International Date Line

If you cross the International Date Line (IDL) on your journey from east to west you tend to earn a day, and when you cross it while traveling from west to east you lose one.

Even though the distance between Tonga and Samoa―both islands located in the south Pacific―is a mere 557 miles, the two places have a difference of 24 hours in their time.

If it is 2200 hours in Tonga at this very moment, the time in Samoa will also be 2200 hours, however, it will be Saturday, November 27 in Tonga, while the same in Samoa will be Friday, November 26.

This difference in time in these two places can be attributed to the fact that they are located on either sides of the International Date Line, the imaginary line which separates two different days on the planet.

The International Date Line (abbreviated as IDL) is an imaginary line, running down the Pacific Ocean from North to South, which determines where a calendar day ends and the next day begins. It is a universally accepted line, which roughly runs along the 180° meridian, with a few diversions here and there to make sure that no country is split into two.

If you cross the IDL on your journey towards east, 24 hours are subtracted from your time, and if you cross it when going west, 24 hours are added to your time. Similarly for every 15° crossed towards east, you earn an hour, and for every 15° crossed towards west, you lose one.

If it were not for this imaginary line, a person crossing over to the west would have taken an extra calendar day to complete his journey; something which the Portuguese navigator, Ferdinand Magellan had experienced when he traveled west during his circumnavigation of the globe.

Even though the International Date Line separates the two calendar dates, each new day starts at Greenwich Meridian as per the resolution passed at the International Meridian Conference held in Washington DC in 1884.

At 180° meridian, the IDL is located right opposite the Prime Meridian located at 0°. Even though its location is not fixed by any International law or treaty, most of the maps and globes follow it along the 180° meridian, with a few instances wherein it goes off the meridian.

This location was chosen with consensus, owing to the fact that this is a relatively sparsely populated region and a couple of diversions were eventually added to ensure that no country is subjected to two different times. If you trace it on a world map, you will realize that it's a zigzag line.

The first prominent diversion is seen near Bering Strait, where it is diverted to the east in order to make sure that the northeastern region of Russia is not separated from the time zone followed by rest of the country.

Similarly, it is diverted to west near the Aleutian Islands to make sure that Alaska in not separated from the United States. The most prominent diversion is seen near the territory of Kiribati, where it circumnavigates this Island Republic by reaching as far as the 150° meridian.

Today we know what happens when we cross the International Date Line, but it was not known until recently and that resulted in furore when Magellan lost a day on his attempt to circumnavigate the Earth in the 16th century. Even today, if you set out on a journey from Tonga to Samoa, you will arrive at Samoa even before you had left Tonga.