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    Re: Lunar Distance in Wikipedia
    From: George Huxtable
    Date: 2007 Aug 22, 10:15 +0100

    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
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    To post to this group, send email to NavList@fer3.com
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