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    Re: How did Sumner navigate in 1837?
    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 
    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 
    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 
    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 
    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

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