<div>On 24 May 2012 17:18, Alan Mintz <span dir="ltr"><<a href="mailto:Alan_Mintz+TZ_IANA@earthlink.net" target="_blank">Alan_Mintz+TZ_IANA@earthlink.net</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<div class="im"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">Note that the nautical date line is different from the international date<br>
line.<br></blockquote><br></div>Where (as of 2010)?</blockquote></div><div><br></div>The "nautical date line," used when not in territorial waters, is the 180th meridian by convention, which splits the 15-degree range from 172.5°E to 172.5°W into two 7.5-degree ranges: One for GMT+12 and one for GMT–12. Obviously, when one is in the territorial waters of a nation, though, one should set clocks to local jurisdictional time instead.<div>
<br></div><div>The "international date line," on the other hand, is simply a line drawn for convenience to show the "locations" of time transitions on the order of 24 hours. The line is itself not directly legislated by any governing body, nor does it have any meaning intrinsic to itself, except to distinguish where a roughly day-long disparity exists between adjoining "time zones" (whether nautical or jurisdictional). Regardless of their geographical location, nations can "move" this line so as to put themselves on one side or the other simply by making their own UTC offset positive or negative. As such, crossing into territorial waters could mean a clock shift of more than 24 hours; for instance, if one were to enter the UTC+14 portion of Kiribati from international waters which are geographically in the GMT–12 zone (ignoring, for now, the minor differences between the two timescales), one would shift clocks ahead by 26 hours.</div>
<div><br></div><div>This is why the international date line is so jagged. Where appropriate, it is equivalent to the nautical date line on the 180th meridian. Elsewhere, it veers to either side to follow the appropriate boundaries of territorial waters, serving only to separate "date changes" (changes of about 24 hours) as a visual cue on a map to travellers. Since the boundaries of territorial waters are often hard to pin down precisely, different cartographers will tend to draw these boundaries on maps slightly differently, often with clean diagonal line segments to be aesthetically pleasing, as in <a href="http://upload.wikimedia.org/wikipedia/commons/6/61/International_Date_Line.png">http://upload.wikimedia.org/wikipedia/commons/6/61/International_Date_Line.png</a>. This does not mean these diagonal lines represent the actual geographical and logical boundaries where "date changes" occur; rather, the actual international date line is the result of the sum total of all the nearby nations' time zone legislation.</div>
<div><br></div><div>Theoretically, there could even be geographical "pockets" where negative-offset nautical time is used, surrounded completely by positive-offset jurisdictional time (or, conversely, positive-offset nautical time surrounded by negative-offset jurisdictional time). Such a scenario would mean the international date line wouldn't actually be a line at all. (I do not know whether such a scenario actually exists at present, nor do I believe it would be an easy determination to find out.) Technically, a vessel travelling straight through such a region should have several "date changes" back and forth; however, a reasonable person travelling through such a region would probably just choose to "stay on one day or the other" as they found it convenient (keeping in mind the potential for disparity amongst others who may have made the inverse decision), until reaching a region where the date doesn't keep changing on them anymore. <b>;)</b></div>
<div><div><br clear="all">--<br>Tim Parenti<br>
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