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    Re: Lunar Distance in Wikipedia
    From: George Huxtable
    Date: 2007 Aug 24, 11:08 +0100

    This discussion about lunar distances is widening, I'm pleased to see.
    
    Fred commented-
    
    | The Shovell disaster itself was due to a combination of latitude
    | error and mapping error, but the incident provided the impetus for
    | the Longitude Prize, or the excuse the proponents of the prize needed
    | to push it through to government approval.  So it was important to
    | the development of lunars and chronometers.
    
    What Fred says here is perfectly correct. However, that doesn't make the
    wrecking of the Shovell fleet a suitable topic to introduce into an
    encyclopaedia article on lunars. That would only mislead its readers, just
    as it misled the public at that time. Renee is right to drop it.
    
    Incidentally, although the mis-charting of the Scillies was so dangerous,
    and Halley had warned about it at the time, it still remained uncorrected on
    charts that were published as late as 1753; even charts to which Halley's
    name had been attached.
    
    ===============
    
    Jim wrote-
    
    I didn't like this statement:
    
      At that moment, anyone on the surface of the earth who can see the
      same two bodies will observe the same angle (after correcting for
      errors).
    
    Strictly speaking, the only significant "errors" are sextant and
    reading errors, and the mariner has no way of "correcting" those.
    Aside from that, the lunar distance observed by the mariner differs
    from what would be seen by the imaginary observer at the center of a
    transparent earth because of parallax and refraction.  However, I'm
    uncomfortable calling one of those observations more "erroneous" than
    the other.  They are observations of different things.
    
    My first thought was to change the parenthetical statement to "(after
    corrections)", but I still didn't like it.  The only way two people
    will both "observe" the angle listed in the almanac is if they are
    both at the center of the earth, and *not* "on the surface".
    
    One could say "(after adjustments)", but it would still be wrong - the
    people at different places are not actually "observing" the same
    angle, they are *calculating* the same angle.  At that point I decided
    the sentence had too many problems and deleted it.  However, I didn't
    record any rationale on the "discussions" page, and it was restored.
    
    Could we agree to something like this:
    
      At that moment, anyone on the surface of the earth who can see the
      same two bodies can observe that angle and determine what time it
      is.
    
    ====================
    
    I agree with Jim that the clearing operation allows for corrections, and not
    "errors".
    
    My suggestion would be "At that moment, anyone on the surface of the earth
    who can see those same two bodies will determine the same angle between
    them, after making certain important corrections (see "clearing", below),
    and will therefore agree about the Greenwich Time.
    
    But there's another problem with that statement, and I can't think up how to
    express it simply. It's perfectly true in itself, but it doesn't go far
    enough. It isn't necessary for observers, at the same moment, at different
    points on Earth, to be able to see the SAME two bodies, in order to agree
    about the corrected position of the Moon in the sky, and therefore agree
    about Greenwich Time.
    
    That statement, above,  is no more than an EXAMPLE that provides a simple
    picture; NOT a definition of the lunar distance method as such. In practice,
    at the same moment one observer may be in daylight, measuring a Sun lunar.
    Another may be in darkness, measuring distance to a star. For a third
    observer, that star  may have already set, and distance to another star,
    perhaps on the opposite side of the Moon, may be called for. All that's
    really necessary is that each should be able to see the Moon, and some other
    tabulated body.
    
    Even insisting that all such observers must be able to see the Moon at the
    same moment would limit them to just half of the World, and isn't necessary,
    provided there's some sort of timepiece to bridge any gap in time until the
    Moon becomes visible. So it needs to be clear to the reader that the
    statement is no more than an illustration. Should we precide it by "For
    example ..."? Perhaps it would be best if we could think up a different way
    of expressing that whole concept. We have a number of sharp NavList minds
    pondering these matters now. Any ideas?
    
    One could add reams of detail here, but this is an encyclopaedia article,
    not a treatise.
    
    George.
    
    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.
    
    
    
    
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