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    Re: Jack Aubrey's fixing of longitude
    From: Geoffrey Kolbe
    Date: 2011 Jun 25, 11:48 +0100

    >At the moment I tend to another interpretationn 
    >of this literary fantasy. As no lunar data for 
    >planets was available, I think that the 
    >occultation of Venus can also be ruled out, as 
    >an occultation (please correct me) is a special 
    >case of lunar (distance = 0). Nor was it a 
    >normal lunar, as the team was waiting for 
    >something to happen and a lunar can be taken any time.
    >To sum up: they waited for the occultation of a 
    >star. And again, here comes the "azimuth man"�
    >By measuringg the exact azimuth you can reduce 
    >the local hour (angle), as now you have the 
    >precise declination and the local latitude. The 
    >Nautical Almanac gives you the Greenwich time for this event.
    Albert, a couple of points.
    The Nautical Almanac of the period gave the 
    positions of the moon and also the positions of 
    Venus. The declination of the moon and Venus at 
    any given time would have been known. It would 
    have been possible to calculate the time of an 
    occultation of Venus using the data in the 
    Nautical Almanac. It follows that it would have 
    been possible for a mathematically inclined 
    master of a naval ship of the day to use a 
    convenient Venus occultation to fix the position of an island.
    The second point is that if he had his local 
    latitude, which our mythical master would 
    doubtless have had written down to the nearest 
    second from a noon altitude of the sun, then a 
    measurement of azimuth of the moon at the moment 
    of occultation is not necessary. In any case, 
    when the observer is near the equator (as our 
    mythical master was) then the azimuth would 
    change very slowly with time - especially if it 
    was around "two seven four" degrees - and so 
    would not be of much use in fixing a position.
    Geoffrey Kolbe

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