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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: What time is it?
From: Trevor Kenchington
Date: 2004 Nov 10, 10:36 -0400
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. 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 -- Trevor J. Kenchington PhD Gadus@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 http://home.istar.ca/~gadus