The International Date Line (IDL) is an imaginary line that separates the Eastern and Western Hemispheres, where one day is divided from another. Time zones are set by each country independently, and the location of the IDL was chosen to avoid confusion in inhabited areas. The line has been moved several times for economic reasons. Traveling through the IDL can be confusing, but airlines and travel agencies keep it in mind when informing travelers about schedules and itineraries.
The International Date Line (IDL), is an imaginary line that runs roughly along the 180° line of longitude in the middle of the Pacific Ocean. International convention accepts the line as the location where one day is divided from another, with the area of the Eastern Hemisphere one day ahead of the Western Hemisphere. This line is needed to deal with some quirks that occur during travel; people traveling the world perceive themselves as gaining or losing a day, depending on which direction they traveled.
How time zones work
In theory, the world could be divided into 24 time zones, one for each hour of the day. Time zones are actually set by each country independently, however, and local time doesn’t necessarily match these ideal standard time divisions. Ideally, time zones are designed so that local noon falls when the sun is more or less overhead. Traveling west, a person would lose one hour for every 15° of longitude; by traveling east, towards the sun, he gains an hour.
Time zones are described by their relationship to the prime meridian, which falls at 0° longitude in Greenwich, England. If someone were 45° from Greenwich, they would experience a time difference of three hours; 45° West would mean the person was in Coordinated Universal Time (UTC) -3, while someone 45° East would be in UTC +3. The International Date Line falls directly across from the Prime Meridian in the UTC +12 and UTC -12 time zones. Coordinated Universal Time was formerly known as Greenwich Mean Time (GMT), and is still sometimes written with this abbreviation.
How was the location of IDL chosen?
The International Date Line’s location was chosen because it is in a relatively uninhabited area of the Pacific. The presence of inhabited areas – including Alaska, Russia and a number of islands – causes the line to deviate in some areas. If the line were straight, eastern most of Russia would be one day behind the rest of the country, while a number of islands that are part of Alaska would be one day ahead of the state. This would likely lead to confusion and administrative chaos.
Move the line
The line has been moved several times putting one country on one side or the other. The islands of the Philippines, for example, were on the eastern side of the Date Line – with the western hemisphere – until the mid-19th century, despite being located at about 19° east longitude. In the late 120s, the islands of Samoa and Tokelau, which are very close to the line, moved from the east to the west side of the line, rejoining the eastern hemisphere. Such changes are often made for economic reasons; Samoa and Tokelau have shifted due to growing trade links with Australia and New Zealand.
Crossing the Line
Traveling through the International Date Line can be confusing, especially for people taking a short trip, such as from Fiji to Hawaii. According to the clock, a traveler would end up arriving in Hawaii before leaving Fiji, because Fiji is one day ahead of Hawaii and two hours behind – when it is noon in Hawaii, it is 10am in Fiji the day after . The confusion of days and times sometimes leads to setbacks, but most airlines and travel agencies keep the line in mind when informing travelers about schedules, estimated times of arrival, and itineraries.
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