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    Re: Nautical astronomy was different
    From: Bruce Stark
    Date: 2004 Oct 20, 13:16 EDT
    Herbert & Frank,

    I stand by what I wrote (except for an error in point # 5 that was noticed too late).

    Please don't think these are notions I've cooked up. What I know about the old nautical astronomy has been absorbed over the years, from reading old navigation manuals and working problems in them. In the 1980's a historian specializing in Vancouver's voyages sent me a copy of part of the journal of one of Vancouver's officers. It had page after page of numbers. Navigation authorities felt sure the numbers had to do with lunars, but couldn't make head nor tail of them. I had the Almanac for that year, so puzzled the journal out, and worked a good many of the observations. Either came out with the same result as the journal recorded, or else acceptably close. It was a good beginning. Over the years I've become convinced that, if you want to understand the old nautical astronomy, you have to stop forcing present-day logic on it and take it as it is.

    In (1),(2), and (3) I'm saying that the point of view of the old navigators regarding local and Greenwich time was the opposite of ours. Suppose we take a time sight. How much thought do we give to the time found? Not much. It's time all right, local apparent time. But local apparent time is not THE time. It's just A time. THE time is GMT, or else a zone time that instantly converts to it. In our view a time sight doesn't give the time. It gives the longitude. The chronometer gives us the time. The old navigators saw things the other way around.

    They tracked the rotation of the celestial sphere with local time. Their Almanac tracked changes within the sphere with Greenwich time. Excepting the moon, things are pretty slow. The majority of the old navigators got local time and latitude almost exclusively from the sun. For that they only needed the sun's declination. Greenwich time an hour off can cause an error of as much as one minute of arc in declination. But only near the time of the equinoxes.

    My mistake was in (5). Wish I hadn't written it, or had at least given it more thought. The problem is, chronometer navigators have to have the equation of time, and I don't recall ever seeing an accurate table of it in a manual. Only a small, one-year-fits-all table to help find when a star would be on the meridian for a latitude shot. But it does seem to me that the four-year tables of the sun's declination could have been expanded to include the equation of time.

    Bruce
       
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