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    Re: Jack Aubrey's fixing of longitude
    From: Frank Reed
    Date: 2011 Jun 22, 21:38 -0700

    Albert,

    First, welcome aboard.

    I don't think it's at all likely that this passage was supposed to describe the azimuth of moonset. First, moonset is extremely difficult to observe. The Moon nearly vanishes at the horizon. Many of us have seen the Moon disappear behind a low hill, and we then remember this as having seen the Moon set. But actually observing the Moon go down over a real sea horizon... well, good luck! The extinction is so high right at the horizon, that you can't get the time to within better than a couple of minutes if you're lucky. Even with the Sun, the exact time of sunset is variable by up to a minute thanks to the variability of refraction near the horizon. Finally, your suggestion that "two seven four" was supposed to be an azimuth just doesn't fit. An azimuth wouldn't be useful for finding longitude. In fact, the latter is more sensitive to latitude in the Tropics. You also suggested that a theodolite might have been used to find the moment when Venus and the Moon were at the same azimuth. That wouldn't work very well either. Both of these methods would be low accuracy methods for finding longitude. We know they knew how to shoot lunars back then, and we know that O'Brian knew this. Nearly all officers in the Royal Navy would have known lunars in this era. So either they shot lunars, or they might have tried something more accurate than lunars --not something less accurate (assuming it's not just a literary fantasy, which is always possible, even for O'Brian).

    You wrote:
    "Regarding the lunar occultation, why should he rely on such a seldom event?"

    He didn't "rely" on it really, at least not the way it's described in the book. Rather, he took advantage of a moderately rare event. He did something unusual because he had it in his head to settle the longitude of this poorly charted atoll. If someone with a solid understanding of nautical astronomy knew a lunar occultation of Venus was about to occur (and it was not hard to predict), he might go out of his way to take advantage of that. The exact timing of the local apparent time when Venus disappears or reappears in a lunar occultation will usually yield an excellent longitude superior to almost any other astronomical method. Otherwise, in this era, most Royal Navy vessels were already "relying" on chronometers for longitude and these were only occasionally checked by lunars. There's good evidence (though I would like more) that lunars and other astronomical checks on the chronometers were already rapidly fading away in the Royal Navy in the decade in question. Aboard American vessels and presumably in other navigational cultures with fewer resources (lower funding) than the Royal Navy, longitude by lunars lingered on until 1850 or so.

    -FER


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