International Date Line - Geography

Geography

For parts of its length, the International Date Line follows the meridian of 180° longitude, roughly down the middle of the Pacific Ocean. To avoid crossing nations internally the line deviates around the far east of Russia and then around various island groups in the Pacific. These various deviations (east or west) generally accommodate the political and/or economic affiliations of the affected areas.

From the north, the date line first deviates to the east of 180° to pass to the east of Russia's Wrangel Island and the Chukchi Peninsula which is the easternmost part of Russian Siberia. The date line then passes through the Bering Strait between the Diomede Islands at a distance of 1.5 km (1 mi) from each island. The line then bends considerably southwest, passing west of St. Lawrence Island and St. Matthew Island. It then passes midway between Alaska's Aleutian Islands and Russia's Commander Islands before returning southeast to 180°. Thus all of Siberia is to the west of the International Date Line, and all of Alaska is to the east of that line.

Two uninhabited atolls, Howland Island and Baker Island, just north of the equator in the central Pacific Ocean (and ships at sea between 172.5°W and 180°) have the latest time on Earth of UTC-12 hours. The date line then circumscribes the country of Kiribati by swinging far to the east, almost reaching the 150° meridian. Kiribati's easternmost islands, the southern Line Islands south of Hawaii, have the most advanced time on Earth, UTC+14 hours. South of Kiribati, the date line returns westwards but remains east of 180°, passing between Samoa and American Samoa; accordingly, Samoa, Tokelau, Wallis and Futuna, Fiji, Tonga, and New Zealand's Kermadec Islands and Chatham Islands have the same date, while American Samoa, the Cook Islands, Niue, and French Polynesia are all one day earlier.

A person who flies (or sails) around the world from east to west (the same direction as Magellan's voyage) would lose one hour for every 15° of longitude crossed, and would lose 24 hours for one circuit of the globe from east to west if they did not compensate by adding 24 hours when they cross the International Date Line. In contrast, a west-to-east circumnavigation of the globe requires subtracting 24 hours when crossing the international date line. The International Date Line must therefore be observed in conjunction with the Earth's time zones: on crossing it in either direction, the calendar date is adjusted by one day.

For the two hours between 10:00 and 11:59 (UTC) each day, three different days are observed at the same time in different places. For example, at UTC time Thursday 10:15, it is Wednesday 23:15 in American Samoa, (UTC-11), and Friday 00:15 in Kiritimati (UTC+14). For the first hour (UTC 10:00–10:59), this is true for both inhabited and uninhabited territories, but during the second hour (UTC 11:00–11:59) it is only true in an uninhabited maritime time zone twelve hours behind UTC (UTC-12).

According to the clock, the first areas to experience a new day and a New Year are all islands that use UTC+14, that is the Line Islands and Tokelau, and in the southern summer also Samoa. The first major city is Auckland, New Zealand.

The areas that are the first to see the daylight of a new day vary by the season. On 1 July, it is a large part of the Chukchi Peninsula which uses UTC+12 and experiences midnight sun on this date. At New Year, the first places to see daylight are the South Pole and the McMurdo Station in Antarctica, which both experience midnight sun at this time. Both use UTC+13 as daylight saving time. The first place to see daylight at equinox is the uninhabited Caroline Island which is the easternmost land west of the International Date Line.