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    Re: What time is it?
    From: Trevor Kenchington
    Date: 2004 Nov 10, 10:36 -0400

    Jim Thompson wrote:
    > The statement in the CPS Student Notes, "at the instant of GMT 1200, the
    > date is the same all over the world" puzzled the heck out of me, because I
    > could never prove it using the celestial navigation time conversion rules
    > taught in the rest of the course, and in Bowditch and Dutton's.  It has
    > taken me a year to settle the issue in my own mind:
    > http://jimthompson.net/boating/CelestialNav/CelestNotes/Time.htm#DateSame
    > as I mentioned in the other post just now.  In short, I believe that the
    > statement is wrong, at least from a pure navigator-at-sea point of view.
    I have now worked through most of the CPS CN Student Notes and I have
    found them to contain far fewer errors than most other CPS course
    materials (though, perhaps in compensation, the presentation is
    hopelessly confusing -- which is unusual for CPS). But while the errors
    are few, they are certainly there. I don't think you should be surprised
    at the one about the uniformity of date at 1200 GMT. [I'll take a guess
    that a close search of Bowditch would reveal the sentence that has
    been misinterpreted or oversimplified to produce the erroneous wording
    in the Student Notes.]
    Turning from the specifics of the CPS course: One way to look at this
    issue would be that at the instant of 1200 GMT, the LMT date is the same
    everywhere. At any one time, there is an infinitely-thin line from pole
    to pole along which LMT is _exactly_ 0000 -- that line normally dividing
    areas with different dates, just as the 180 degree meridian divides
    areas with different dates (the combination accommodating the two dates
    which must simultaneously occupy this one globe. At _exactly_ 1200 GMT,
    that "midnight" line lies over the 180 meridian and everywhere on earth
    has the same LMT date -- the line being infinitely-thin and thus not
    occupying any space.
    Once you step from time and date in LMT to those in ZT, this breaks
    down, of course and (as you have identified) it is never possible for
    the entire planet to have the same ZT date at the same time. But in
    terms of LMT, there is one infinitely-brief instant per day when we all
    share the same date.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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