Int. Dateline Puzzle

Factual Questions
1

This concept eludes me. Maybe someone can explain…

{If you were watching CNN’s millennium celebration around the world on New Year’s Eve, then you should be wondering why, too!}

Midnight divides one day from the next. And, midnight is a local event - occurring as the earth revolves.

How, then, is it possible for an arbitrary imaginary line fixed roughly along the 180 degree longitude to divide the dates?

For example, there is some brief window when both Japan and the US (e.g. Eastern Time) are both IN the same DAY. If I fly across the dateline, shouldn’t the DAY be the same?

Also, consider what if it’s 12:01am ET, but 11:01pm CT? If I fly West, didn’t the DAY change without crossing the dateline?

Something just doesn’t jive here! Hints?

I’d rather have a bottle in front of me than a frontal lobotomy - Hawkeye 4077th

2

The international date line is the place that the new day hits first. You are right that the new day spreads around the world, but it starts at the date line, moves west, and dies at the date line again.

I think this is because the people in Greenwich, UK decided to reckon everything assuming it was noon where they were. From their perspective, the Prime Meridian was the middle of the world, and the International Date Line was the edge of the world. So, a new day, say January 15, 2000 for example, will first arrive on earth at 12:00 noon, Greenwich Mean Time. It will start out in Kamchatka and the other extremely Eastern places, at midnight local time.

3

Okay, I’ll take a shot. Please be gentle with me when I get part of this wrong.

Warning up front – this makes a lot more sense if you’re holding a globe as you read this.

Do you agree that there has to be some random place where the day “starts” (in other words, Thursday is separated from Wednesday)? If not, stop reading now.

When the powers that were got together to standardize time, they divided the earth into 24 different zones, because it takes the earth 24 hours to make a complete revolution. Then, they stuck the aribtrary line dividing Thursday from Wednesday in the Pacific Ocean, and zig-zagged it, so it wouldn’t bother anyone.

The dateline does not mean that it’s always midnight at that place. It means that that’s the place where the calendar officially rolls over for the world. Your local calendar will continue to roll over at whatever midnight you observe.

The result is that if you start at 12:01 a.m. Thursday on the west side of the line and instantaneously go westward through all 24 time zones (because the sun also goes westward) you will wind up on the east side of the line at 12:01 a.m. Wednesday – because you have passed backward through 24 time zones, each of one hour.

Regarding Tokyo and New York, I believe Tokyo is 14 hours ahead of New York (14 time zones EAST.) When it is 6:00 a.m. Wednesday in NYC it is 8:00 p.m. Wednesday in Tokyo. So yes, there are 10 hours during the day when both New York and Tokyo are on the same side of the Wednesday/Thursday line.

4

I follow your logic, but I’d like to have a better understanding as to the practical application of the Int. Dateline.

When it was taught to me in school, as a lad, they just said “add a day/ subtract a day”.
Now older and wiser, this generality just doesn’t always hold water…even if I am travelling from the UK!

Perhaps it’s just another general rule of thumb taught by the school system, and one should apply with caution.

Or, maybe the rule is a USA-centric thing? The exception is such a small window, and the flight is so long, maybe we just don’t talk about it??? So the rule works?

Many a science fact gets lost in the schools!

In college, I learned in spite of my profs!

  • Jinx
5

For simplicity’s sake, assume there are only 24 time zones and that only increments of one hour are used (unfortunately not the case, but it’ll do for purposes of the illustration).

Clocks in every time zone will read one hour behind those in the time zone immediately to the east, and one hour ahead of those in the time zone immediately to the west.

EXCEPT at one point: the IDL. Here, the bordering timezones are actually 23 hours apart, not one. If the clocks include a dial for the date, the ones in the more easterly one will be one day behind those in its western counterpart except for a period of one hour (from midnight to one AM Eastern International Date Zone time, or eleven PM to midnight Western International Date Zone time).

(I’m using east and west in the local sense here, not to mean Eastern vs. Western hemisphere.)

6

I should add the following:

Everytime I look at an atlas, globe, etc. which shows little (analog) clocks for the time zones, they ALWAYS set up their little visual example such that the 0hr falls at the IDL…nice and easy example, huh?

This never served to help me understand because the example just simplified the issue.

Thanks to all foer their input!

I’d rather have a bottle in front of me than a frontal lobotomy - Hawkeye 4077th

7

The little example puts 0 hour (midnight) at the date line because that’s the only time of day where the whole world has the same date. So the date line is the “border of the day”.

8

Look at it this way: every time you cross into a time zone, the time changes by an hour. Clearly, there must be some line you cross that causes the time to change by 23 hours in the other direction. It’s logical if you think it through. That line is the International Date Line.

9

I imagine the IDL kinda like a radar screen. Just imagine the green line that goes around the radar screen is the line where midnight begins. This line moves around the screen, just like the midnight hour moves westward around the world, never stopping.

Imagine now that the “12 o’clock” or top point of the round radar screen is the International Date Line. The green radar line on the screen hits it: It’s midnight at the IDL, and the time zone to the WEST of the IDL has begun a new day (say July 9th.) This is the first part of the globe that hits July 9th, as the eastern hemisphere of the globe is in a time zone one or more hours ahead of Greenwich time, and the other half is in a time zone one or more hours behind Greenwich.

This time zone the rolls over, from being 00h00m something to 01h00m something. Meanwhile, the next time zone to the WEST rolls over to midnight, making it July 9th local time as well. And so on, and so on. Through Asia, Europe, Africa, across the Atlantic, into the Americas, and across the Pacific, the rollover continues.

The last time zone to roll over is the one immediatley EAST of the IDL, or in other words, the westernmost time zone in the world, the one most retarded behind Greenwich time. This is the last zone where the rollover to midnight, and the entry into July 9th takes place. For until now, even though the time zone across the dateline has been on July 9th for 23 hours, it has all this time still been July 8th.

So finally, the last time zone crosses into midnight, and hits July 9th. Then what happens? The midnight line, like the green radar line, crosses the IDL again, and in the eastern hemisphere it starts to be July 10th. July 10th creeps around the world, time zone by time zone, turning July 9th into July 10th, like the radar line on the screen, until it hits the top of the screen (the IDL) again.

The radar line, or the midnight line, divides the earth as it moves around the globe into a wedge of “July 9th”, and a wedge that is still July 8th. Of course, once the midnight line reaches Greenwich and the prime meridian, the two “wedges” are of equal size, and then the July 8th “wedge” shrinks as the July 9th “wedge” takes over. But there is always a point between the two wedges that stays a boundary between the two wedges: The place where the rollover first took place, or the International Date Line.

Another analogy is the countdown to a movie, where the line goes around a circle, revealing “10”, “9”, “8”, etc. The “12 o’clock” position of the circle is where “8” begins, and “8” continues to move around the circle until it reaches the start point, where it suddenly turns into 7 again.

http://members.xoom.com/labradorian/

10

Sorry, but nope; the time zone does not change the hour when one crosses the IDL. The time zones immediately on either side are 1/2 the width of the other standard time zones. What does happen is that when crossing the IDL going West, the clock is advanced 24 hours. When crossing the IDL going East, the clock is retarded (set back) 24 hours. The hour, though, is still the same hour of the day, just different days.

11

Thanks, Monty, I was writing this and looked back at your response.
IDL time zone is divided in half, IDLW & IDLE.
Give you guys a Zero and you still don’t know what to do with it. There are 25, not 24, integer time zones. GMT is the Zero time zone, split right down the middle by the Zero meridian. There are 12 positive and 12 negative zones from there. I know there are time zones on the fraction of an hour and don’t even try to explain Tongo(sp?) being at GMT +14.

Anyway, the 15[sup]o[/sup] section that is split by the IDL (180[sup]o[/sup] meridian)is into 2 different time zones. And you can always step across it and change days. The globe is never all in the same day.

When it’s midnight in London, it’s 1am in Germany of let’s say the 5th of July. In the Azores or somewhere over to the west, it’s 11pm of the 4th of July. Moving east, it gets later and later in the morning and in the time zone called IDLW (GMT +12, around Figi) it’s noon of the 5th. Moving west from London, the time gets earlier and earlier on the 5th till you get to IDLE (GMT-12, the hell if I know what’s out there, some other island) and it is noon on the 4th.

Hope that helps. If it’s wrong jump on it.

12

Oh, crap, that last 5th should have been a 4th.

13

Hi Jinx

If you’re still around, try this thought. Frig the dateline. Imagine yourself at Greenwich at 11 in the evening, 11:05PM Wednesday. When you jump east over to Europe, you’ve moved away from the sun, so it’s an hour later there, 12:05AM Thursday. Set your calendar watch on your left wrist. Jump 12 times, going west, and you’ll lose 12 hours, ending up at 11:05AM on Thursday.

Now start over, going west. The first jump is to 10:05PM. Set your calendar watch on your right wrist. After 12 jumps, your last jump was from 12:05PM (just after noon) Wednesday to 11:05AM on Wednesday.

But you’re in the same time zone as you were when you went east! You went half way around the world both times. Your watches are a whole day apart. You don’t need no stinking dateline. Shit just happens.

.

14

BTW, the Greenwich meridian wasn’t “arbitrary”, although perhaps it might be deemed “ethnocentric”. For celestial navigation, the one sure thing you have is latitude, known by declination of the sun & time (12 noon). In the case of a known latitude (Greenwich, England) at a given longitude (Zero), you can set your chronometer and sail off around the Empire and know where you are. Couldn’t set your watch at midnight, you see, not until the advent of radio transmitted time signals.

A corollary question: had any other major seafaring nations proposed some other standard by which to set time before the worldwide acceptance of GMT ?

15

I don’t know about setting time, but I do know that when reading certain exploration narratives, you have to bear in mind the possibility that the longitude is given relative to somewhere other than Greenwich. I’ve seen Paris longitudes; there may have been others.

http://members.xoom.com/labradorian/

16

The point of it all is this: Imagine that you have a fast airplane that can circle the world in 24 hours. You take off at Noon on Wednesday and head west. Wherever you are, it’s Noon. Finally, you get back to where you started, and it’s still Noon. But 24 hours have passed, and it’s now Thursday. Now, if it was Noon all through your trip, when did it change from Wednesday to Thursday? It must have changed somewhere…

The line was put in the Pacific ocean because that’s where it caused the fewest problems. It was aligned with 180 from Greenwich because, since it was in the neighborhood anyway, putting it at 180 (with a few detours) made the most sense.

It is worth noting that Lewis Carroll found the above Gedankenexperiment insoluble in A Tangled Tale – the matter had not yet been formalized, and he couldn’t find any place where it was actually written down. It’s also a plot gimmick in Around the World in 80 Days, though I can confess it has always stretched my imagination to the breaking point to suppose the Fogg never noticed what day it was during the entire American and Atlantic legs of his journey.

John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams