International Date Line explained

The International Dateline Explained

I've received several questions about the international dateline (IDL). "How does it work?" "What happens when you cross the IDL on a ship or airplane?" "Could Superman really go back in time by flying around the Earth and crossing the IDL multiple times?" As with most questions about "time" this one is tough to answer without an animation. And the two best animations I found online were at the links below:

They're worth looking at for an introduction. Still, each was lacking, imho, so I decided to do my own and present it here. But before you hit the play button and watch my animation, there's two things you should know about the IDL.

First, at any point in time, there are two sequential days/dates in effect on the Earth. Those days are separated by the IDL, which runs from the North Pole to the South Pole (approximately) along the 180° meridian of longitude. For most of its length, it zig-zags along east edge of the GMT±12 time zone. Whatever time of day it is, when you cross the IDL, the day and date will change. When you cross it traveling to the east, the day goes back one, and the date decreases by one. Traveling westward, the opposite happens.

Second, those two days/dates are also split by the midnight line, the meridian exactly opposite the Sun. For any given location, the day/date always changes at midnight. So there's really two "datelines" on Earth — one rotates with the planet (the IDL), and the other remains fixed at the midnight meridian. On opposite sides of both, the day/date are different.

There is one exception to the above scenario. The entire planet is on the same day/date for an hour every day. It's the hour just before the IDL reaches midnight. You'll see how and why that happens when you run the animation. But that's only what my "idealized" animation shows. Around Oceania, the time zones and IDL have been gerrymandered to the point where only a few locations on opposite sides of the IDL ever share the same day and date.

Now study that (paused) first frame of my animation. It shows the IDL (white line) at the midnight point. The GREEN wedge represents the first hour of the new day. That wedge is the first time zone west of the IDL. It runs from longitude 172.5° W to longitude 172.5° E, is centered on the 180° meridian, spans 15° of longitude, and (along its east edge) coincides with the IDL. That's the "idealization" I spoke of. But as you've seen, the real geometry is not quite that simple.

West is clockwise seen in this view from above the North Pole. The instant the IDL passes midnight, that entire time zone registers the start of the new day. That's because all locations in a given time zone are on the same clock time.

Let's call that new GREEN day Saturday. West of that time zone, it's still BLUE Friday. Start the animation now, and watch Saturday grow as Friday shrinks.

For simplicity, I made all 24 time zones correspond to exact meridians of longitude. If you clicked on that time zone link above, you saw this is not the case. Real time zones have been "adjusted" to avoid causing different parts of major cities, or island nations, to be on different clock times.

They had to choose some longitude to start the new day, and the 180° meridian was a logical choice. That meridian runs over the ocean between groups of islands. So the choice was logical in terms of convenience, commerce, and travel, but it was really an arbitrary choice.

New Zealand is just west of the IDL, so New Zealanders are the first to experience each new day/date. As they're carried along on the rotating Earth, their clock time steadily advances toward their next midnight.

Note what happens when the IDL returns to the midnight point and the next day/date begins. If the GREEN was Saturday, the RED is Sunday. You'll see Sunday growing and replacing Saturday as the Earth rotates. Use the pause button and the slider to go back and forth and watch how the IDL works.

Now let's talk about that Superman myth. Whether you're flying in a blue and red uniform, or a fast jet, going around and around the Earth in either direction simply causes the two days/dates to alternate. The day changes when you cross the IDL, and again as you pass through midnight. The faster you fly, the faster the days alternate. But even if you're faster than a speeding bullet, you can't really go backward in time.

Interestingly, the International Space Station (ISS), moving at a speed of 7.7 km/s (4.7 miles/s) makes one trip around the Earth every 90 minutes. So in a 24 hour day, occupants experience 32 day/date alternations, and get to see 16 sunrises and 16 sunsets. To keep things simple, their clocks are set to Greenwich Mean Time (GMT). Some scientists in Antarctica do the same thing. As the meridians converge near the South Pole, it becomes possible to literally walk between time zones on a short hike. One kilometer from the pole, time zones are only 262 m wide.

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