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    Re: Lunar Distance in Wikipedia
    From: George Huxtable
    Date: 2007 Jul 26, 00:51 +0100

    Renee Mattie is applying a bit of discipline to the Wikipedia entry on Lunar
    Distance (Navigation), and a good thing too.
    
    But it's still not right, as I see it, and a bit more tinkering may be
    called for.
    
    It now reads-
    
    "Theory
    The local time at Greenwich is always one hour later than the local time 15
    degrees West of Greenwich. Suppose one person observes the moon at Greenwich
    at local apparent noon (when the sun crosses the observer's meridian). Then
    suppose a second person observes the moon 15 degrees West of Greenwich, also
    at local apparent noon. The two observations take place one hour apart. The
    second observer will see the moon in a different position than the first
    observer did because, during that hour, the moon would have moved
    approximately its own diameter across the sky."
    
    But in that hour, the Moon, just like any other object near the ecliptic,
    moves through something like 15 degrees across the sky; far more than its
    diameter.
    
    What the observers have to measure is the position of the Moon with respect
    to the star background, or the Sun, which is where the lunar distance comes
    in. It's that angle that changes by a Moon diameter in an hour, roughly
    speaking. And of course, there's no call for that mythical observer at
    Greenwich anyway; what he would measure has been predicted in the almanac.
    
    What I'm suggesting is that the whole paragraph could do with a wholesale
    revision, rather than tinkering with one clause at a time. What's the best
    way for that to be done, collectively, by this list? We're as clueful a body
    as any to apply our wisdom to the job. It seems counterproductive for Renee
    to make a change, then another of us to follow it up by another piecemeal
    edit process, and so on. Wouldn't it be better for us, on this list, to
    thrash out a text we all can agree on, if that's possible, and then Renee,
    who has shown herself to be so adept, can act as our ambassador and make the
    change on the list's behalf. Does that make sense? Would anyone like to set
    the ball rolling?
    
    And it's not just that paragraph. The introductory paragraph to the whole
    article reads-
    
    "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."
    
    Indeed, taking a lunar, it is necessary to observe "the height of the
    Moon and the second celestial object", but that's mainly in order to get
    information for "clearing" the lunar distance: correcting it for the effects
    of parallax and refraction. It's quite true that under certain
    circumstances, either the Moon altitude or the star altitude (no need for
    both), might be used
    to determine local timebut usually local time will be
    found separately, from a different observation. The text gives quite a
    misleading impression of why those altitudes are needed.
    
    ===================
    
    If we took a rigorous look at the whole piece, it's likely we would uncover
    more deficiencies. All I have done, so far, is to quickly scan it over.
    
    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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