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    Re: Jack Aubrey's fixing of longitude
    From: Frank Reed
    Date: 2011 Jun 19, 21:04 -0700

    Hi Geoffrey,

    Yeah, that seems very odd. Clearly they're describing land-based instruments (set up on a "carefully leveled patch of sand") so it's not a sextant observation. That alone implies it's not a standard lunar distance observation. In addition, assuming the story is set in the first decade of the 19th century (which is what I understand is implied for that specific novel), lunar distances for the planets were not available. They were tabulated in the standard Nautical Almanac starting only in 1834.

    What else could we do with Venus to get a longitude, if not a proper "lunar"? A carefully timed lunar occultation of Venus can yield an excellent longitude. That would be a rare enough event that the commander of a vessel with an astronomically-sophisticated navigator might try to find a good land location to set up a telescope for the observation. The calculations to reduce it would be long and difficult and so the idea of two calculators both ending with "274" might make some sense. Or, maybe closer to the description, they might have pre-calculated the local time of the immersion or emmersion of Venus and they were reporting the number of elapsed seconds after some chosen start time --maybe the number of seconds after an observation for local time had been taken. The description also makes it plain that this was a daylight observation since you have to know "where to look" to spot Venus. The description of Venus "clear above" the Moon could make sense if it's a crescent moon with Venus emerging from behind the dark limb. But there's just not enough detail in the description to be sure. I googled the phrase "Venus clear above here" and found some previews of the book online. There's not much more in the complete version except a comment that this was done on an island in the Indian Ocean. Anyone care to check: was there an occultation of Venus visible in daylight in the Indian Ocean in the first decade of the 19th century? It wouldn't be a "surprise". :)

    The line about the chronometers strikes me as a little unhistorical. The chronometers (plural) would not be out by some single number unless they're referring to the average longitude implied by two or more chronometers. Rather, one chronometer might be out by 20 miles West, another out by 32 East, for example.

    -FER

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