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    Re: Lunar Distance in Wikipedia
    From: Fred Hebard
    Date: 2007 Aug 22, 08:48 -0400

    I agree with George's comments.
    
    Another place with an unnecessary "citation needed" spot is the lead
    sentence of the errors section, "A lunar distance changes with time
    at a rate of roughly half a degree, or 30 arc-minutes, in an hour
    [citation needed]."  One could add, "since it takes about 27.3 days
    for the Moon to orbit the earth (360 degrees per 27.3 days is
    approximately 30 arc-minutes per hour)."
    
    In the lead sentence, one could also change "30 arc-minutes, in an
    hour" to "30 arc-minutes, per hour", substituting "per" for "in an".
    
    I believe a section on lunars leading to the change from quadrants to
    sextants and to increases in sextant precision and accuracy would be
    appropriate.  It might be entitled: "Effect of the lunar method on
    navigation instruments and methods," or some such.  I believe it
    would be important to emphasize that the calculation itself might
    take a half hour or more. Here, as I argued previously, I believe
    some mention of clearing methods would not be inappropriate,
    explaining, among other things, Bowdich's true contribution to lunars
    rather than letting rest the myth that he was integral to their
    development and implementation.
    
    Fred
    
    On Aug 22, 2007, at 5:15 AM, George Huxtable wrote:
    
    >
    > Jim van Zandt and Renee Mattie have both attended to that Wikipedia
    > article,
    > and between them they are transforming it into something really
    > worthwhile.
    >
    > I have a few comments, though.
    >
    > Can anyone see a way to credit Clive Sutherland with that nice
    > drawing? Not
    > that he has any wish to copyright it; he doesn't.
    >
    > ===================
    >
    > The article states, under "History",  "Lunar distance tables last
    > appeared
    > in the British Nautical Almanac for 1904, and in the USNO Nautical
    > Almanac
    > for 1912". I don't know about the US version, but have suspicions
    > that the
    > 1904 date is a bit out.
    >
    > Lecky, in "Wrinkles" (actually, a later editor, in my 1917 edition)
    > states
    > on page 758 "The Nautical Almanac for 1907 ... is without tables of
    > lunar
    > distances ..."
    >
    > May, in "A history of marine navigation", 1973, page 40, states "In
    > 1909 the
    > Nautical Almanac ceased to publish the necessary tables..."
    >
    > Hewson, in "A history of the practice of navigation", 1983 ed.,
    > page 241,
    > states "... in 1908, lunar distances were omitted from the Nautical
    > almanac"
    >
    > All three authors were writing from a British background, and would
    > have
    > been referring to the British almanac.
    >
    > So here we have three authors, all differing from that 1904 date,
    > and from
    > each other! It isn't a vitally important matter, but let's get it
    > right.
    >
    > Has the British "Nautical Almanac" for those years been digitised,
    > and does
    > any listmember have easy access? It would then be the work of only
    > a few
    > minutes to discover the year of disappearance.
    >
    > Otherwise, I will ask Catherine Hohenkerk, of HM Almanac Office,
    > who has a
    > complete stack of back-numbers.
    >
    > ==================
    >
    > The last paragraph, under "Method", now states- "Knowing Greenwich
    > time and
    > the altitudes of the moon and the other body, the navigator can
    > apply the
    > intercept method to find his latitude and longitude. Alternatively,
    > the
    > navigator can first determine local time, and then longitude.[1]"
    >
    > That is unarguably true, now, but gives quite the wrong slant,
    > because the
    > first alternative was not generally available in the period we are
    > concentrating on, ending in the mid 19th century. There were two
    > reasons-
    >
    > First, Sumner's line of position method was not published until
    > 1843, and
    > the intercept method improvements, by St Hilaire, not until the
    > mid-1870s.
    > By this date lunars were in serious decline.
    >
    > Second, Moon predictions, in terms of GHA and dec, were not
    > provided in such
    > a way as to allow Moon altitudes to be accurately calculated, in early
    > almanacs. In the 1767 almanac, and for some time after, these
    > quantities
    > were provided only at Greenwich noon and midnight, between which
    > the Moon's
    > declination could change by more than 2 degrees; far too infrequent
    > for
    > proper interpolation. By 1864, the next almanac I have, Moon dec was
    > predicted for each hour, just as in modern almanacs. I don't know
    > when the
    > omprovement took place.
    >
    > So I think the intercept method should NOT be quoted as the primary
    > method
    > of position finding, to go with a discussion of lunars. Instead, the
    > emphasis should be on direct determination of longitude from
    > difference
    > between local time and Greenwich time.
    >
    > What do others think?
    >
    > ================
    >
    > Under "Errors", there's an explanation of why an error of 1' in lunar
    > distance gives rise to an error of 30' in longitude, and a
    > conclusion "So
    > lunar distance can never be a precise way to determine longitude.
    > After
    > that, someone has inserted {cite}. I wonder why? One follows from
    > the other.
    >
    > Although I am all in favour of rigour, in presenting evidence for
    > statements
    > that are made, I feel that this is being unduly pernickety. And I
    > am not
    > convinced that evidence presented in a citation from a book or a
    > journal
    > article is always of value, anyway, as you can see from the
    > contradictory
    > statements, above, about the date of omission of lunar distances
    > from the
    > almanac.
    >
    > George.
    >
    > contact George Huxtable at george---.u-net.com
    > or at +44 1865 820222 (from UK, 01865 820222)
    > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    >
    >
    > >
    
    
    
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