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    Re: Lunars by Moon declination. was: [NAV-L] Thomas Jefferson and Lunar Obs.
    From: George Huxtable
    Date: 2005 Mar 26, 17:33 +0000

    Well, here's a rum state of affairs.
    Most senders of postings, challenged with an apparent identified error,
    would either defend it or explain what's gone wrong. Not so Frank Reed, it
    Asked (twice) to explain what seems to be an erroneous quantity in his
    recent posting of text from Dunbar, in which 5" was printed, where 15"
    would be expected, he has (twice) declined to say whether it's an error in
    Dunbar's text or an error in Frank's (or another's) transcription. And in
    the following contemptous terms-
    >No one else seems interested in this, so I don't think it's worth the
    >effort, "onus" or not .
    How does he presume to know how many readers may be interested, I wonder?
    If it's too much "effort" for him as author to put the matter right, does
    he then expect his readers to put in equivalent effort to do the same?
    There's an unwritten understanding, on this list as well as in most other
    means of communication, that authors take responsibility for the accuracy
    of what they write. And yet, in regard to his original posting, Frank Reed
    >There are certainly transcription errors, especially on numerical matters,
    >but the original scans are available on the web site so you can check for
    I hope that other contributors to Nav-L will avoid this laid-back
    couldn't-care-less attitude to the accuracy of what they post.
    Perhaps Dunbar got it wrong: or perhaps Frank got it wrong when it was
    transcribed, and then failed to notice the unlikely resulting value. Is he
    just avoiding admitting that?
    If any other reader happens to take an interest in resolving this minor
    matter, Frank's instructions (which didn't work on my old Mac) were-
    "So let's try this web  link:
    _http://memory.loc.gov/ammem/_ (http://memory.loc.gov/ammem/)
    At that address, enter "lunar Dunbar" (without quotation marks) in the
    "Search all collections" box. That will call up the letters back and forth
    Dunbar and Jefferson on this topic."
    Frank doesn't identify the particular letter his posting referred to,
    except as "From William Dunbar to Thomas Jefferson in 1804 (Library of
    Congress web site)". Nor can I say how many items there are in this
    collection of letters, though Frank may know.
    and the text of Dunbar's letter in question (or perhaps a paragraph from
    it) states-
      "I mentioned in my last that one very simple method had occured to  me of
    ascertaining in certain Circumstances the Longitude of places, which is  much
    better calculated for travellers by land than Voyagers by Sea; the method  is
    such that a Single observer with a good altitude instrument, altho' deprived
    of the use of a time keeper, may still make useful observations for the
    advancement of geographical Knowledge. I shall now just mention the
    principles  &
    shall hereafter Send you some examples of the Calculation. The excellence  of
    the usual lunar method of determining the Longitude depends (supposing her
    theory to be perfect) upon her quick change of place from west to east; but it
    cannot be denied that it requires great dexterity to make good observations,
    which is evident from the disproportion of the times to the distances in the
    hands of the best Observers, and this arises from the slow progress of the moon
    which Causes the Contact to appear to be continued for many seconds of time;
    were this observation similar to a meridian altitude, it might certainly be
    taken to any desireable accuracy, that is, were the motion of the moon from
    North to South in place of from West to east, the moon's altitude when brought
    upon the meridian by the rotation of the earth would furnish an easy & very
    Correct mode of ascertaining the Longitude: Now altho' the proper motion of
    the  moon is from West to East, yet her orbit makes so considerable an angle
    with the  equinoctical circle, that there are two portions of each lunation when
    the  moon's change of declination is very rapid, exceeding 6 in 24 hours, that
    is 5"  of a degree in one minute of time; if therefore under favorable
    Circumstances we  take the moon's greatest altitude near the meridian, we shall
    thence be enabled  to ascertain the moon's declination at the moment of her
    passing our meridian;  we must then find the time at Greenwich when the moon had
    that declination and  also the time when the moon passed the meridian of
    Greenwich, from which data  the Longitude is easily found: this method will
    the use of some  interpolations and an equation for the Correction of the Moon's
    altitude on the  Meridian, because her greatest altitude will not be on the
    meridian, but to the  East or West according as She is increasing or
    diminishing her North polar  distance. I have communicated this method to
    my Worthy
    friend Mr. Briggs who is  pleased with the idea & intends giving it
    The matter in question is simply this; whether Dunbar referred to the
    Moon's change of declination as 5" of a degree in one minute of time, or
    15". Or whatever.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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