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    Re: Patrick O'Brian characters discuss time and longitude
    From: Don Seltzer
    Date: 2016 Jun 26, 11:41 -0400

    On Sun, Jun 26, 2016 at 11:18 AM, Geoffrey Kolbe <NoReply_GeoffreyKolbe@fer3.com> wrote:
    > However, this discussion caught my eye and I wonder if O'Brian is just reflecting how the 18th (very early 19th) century navigator would generally calculate a lunar to the nearest second of arc. With our modern knowledge of statistics and instrument error, we would automatically round this down to a more realistic number to reflect the accuracy of the instrument, the tables and the mathematical methods. However, I suspect that this is a relatively modern attitude to experimental method and the statistical knowledge that underpins had yet to be developed when Aubrey sailed Surprise to the far ends of the Earth.

    Perhaps, but here it the specific passage:

    ‘I have just been gazing at [the moon] too, with my sextant: a perfect lunar, with old Saturn there, as clear as any bell. I have my longitude to within a second.

    Curiously, Jack Aubrey only does night time lunars with stars or planets.  I don't think that O'Brian knew that the lunar distance method could be done with the sun.  And although he understood that time as measured by chronometers was linked to longitude, I don't think that he realized that celestial observations were still required. There is a scene in which bad weather has obscured the skies for days, preventing him from taking a lunar measurement.  He would have simply relied upon his chronometers for longitude, but they were acting up and he couldn't trust them.

    Additionally, Aubrey puts a great importance upon observing Jovian moons for triple-checking both his chronometers and lunars.  Presumably these take place while ashore on some island.  Perhaps I am mistaken, but I did not think that Jovian moons were anywhere near as accurate as lunars for determining Greenwich time in the early 19th century.

    Don Seltzer
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