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    Re: Lunar trouble, need help
    From: Frank Reed
    Date: 2008 Jul 06, 05:31 -0400

    Kent, you wrote:
    "My sources are navigation textbooks from 1795, 1796, 1842, 1845, 1853, 1873
    and 1896. I have also taken alook into Bowditch 1834."
    
    Ok. That helps me to understand where you're coming from.
    
    And you added:
    "What you find in textbooks are good examples which can be used for testing
    and "validation" of your model. That was the very reason for using
    textbooks. It is very rare that you find errors in good texybooks."
    
    Yes. I agree. These textbooks and navigation manuals often provide good
    examples. However, it is CRITICAL that you recognize that these are school
    books written on shore. They tell us about navigational education, but the
    relationship to navigational practice is ambiguous at best. If you want to
    learn about actual navigational practice, you have to dig through primary
    source materials: logbooks, journals, and the scrap paper of traditional
    navigation.
    
    And you wrote:
    "You may have a point that the focus were on "small things""
    
    Also bear in mind that these books were written in a period of fierce
    commercial competition. These navigation textbooks were marketed, peddled,
    "sold" by their publishers. And they exaggerated the importance of small
    details to sell more books.
    
    And you wrote:
    "however I don't agree with you concerning a exclusion of correction for
    earth flatness. One of the real challenges with LD's is to get corrections
    for earth flatness correct."
    
    No. That is not true. You can ignore earth flattening and the cost in the
    position fix will be trivial. It is a MINOR matter. On average, you can
    expect a longitude by lunars to be offset by about ONE nautical mile if
    earth flattening is ignored and around FIVE nautical miles at the high
    maximum (which is very rare). Nautical astronomers did, of course, spend a
    lot of time worrying about this small matter, but it was wasted time.
    
    And further, I would note that the approach you are taking on this 'earth
    flattening' correction is the long way around. Have you read Chauvenet?
    George Huxtable is also taking the long way around; the correction for
    oblateness is not that difficult if you really feel a need to include it.
    Have you tried the calculator on my web site: www.HistoricalAtlas.com/lunars
    ? You can optionally turn off the correction for oblateness and see very
    quickly how small it actually is.
    
    By the way, you asked whether "oblateness" would be the better term in
    English. It's 'technical language', so the community who would use the
    expression is rather small, but yes, generally, you would say 'oblateness'
    in English rather than 'earth flattening'. Nonetheless, I want to say that
    you have been very clear in your wording, and I don't think that there has
    been any confusion.
    
    I noted that Moon-Sun lunars were the most common. You replied:
    "I think the explanation is simple. It is much easier to find the local time
    using the sun than with any other celestial object. You don't need to
    calculate the long way with Aries, RA etc."
    
    Yes. I agree. I think that's most of it. There are some other smaller
    reasons. At a practical level, many people seem to agree that it's an easier
    observation visually (when the Sun is used). And of course, there's no
    identification issue. We today are accustomed to the idea that celestial
    navigators know the stars and can identify them all, but this was apparently
    not the case 150-200 years ago. It was a rare navigator who could find the
    lunars stars...
    
    I wrote: "Watches were not the issue. Plenty of good watches were widely
    available in the period."
    And you replied: "This view is not consistent with advices found in the
    earlier references above."
    
    Yes. And that is a typical textbook bias. The textbooks were generally
    written by mathematicians and theoreticians. They tended not to be practical
    individuals. They didn't like solutions involving mechanical devices. But
    pocket watches were widely available, at least to ships' officers, even at
    the beginning of the EIGHTEENTH century. By the 19th century, they were so
    common that it wasn't even necessary to refer to them. Watches were
    ubiquitous.
    
     -FER
    
    
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