About the Dateline


So what is this line on the globe, where the date changes?

Actually there are two datelines

The obvious first one is the line where it is midnight.  Suppose that where you are located it's Monday evening, just before midnight.  In a few minutes it will be Tuesday.  A bit further East it is already Tuesday.

The change from Monday to Tuesday happens when you pass through the point that's furthest away from the sun:  "the dark side of the Earth"

In the image we view the world from over the North pole, the sun shines at it from the bottom of the page and the Earth turns in the direction of the green arrow.  The red line marks midnight.

It is midnight in central Europe, but not yet in Britain, and midnight is already past in Russia.

On the right side of the red line it is still Monday, on the left of the red line it is already Tuesday.  The red line is certainly a date line, since the date is different on both sides.

The midnight line stays fixed with respect to the sun, the Earth moves under it.


Say Alice and Bob stop the world and then start walking away from the midnight line.  If Alice walks from the left side in the direction of the green arrow, she passes through Siberia and will see the sun again, Tuesday morning.  But if Bob walks from the right side of the midnight line, wades through the Atlantic ocean towards Canada, it will still be Monday for him.  In the USA it is Monday afternoon, the Monday evening dusk has not come yet.

At some point Bob and Alice will meet again, Alice thinking it is Tuesday, Bob believing it is Monday.

So there must be another date line

When Alice and Bob meet they must come to an agreement about where the date changes.  By convention the second date line is fixed to the planet.  It passes through the Pacific ocean and is called the international date line.

We were just lucky to choose the 0º meridian as the one through Greenwich in Britain because the 180º meridian then conveniently passes through a large ocean crossing almost no land.  Here is our second dateline!

The yellow line is opposite Greenwich and fixed to the globe.  The yellowish part between the midnight line and the date line covers the part of the globe where it is Tuesday by convention.  In the image it is somewhat smaller than the uncovered part where by convention it is Monday.

Now let the world turn again and let's take some snapshots:

In the first of these six images it's midnight on the international date line and let's say it's Monday for everyone.  It's very early in the morning in Alaska, and near midnight in Kamchatka.  It's noon in Greenwich and early afternoon in most of Africa and Europe.

In the second image the world has turned by one eight, 3 hours.  Kamchatka has passed midnight, so it must be Tuesday there.  That's why on my image I have covered a part with a yellowish colour.  The international date line (yellow) has also become visible as it moved out from under the midnight line.

Another 6 hours later Australia and most of Asia have passed midnight and gone into Tuesday too.  It's midday or early afternoon in the Americas, still Monday.  Europe and Africa have gone into the evening and the sun has set.

The fourth image shows half the world already in Tuesday and half still in Monday.

Finally in the last two images Monday ends in the Americas and Tuesday now engulfs the planet.

There are always two date lines except for the moment the international date line coincides with the midnight line.  One dateline is fixed to the planet, the other remains fixed with respect to the sun.

And here you can see it as a little video:

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next planned revision: 2009-11