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    Re: Lunar Distance in Wikipedia
    From: James R. Van Zandt
    Date: 2007 Aug 23, 21:53 -0400

    
    
    I agree with George's comments.
    
    I have a couple of additional points:
    
    First is a minor one.  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.
    
    
    
    My second point concerns the "method" discussion. A couple of versions
    ago, it ended this way:
    
      Knowing Greenwich time and local time, the navigator can work out
      longitude.
    
    I changed that to
    
      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.  Alternately, the navigator can [[longitude by
      chronometer|first determine local time, and then longitude]].
    
    At present, it says
    
      Having found the time, and having the altitude of a star as well as
      the Moon, the navigator can now use the techniques of [[Celestial
      Navigation]] to find the vessel's position at the time of the
      observation.[1]
    
    I don't mind removing the reference to the intercept method, which
    came later.  However, the current link to Celestial_navigation has the
    same problem.  That page actually starts by showing two circular lines
    of position derived from two altitude measurements.  But mariners
    could not do that calculation until the intercept method was invented!
    
    My reference to [[longitude by chronometer]] was really to the concept
    that knowing the (Greenwich) time and one altitude lets you calculate
    the longitude. A chronometer is one source of the time. The lunar is
    another source of time. That's why I changed the wording of the
    reference, so what the reader sees is "first determine local time, and
    then longitude".
    
    So, if there is only one cross-reference, I think [[longitude by
    chronometer]] is more appropriate than [[celestial navigation]].
    
    (By the way, I still think the "longitude by chronometer" page needs
    some work.)
    
                 - Jim Van Zandt
    
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