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    Re: Lunar Distance in Wikipedia
    From: Renee Mattie
    Date: 2007 Jul 26, 20:53 -0400

    George,
    
    I think your suggestions are fabulous.  There is NO limit on the length of a
    Wikipedia article.
    If it reads well, it generally gets left as-is.  If people hate it, they
    will tinker with it.
    Sometimes, they will tinker with it even if it is perfect.  That's wikipedia
    for you.  You can always find the "perfect" version and "revert".
    
    I have offered my small tinkerings as targets.  Please do replace the
    sections.
    Nobody will mind.
    
    Renee
    
    -----Original Message-----
    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf
    Of George Huxtable george-at-huxtable.u-net.com |Renee Mattie on NavList|
    Sent: Thursday, July 26, 2007 4:32 PM
    To: .....................
    Subject: [NavList 3040] Re: Lunar Distance in Wikipedia
    
    
    I'm sure we can improve on that Wikipedia page.
    
    Perhaps others may be tempted to join in if I offer a target to be fired at.
    
    Trouble, is, the various sections on that page all interrelate, so it's hard
    to tinker with one without altering others. Nor do I think the way the topic
    is divided under different headings is entirely sensible.
    
    Throughout, there seems to be confusion between the process of determining
    longitude, and the deducing of Greenwich time from lunar distance, which is
    just one step in that process.
    
    Trouble is, it's hard to combine the all-important precision with clarity
    and brevity. I wonder if there are specific rules about the length of such
    contributions. Perhaps Renee or Frank can tell us. I'm a bit out of my depth
    in these Wikipedia matters.
    
    The introduction goes like this.
    
    "In celestial navigation, lunar distance is the angle of the Moon's centre
    from the Sun or a bright star as measured using a sextant. Given a lunar
    distance and a nautical almanac, it is possible to calculate the difference
    between local current time - obtained by observing the height of the moon
    and the second celestial object - and current time at the meridian of the
    nautical almanac - usually Greenwich - which gives the difference in
    longitude between the two places."
    
    What I particularly dislike here is the reference to those two measured
    heights, which are relevant only to the fine details of the "clearing"
    process.
    
    Let's try this instead, as my first shot.
    
    "In celestial navigation, lunar distance is the angle between the Moon's
    centre and the Sun or a bright star, slanting across the sky, as measured
    using a sextant. Such an observation, usually abbreviated to just "a lunar",
    can be made by a mariner, anywhere in the World, if the Moon is visible,
    together with the Sun or a special star. Without needing a chronometer, it
    allowed him to calculate what the time was at some reference longitude
    (usually Greenwich) at the moment of that observation, using data which used
    to be published in a nautical almanac. That was an important step in finding
    his own longitude, from Greenwich."
    
    ===============
    
    The next section goes-
    
    Why Measure Lunar Distances?
    In Celestial navigation, precise knowledge of the time at a reference point
    and the positions of several celestial objects are combined with careful
    observations to calculate latitude and longitude. But reliable marine
    chronometers were not invented until 1761, and were not generally available
    for many decades afterwards. For nearly one hundred years (from about 1767
    until 1850), the method of lunar distances was used to determine longitude
    at the time of the lunar observation. Longitude information could also be
    used to check chronometer error, so that chronometer time could be used for
    longitude calculation at moments when lunar observations were not practical.
    
    Not too bad, in my view, but surely, that last bit is quite misleading. If
    you had a chronometer, you would use to measure longitude all the time, not
    just "when lunar observations were impractical". So I would excise that last
    bit, to leave-
    
    Why Measure Lunar Distances?
    
    In Celestial navigation, precise knowledge of the time at a reference point
    and the positions of several celestial objects are combined with careful
    observations to calculate latitude and longitude. But reliable marine
    chronometers were not invented until 1761, and were not generally available
    for many decades afterwards. For nearly one hundred years (from about 1767
    until 1850), the method of lunar distances was used to determine Greenwich
    time, in order to deduce the longitude at the time of the lunar observation.
    
    Such time information could also be used to check chronometer error.
    
    =============================
    
    Next comes-
    
    Method
    
    The method relies on the relatively quick movement of the moon across the
    background sky. Although the moon appears to circle the earth once a day due
    to the rotation of the earth, it actually circles the earth in 27.3 days. So
    from a stationary observer (i.e. an observer keeping stationary with respect
    to the stars) the moon completes a circuit across the background stars every
    27.3 days; In other words, the moon moves approximately by its own diameter
    across the background stars every hour.
    
    Does this really call for a section all to itself? And if so, should it be
    headed "method"?
    
    Anyway, I would tinker with it as follows-
    
    "This method relies on the relatively quick movement of the Moon across the
    background of the stars. Although the Moon, with every other body, appears
    to circle round the sky in about 1 day, with respect to the star background
    it completes a circuit in 27.3 days, and with respect to the Sun in 29.5
    days. This implies that with respect to the Sun and to stars that lie near
    its path in the sky, it is moving by approximately its own diameter, about
    half a degree, every hour. So lunar distances to those bodies are generally
    changing at about that rate, some increasing, others decreasing. That motion
    of the Moon is by far the fastest such change that can be seen in the sky,
    and because it is predictable in advance, it can be used as a measure of
    time. Wherever on Earth the Moon is seen from, at that moment, observers
    will agree about that time. If the predictions they use are based on
    Greenwich, that time will be Greenwich Time.
    
    ============================
    
    As for the bit that caused the trouble to start with; the section headed
    "Theory", I think it's awful, in so many ways, that just tinkering with it
    will not do. For example, why invoke two observers, when in reality there's
    only one? It needs a complete rewrite, and I will think about that.
    
    Meanwhile, I will stop at that point, and leave the above, with a bit of
    diffidence, as tentative suggestions for anyone else to mess about with, or
    comment on.
    
    
    
    
    
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