International Date Line

The International Date Line is an imaginary line that runs between the South and North Poles. It marks the place where one calendar day ends and the next begins. This line goes through the Pacific Ocean and mostly follows the 180.0° line of longitude, but it bends around some islands and areas to suit local needs.

Crossing the International Date Line from east to west means losing a day on your calendar. If you travel west across the line, you will gain a day. This line is not a real, legal boundary, but a suggestion made by mapmakers to help keep time organized around the world.

Because it is just a suggestion and not a law, mapmakers have sometimes disagreed about exactly where the line should be. Some countries have even asked to move the line to make things easier for their own schedules and plans.

Geography

Circumnavigating the globe

When traveling around the world, people moving westward need to set their clocks back by one hour for every 15 degrees of longitude they cross. But when they cross the International Date Line, they must move their clocks forward by a whole day.

People traveling eastward set their clocks forward by one hour for every 15 degrees of longitude they cross. But when crossing the International Date Line, they must move their clocks back by a whole day. Changing the clock by a whole day also means changing the date by one day.

Description

The International Date Line is mostly based on the 180-degree line of longitude, running through the middle of the Pacific Ocean. It’s about halfway around the world from the meridian that runs through Greenwich, England. In most places, the Date Line follows the 180-degree meridian exactly. But in some areas, it moves east or west to match the political and economic ties of nearby lands.

Starting from the north, the Date Line first moves east of Wrangel Island and the Chukchi Peninsula in Russia. It then passes through the Bering Strait between two islands called the Diomede Islands. After that, it bends west of the 180-degree line, going west of islands like St. Lawrence Island.

The Date Line crosses between the U.S. Aleutian Islands and Russia’s Commander Islands. It then bends southeast to return to the 180-degree line. This means all of Russia is to the west of the Date Line, and almost all of the United States is to the east, except for places like Guam and the Northern Mariana Islands.

The Date Line stays on the 180-degree meridian until it reaches near the equator. Two tiny U.S. islands, Howland Island and Baker Island, are among the first places on Earth to start a new day. The Date Line then swings far east around Kiribati, an island country, almost reaching the 150-degree west meridian. Kiribati’s farthest eastern islands are among the last places on Earth to start a new day.

South of Kiribati, the Date Line returns westward but stays east of the 180-degree line, passing between Samoa and American Samoa. This means places like Samoa and Tonga are on the same date, while American Samoa is one day behind. The Date Line then bends southwest to return to the 180-degree line and continues until it reaches Antarctica, where it isn’t usually shown on maps.

Facts dependent on the IDL

Because of the International Date Line, some places are the first to welcome a new day. Islands that use UTC+14:00 time, such as parts of Kiribati, are often the first. Major cities like Auckland and Wellington in New Zealand are also among the first.

In 2000, the Date Line was changed so that Caroline Island in Kiribati was one of the first places to reach January 1. It was renamed Millennium Island because of this.

Every day, for a couple of hours, there are three different dates happening at the same time on Earth. For example, while most of the world is on a Tuesday, some places are still on Monday and others are already on Wednesday.

The first places to see sunlight on a new day change with the seasons. Around the June solstice, it would be places in the Kamchatka Time Zone that have midnight sun. Around the equinoxes, it would be Millennium Island in Kiribati. Near the December solstice, it would be Antarctic research stations using New Zealand Time during their summer, when they also experience midnight sun.

De facto and de jure date lines

Time zones and the location of the International Date Line can be determined in two ways: on land and nearby waters, and on the open seas. Each country decides its own time zones for its land and nearby waters. This is called the de facto date line because it is based on each country’s laws, not international rules. These time zones do not extend into international waters far from land.

There is also a de jure date line for ships, agreed upon internationally. It follows the 180° meridian and was set up so ships can keep track of time while sailing. Ships usually follow the time of the country they are near, but when far out at sea, they use international time zones for communication. Maps show the date line based on each country’s choice, and it can change over time.

History

The 14th-century Arab geographer Abulfeda predicted that people traveling around the world would find their date one day different from the place they started. This was proven in 1522 at the end of the Magellan–Elcano expedition, the first successful trip around the world. After sailing westward from Spain, the ship stopped at Cape Verde on Wednesday, 9 July 1522, but the locals told them it was actually Thursday, 10 July 1522. The crew was surprised because they had kept track of each day during their three-year journey. Cardinal Gasparo Contarini, the Venetian ambassador to Spain, was the first European to correctly explain why this happened.

Historic alterations

Philippines (1521 and 1844)

Main article: Philippine Standard Time

Ferdinand Magellan claimed the Philippines for Spain on Saturday, 16 March 1521, after sailing westward from Seville across the Atlantic Ocean and the Pacific Ocean. Because the Philippines was on the eastern side of the International Date Line, it was one day behind its Asian neighbors for many years. In 1844, the Philippines decided to change to match its Asian neighbors by removing a day from the calendar.

Tahiti & French Polynesia (early 1797 and late 1846)

Missionaries arrived in Tahiti in 1797 and brought the date from the eastern hemisphere. After the Franco-Tahitian War, Tahiti changed to follow the western hemisphere's date in 1846.

Pitcairn Islands (1814)

The Pitcairn Islands were settled by mutineers from HMS Bounty. They initially used dates from the eastern hemisphere but later switched to dates from the western hemisphere.

Alaska (1867)

See also: Alaska Time Zone and Time in Alaska

When the United States purchased Russian America in 1867, Alaska moved to the eastern side of the International Date Line. This changed both the date and the time.

Samoan Islands and Tokelau (1892 and 2011)

See also: Samoa Time Zone

The Samoan Islands changed their date in 1892 to match American dates. In 2011, Samoa moved back to the western side of the International Date Line by removing a day from the calendar. Tokelau made the same change at the same time.

Cook Islands and Niue (1899)

In 1899, the Cook Islands and Niue changed to the eastern side of the International Date Line when they became British protectorates.

Kwajalein (c. 1945 and 1993)

Kwajalein changed to the eastern side of the International Date Line when the United States established a military base there. It later switched back to the western side in 1993.

Eastern Kiribati (1994)

Main article: Time in Kiribati

In 1994, Kiribati changed the date for its eastern islands by removing a day from the calendar. This moved the International Date Line to go around the entire country. After this change, Kiribati's easternmost islands started the year 2000 before any other country.

Date lines according to religious principles

Christianity

Christians around the world celebrate holidays like Christmas on the same calendar day, even when they are on opposite sides of the International Date Line. For example, Christians in Samoa, west of the line, celebrate a day earlier than those in American Samoa, east of the line.

Some Christian churches face special questions when they are near the International Date Line. Because the line separates days on calendars but not on actual sunrises and sunsets, it can be unclear which day is the true Sabbath. This is especially important for churches that observe Saturday as their holy day of rest. In Tonga, which lies east of the main line, many Christians worship on Sunday instead.

Islam

Muslim communities also follow the International Date Line. The day for special Friday prayers, called Jumu'ah, is observed locally as Friday everywhere. The start of Islamic months, like Ramadan, depends on when people see the new crescent moon, not on the date line.

Judaism

Jewish law also considers the International Date Line. Jewish communities in places like Japan, New Zealand, Hawaii, and French Polynesia all observe their holy day, Shabbat, on local Saturday. However, some Jewish leaders have different ideas about exactly how to follow these rules, especially for travelers crossing the date line. The actual rising and setting of the sun still matters for counting days, even when the calendar date changes.

Cultural references and traditions

See also: Line-crossing ceremony

Special ceremonies on ships have celebrated when someone crosses the International Date Line for the first time. These traditions have been enjoyed by sailors and passengers for many years.

The idea of the date line has appeared in many stories. For example, Edgar Allan Poe used it in a tale where two captains meet but disagree on what day it is. In Jules Verne’s book Around the World in Eighty Days, the main character almost misses his bet to travel around the world in 80 days because of the date line. Umberto Eco also wrote about a character who thinks deeply about what the date line means.