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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.



Jim:

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


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