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International Date Line: Definition, History & Location

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Lesson Transcript
Instructor: Josh Corbat

Josh has taught Earth Science and Physical Science at the High School level and holds a Master of Education degree from UNC-Chapel Hill.

There is a line on Earth that causes you to change days when you cross it. This lesson will explain what the International Date Line is, how it affects the time zones on either side, and the history of how we decided on it.

What Is the International Date Line?

Did you know there is a place on Earth where you can go back in time? Okay, not really go back in time, but it's almost the same thing. When you cross the International Date Line, the date changes. What's happening here? You've probably noticed that different places on Earth have different times. If one person in New York and another in California are talking on the phone, the time for each is different. Three hours different, to be exact. This is because New York and California are in different time zones.

At any given time, the sun is shining on some part of the earth. In fact, if you left your home at noon and ran westward at over 1000mph, you would outrun the sun and it would seemingly never set. Time zones are a way to make everyone's daylight hours roughly the same, so the sun rises in the morning and sets in the evening no matter where you live (with a few exceptions, but we won't worry about those right now).

The earth is divided into slices that run from pole to pole, kind of like an orange. The imaginary lines that divide these slices are called lines of longitude. Anything circular, like a sphere, can be divided into 360 degrees. The earth is roughly a sphere, and therefore can be divided into 360 equal slices. We give time zones their own sections 15 degrees wide.

If you do the math, 360 divided by 15 is 24, so there are a total of 24 different time zones. Each one increases by one hour if you go east or decreases by one hour if you go west. 24 hours in a day, 24 time zones. Easy, right? Well, eventually you will have to start over, and this is where it gets a bit weird. If you travel all the way around the world, you will eventually reach a line where you leap backward or forward a day. You're not literally traveling through time, of course, but the date, like the one you would write in a journal, changes.

Let's take a look at a world map together. First, let's go to Hawaii. It's 9 in the morning on Thursday there. Now let's head east to England. There it would be 9 in the evening on Thursday, 12 hours ahead of the time it currently is in Hawaii. You moved over 12 time zones. Now, let's keep going east to New Zealand, where you're almost back to the same slice that Hawaii is in, and it's actually 9 in the morning on Friday, the next day!

The Prime Meridian (center red line) and International Date Line (red line at right). Notice how it does not perfectly follow the 180 degrees longitude line.
IDL and prime meridian

That's because in between Hawaii and New Zealand is a line called the International Date Line. The International Date Line is an imaginary line that runs from the North Pole to the South Pole and separates one day from the next. So in some places, locations right next to each other are actually separated by an entire day. Weird, right?

The Location of the International Date Line

Scientists decided long ago that time would be calculated from a special line of longitude located in Greenwich, England. This line is called the Prime Meridian. The International Date Line is located halfway around the world from the Prime Meridian, and roughly follows the 180 degrees longitude line. It runs down the Pacific Ocean, in between Alaska and Russia, and down between Hawaii and New Zealand.

You'll notice that there are places where the line shifts significantly. These are areas where nations and islands called for the line to be drawn differently. If one nation has an island group that they want to be in the same time zone as them, the International Date Line may be drawn to include that island group.

The History of the International Date Line

The International Date Line is a relatively new concept that has been created to help with international timekeeping. Before worldwide travel by ship, and now by plane, there wasn't much need for different time zones and the International Date Line. Now, they are incredibly important.

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