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    Re: Nautical astronomy was different
    From: Frank Reed CT
    Date: 2004 Oct 20, 18:26 EDT
    Bruce S wrote:
    "Please don't think these are notions I've cooked up."

    Your specific interpretation (that lunars did not yield "time") is clearly your own culinary product. Unfortunately, I think you may have used too much salt. Your contention that they didn't get "time" from lunars is simply an odd way of wording things. I can show you numerous worked examples in real logbooks where the penultimate result is labeled Greenwich Time. They weren't confused by this. They knew that they were comparing local time with the time in England. And if it's 6am where I am, for example, and it's noon in England then that difference (6 hours ahead) yields my longitude (90 degrees west) at the rate of 15 degrees per hour difference in time. Lunars were not conceptually difficult, and they were understood pretty much the way they are understood today.

    And wrote:
    "What I know about the old nautical astronomy has been absorbed over the years, from reading old navigation manuals and working problems in them."

    As I've mentioned previously, studying the navigation manuals teaches you a certain idealization. It teaches the reader how lunars were *taught* in books. There are differences between that idealization and real-world practice.

    And:
    "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."

    I agree with that sentiment 100% (of course, that doesn't mean I yet agree with your personal interpretation). Maybe you could explain what you mean by "forcing present-day logic on it". In other words, what do you think other people have said about lunars that is an example of present-day logic being applied in error?

    Bruce expanded on his earlier post:
    "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."

    I don't agree that 19th century navigators necessarily thought this way. They understood the concept of local time. They knew that local time varied continuously with longitude. They knew that a lunar could tell them an absolute time, which happened to be the local time in Greenwich. I DO agree that many modern navigators approaching these concepts have somewhat muddled notions of time because of the use of "zone time". They also have had experience with the modern Nautical Almanac format which may give the erroneous impression that you must have exact Greenwich Time to pull any data out of the almanac. That change in format of the almanacs is only 60-70 years old --so convenient in practical terms yet perhaps confusing from an educational standpoint.

    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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