Why an International Date Line?

Why an International Date Line?

I can't seem to defeat a circular flaw in my own understanding for the need for an international date line. Maybe somebody can help.

For the longest time, since high school I suppose, I was puzzled by the apparent need for it since it always seemed to me that midnight travels around the world constantly, and at those places where midnight is striking, day 1 changes to day 2. There are no exceptions to this anywhere including at the date line itself. It seems to take care of itself.

Setting out to determine what it was that I was missing, this [the following] is where I get and when I get there I think I've 'got it', but then....

So we know that day 1 changes to day 2 at midnight. Actually, it is not a flaw in my thinking, I'm pretty sure, that it seems to take care of itself. Locally is not the place where you realize the problem. But when does day 2 change to day 3 then? On a world scale, it has to be 'from where you started until you get back there again' with midnight striking, then it is a new day. If we start then at 180 degrees away from Greenwich and say 'this is where we started' then we have a constant starting point for when day 1 changes to day 2, 24 hours later, day 2 to day 3, etc. I feel like I get it.

I can even look at the gif on the wikipedia page and say to myself, I get it.



Alas, it is more of a matter of 'I can believe it'. I can believe it is necessary to have one place where day X changes to day Y, but this is where the circular nature of my own logic kicks back in. If locally there is no apparent need to think about what is going on in Somoa when midnight hits in my own locality, then what is the real problem? The day changes to the next day just like it did for the caveman marking scratches on his cave wall. It started around Somoa? Why should I care. Why should the world care? And why does a ship traveling across that line in the middle of the day have to go from, say, Wednesday back to Tuesday? (actually, I think I get that, once it has been established that we are going to have this)

I suppose this can be a case of intuition failing in the face of counter-intuitive facts. Intuition is supposed to fail in that circumstance, something the human brain always resists.

Which is why I expect somebody trying to explain it will indicate that his intuition has accepted it, but have trouble explaining it to me. The Wikipedia page does that to me, for example.

Your thoughts?

Quote: odiousgambit

Alas, it is more of a matter of 'I can believe it' is necessary to have one place where day X changes to day Y, but this is where the circular nature of my own logic kicks back in. If locally there is no apparent need to think about what is going on in Somoa when midnight hits in my own locality, then what is the real problem? The day changes to the next day just like it did for the caveman marking scratches on his cave wall. It started around Somoa? Why should I care. Why should the world care? And why does a ship traveling across that line in the middle of the day have to go from, say, Wednesday back to Tuesday?

Your thoughts?

All about record keeping. No nation would want to be split over two days except maybe as a tourist gag. When a ship travels from Oz to Long Beach they the least have to pay the crew for x days. Now, the ship itself probably does not really care and the crew is probably on 12/12 schedule. But other things do matter. Banking transactions for one. You need a line somewhere and it happens to be in the middle of the ocean where it is.

Plus it is 180 degrees from Prime Meridian, which runs through the living room of the guy who invented the whole system.

For what it's worth, my mother and wife always get confused when they cross the International Date Line.

Would it help if I said that usually when you cross the IDL it doesn't became a "new day," just a different day? For example, if you cross going west bound, it might change from 3 PM on Tuesday to 2 PM on Wednesday.

Quote: Wizard

For what it's worth, my mother and wife always get confused when they cross the International Date Line.

Would it help if I said that usually when you cross the IDL it doesn't became a "new day," just a different day? For example, if you cross going west bound, it might change from 3 PM on Tuesday to 2 PM on Wednesday.

This is how I explain it to my clients
Going from the USA to Asia, always arrive the next day due to the dateline
Returning Asia to USA travel is all done the same day
The time you lost going to Asia, you are now getting back on your return home
Sometimes I explain a flight such as UA 850, departs Beijing Fri afternoon 410pm and arrives Chicago 415pm Fri afternoon
Time and date don't change because just following the afternoon sun around the planet

Try this scenerio Odious and see if it gets you there.

You get in a plane at noon at your place. Your jet is fast enough that it can keep up with the sun (earths rotation) including time for fuel stops (or in flight fueling if you prefer). You follow the sun for a week and it is always noon. Without the date line it would still be the same day after that week.

that'd be 1000 miles per hour, roughly

supersonic!

yeah, yeah, I think I am back to 'getting it' now . Will it last? I think hashing it out is working.

also, the inconvenience of different places deciding when it should be 'next day' ... nah, you need that to be just one longitude in the world for that [not that the date line is a straight line or a single longitude, but you know what I mean]

about 1000mph at the equator.

You could walk around the north or south axis of rotation (not the magnetic poles) and cover a timezone with every step if you wanted.

You could step into tomorrow, and then with each step go backward in time one hour until you arrived back in your starting time, today.

Quote: odiousgambit

that'd be 1000 miles per hour, roughly

Well it would be 1000 miles per hour on the equator. At New York and San Francisco it would be more like 750 mph. The speed of sound at 30,000' is 678 mph, so you would still be over the speed of sound, but just barely. Speeds between Mach 0.8-1.2 are often called Transonic.

If you were travelling westward at 750 mph, and you left at time XX:XX local times then each time zone would take you one hour to cross, but the clock would reset by exactly one hour. So if you left NYC at 10:00 local time you would arrive in San Francisco at 10:00 local time.

Now suppose you had a hypothetical plane that could carry enough fuel to circle the world and maintain that speed. By the time you got back to New York City it would still be XX:XX local time. But you would have been travelling 24 hours. At some point you would have to adjust by one day.

If you wanted a scenario that is probably possible with an experimental plane, the Arctic circle is 10,975 miles, so a plane capable of maintaining 457 mph and with enough fuel capacity would circle in exactly 24 hours. By the time you get back to Iceland one day would have elapsed, but you would always be at the time you left (local time) give or take 30 minutes. The opposite of the world from the prime meridian is just a convenient place to change the day.

One advantage to using London as the Prime Meridian is that the opposite side of the world has very few inhabited places. If the Arabs succeed in getting it changed to Mecca, that will no longer be true.