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    Re: Jack Aubrey's fixing of longitude
    From: Albert Rodok
    Date: 2011 Jun 22, 02:36 -0700

    Geoffrey

    In my oppinion, Patrick O'Brian's description is not just fiction. The situation on the island looks very real and reflects the considerable efforts made at the time to lay down far distant islands. Too bad that he does not say a word about the method…

    As with any other method to find the Greenwich time, Jack's procedure surely did not require the use of chronometers. Lunars can be ruled out as the necessary data was not available at the time. Regarding the lunar occultation, why should he rely on such a seldom event?

    I would like to slightly modify my first suggestion and offer two possible methods:

    a) By using a precision theodolite, he measured the azimut of the moon when her lower limb touched the horizon. Under certain circumstances, this value may differ as much as 0.2 minutes of arc for two observers 30 miles apart. And again "two seven four" would be the exact azimut.

    b) He measured the exact azimut of the moon and Venus just as they were vertically in line ("with Venus clear above her…). This value also differs for two observers.

    Albert

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