the very old general double-altitude method and this latitude is free of errors of the "lunar" GMT. I saw such procedures in old handbooks. This is the same as to use the intersection of two LOP's only for determining its latitude, neglecting the longitude of the intersection point.) And LD's were much more important for avoiding dangers than for finding ports, as George Huxtable had written some months ago. A navigator could be quite sure with LD's that he was out of a dangerous island, cape or continent and if he made a sufficient margin of safety (e.g. 1-2 degrees of longitude) owing to the known general inaccuracy of LD's, he could always change his heading without fear when necessary and run for another latitude of his target. Herbert Prinz asked for some examples from the old navigation practice. I can provide one (according to P.B.Andresen, Die Geschichte der Monddistanzen, Hamburg 1924, p.55): During the Napoleonic wars (the year is not stated) the East-Indian merchant sailing ship Arniston took part in the convoy from Ceylon to England with cca 300 disabled ex-servicemen, 50 women and many children on deck. The convoy was escorted by two corvettes and merchant masters relied on them in their navigation. But the Arniston lost the convoy in the storm near the Cape, bound westward. The master hadn't a chronometer and didn't use LD's. Each day he had diminished his assumed daily run by 20 miles in his calculations so as to make provision for any navigational uncertainty. When he supposed on the basis of such DR that the Cape of Good Hope had been rounded, he changed the heading northwards to St.Helena island. But after another twelve hours the Struysbey bay appeared on the north horizon as leeward shore in the storm, 100 nautical miles eastward from the Cape of Good Hope. Only five persons survived. --------------------- And still another remark. Herbert Prinz assumes that the use of LOP's was fostered by the fact that they can be graphically resolved and used on charts, e.g. while making a landfall. It is, in the event, more difficult to use LOP's without a chart, only by calculation. And this was a great hindrance for them - sailors refused to use charts for plotting LOP's for a long time. And one cannot be surprised by it. On cramped, filthy wooden ships the chart work should have been a torture for both men and charts. The proof of this my statement is that Sumner himself pays great attention to explain, how his LOP's can be evaluated by calculation, without any plotting. He uses the proportional logs (known from LD clearing) for it, calculates altogether four hour angles of two bodies for two different latitudes and uses proportions to find observer's true position on the basis of two Sumner lines. He doesn't use azimuths of bodies yet. "Cloudy Weather Johnson" (A.C.Johnson, named so by his famous "On Finding the Latitude and Longitude in Cloudy Weather", published many times in 1880 - 1910) improved this method by using ABC-tables for finding the azimuth of Sumner lines of both bodies and so calculating observer's true position much more quickly and simply than Sumner, again without any use of a chart. Lecky in his renowned "Wrinkles in Navigation" popularized this Johnson's method widely. Both stress that the great advantage of this procedure is that any chart is superfluous while using this method. Only after Saint-Hilaire had won (and iron steamers with their comfortable navigation rooms), the charts started to be used for reducing astronomical sights widely. And even for that method I read he rules permitting to avoid a chart in using St.-Hilaire LOP's, although it was even more difficult than in Sumner's procedure. Jan Kalivoda