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    Nautilus Navigation
    From: James N Wilson
    Date: 2012 Jun 18, 17:22 -0700
    In 20,000 Leagues Under the Sea, Jules Verne describes Captain Nemo’s voyages over many oceans. I read it with fascination, but I was interested in how he navigated the Nautilus.
     
    His latitudes and longitudes often lack cardinal descriptors. Perhaps this was a custom when in familiar waters. He notes that latitude was determined from the sun by measuring its altitude with a sextant, and longitude determined by a chronometer. Verne makes no note as to how the chronometer was used to determine longitude. No mention of almanac data is made.

    He notes that chronometers were set for prime meridians at Paris, Greenwich and New York. No mention is made as to which of those his charts used.

    Speeds are given in miles per hour, knots never being mentioned. This may be a consequence of having been translated into English. One passage notes: "Accordingly, our speed was twenty-five miles (that is, twelve four-kilometer leagues) per hour." An unknown unit of speed. Occasionally, it is noted in meters per second.

    In determining his position in Antarctica, he determines he is at the south pole at the equinox, the last day that the sun was visible, barely rising above the horizon:

    "Arriving at the summit of this peak, Captain Nemo carefully determined its elevation by means of his barometer, since he had to take this factor into account in his noon sights.

    "At 11:45 the sun, by then seen only by refraction, looked like a golden disk, dispersing its last rays over this deserted continent and down to these seas not yet plowed by the ships of man.

    "Captain Nemo had brought a spyglass with a reticular eyepiece, which corrected the sun's refraction by means of a mirror, and he used it to observe the orb sinking little by little

    along a very extended diagonal that reached below the horizon.

    "I held the chronometer. My heart was pounding mightily. If the lower half of the sun's disk disappeared just as the chronometer said noon, we were right at the pole.

    ""Twelve!" I exclaimed.

    ""The South Pole!" replied Captain Nemo, in a grave voice, handing me the glass, which showed the orb cut in exactly equal parts by the horizon."

    Defining the instant of transit is fraught with uncertainty. Perhaps the reticular eyepiece helps to better define it, but there remains the problem of measuring with precision a slowly moving object. I think it’s technical mumbo-jumbo. A different translation refers to a lenticular glass, which I read gives two different images. That doesn’t help.

    No matter to which prime meridian the chronometer had been set, it would not register local noon. So some calculations would be necessary to compensate. Now, all meridians converge at the poles, but Nemo couldn’t count on that unless he was sure he was at the pole. There’s a chicken and egg problem here.

    Further, he was within sight of the Nautilus, so was on shore at the time. We know that the pole is quite some distance from any shore, so this part is pure fiction.

    He navigates Antarctic waters with a magnetic compass, since invention of the gyrocompass was still in the future. But with variations of over 80°, that instrument is virtually useless, especially since no one before had produced charts noting this. Verne notes:

    "Our compass indications no longer offered any guarantees. The deranged needles would

    mark contradictory directions as we approached the southern magnetic pole, which doesn't coincide with the South Pole proper. In fact, according to the astronomer Hansteen, this magnetic pole is located fairly close to latitude 70 degrees and longitude 130 degrees, or abiding by the observations of Louis-Isidore Duperrey, in longitude 135 degrees and latitude 70 degrees 30'. Hence we had to transport compasses to different parts of the ship, take many readings, and strike an average. Often we could chart our course only by guesswork, a less than satisfactory method in the midst of these winding passageways whose landmarks change continuously."

    But that really doesn’t address the problem of large variation. The daily noon sun shot was their primary means of location, and directions had to be deduced from those.

    He calls the Southern Cross, "The polar star of the Antarctic regions," but it’s not at all near the South Pole.

    He describes the gulf stream:

    ". . . it heads north up the American coast, advances as far as Newfoundland, swerves away under the thrust of a cold current from the Davis Strait, and resumes its ocean course by going along a great circle of the earth on a rhumb line."

    Now a rhumb line and a great circle can only co-exist at the equator and on a meridian. Verne’s description fits neither.

    I got the impression that Verne understood quite a bit about navigation, but his knowledge of flora and fauna was much more impressive.

       
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