The international day line is an imaginary line the runs north-south across the Pacific s from the north Pole come the south Pole.


The international date line (IDL) is an imaginary line the runs along the Earth"s surface ar from the phibìc Pole to the southern Pole in the middle of the Pacific Ocean. When you overcome it, friend either gain or shed a day relying on which method you space traveling. If you room traveling westward, you acquire a day, and if you are traveling eastward, you lose a day.

You are watching: The united states spans ________ time zones.

For example, if a traveler move eastward throughout the Pacific s from wake Island come the Hawaiian archipelago on June 25, they will certainly jump behind to June 24 as quickly as they overcome the IDL. If, however, they are traveling in opposing direction, they will arrive at wake up Island ~ above June 26.

Although the traveler seems to relocate backward or forward in time, there"s no physics-defying magic going on here. The international day line is based on a rational, useful system of global timekeeping the takes into account the motion of Earth roughly the sun.

Related: How can Samoa hop over the international day line?

Why execute we require the international date line?

Earth rotates counterclockwise (from west come east) ~ above its axis together it travels roughly the sun. This method that various parts the the world receive the sun"s direct rays at various times, so when it is noon on one component of planet — the duration when the sunlight is in ~ its highest in the sky — another component of the planet may be totally in zero or enduring sunrise or sunset. That also method that, about theory at least, if you to be somehow able to outrun Earth"s rotation, i m sorry is a rate of roughly 1,000 mph (1,600 km/h), you can experience perpetual daylight and also never check out darkness.

But people don"t take trip at such speeds (unless you space on the International an are Station), and when not traveling people generally stay in one place. So, to permit people to experience daylight hrs in roughly the very same amount — the is, to suffer a common day from sunrise come sunset — time zones space in use roughly the world.

The first system that time area was propose by sir Sandford Fleming in 1876, follow to an write-up in Globe and Mail, a Canadian news publication. Fleming was a Scottish engineer who helped architecture the Canadian railway system. He want to make the railroad more efficient and also avoid any complications resulting from the different schedules set by the various train stations, which collection the time according to their local huge conditions, according to Encyclopedia Britannica. Accordingly, that proposed a mechanism of 24 standard time zones that would expectations the one of the Earth. Within each time zone, all clocks would certainly be collection to an typical time that ideal represented where the sunlight was situated in the sky. The practicality of his mechanism soon caught on, so that by 1900 most of the established nations of the world had adopted it.

People walking in ~ clocks displaying global time area in the Warsaw share Exchange structure in Warsaw, Poland. (Image credit: Getty Images/Bloomberg)

Today, through a number of far-ranging modifications, us still use the 24-time-zone mechanism originally suggest by Fleming. The logic of dividing Earth into 24 area is acquired from the truth that the earth is a sphere, and also like any kind of sphere it deserve to be divided into 360 same sections, or 360 degrees. Every time ar is measured together 15 degrees wide, and 360 separated by 15 is 24.

All of this zones are numbered consecutively eastward indigenous what is referred to as the prime meridian, i beg your pardon is a line of longitude that runs directly through the British imperial Observatory in Greenwich, in the joined Kingdom. The prime meridian was created in 1851 in ~ a time when the U.K. Was one of the most powerful and technologically advanced nations top top Earth. With a powerful navy, the British were well progressed in navigation and also timekeeping and also were using the most sophisticated devices accessible to reckon global position and time. The element meridian, at 0 degrees longitude, is the suggest from which every other dimensions of longitude room taken. The element meridian is additionally where the device of 24-hour timekeeping that is dubbed Greenwich median Time (GMT) originated.

As girlfriend go eastern from the element meridian, the moment zones boost consecutively by one hour, or whereas decrease consecutively through one hour together you walk west. Yet Earth, of course, does not go ~ above indefinitely; eventually, a allude is reached once you have to start end — or run backward or forward in time relying on your direction that travel.

What happens once you cross the international day line?

Here"s an instance of just how time zones and also the international day line works.

London, U.K. Is eight time area away from san Francisco, California. Going eastward from mountain Francisco come London, you move ahead eight time zones, i m sorry really method you skip front eight hours. So, as soon as the time in san Francisco is 1 p.m, the time in London is 9:00 p.m. Top top the same day.

However, if you were to relocate eight time zones in the opposite direction from mountain Francisco at 1 p.m. — the is, west across the Pacific ocean to the Siberian city of Yakutsk, because that example, things acquire complicated. It"s not simply a matter of subtracting eight hrs from the san Francisco time. You start by individually an hour because that each time zone moving west, making it to 9:00 a.m., however at that point you"ll hit the IDL, where whatever changes. Currently you have to add a complete 24 hrs to your calculations and also jump a complete day ahead, making the 9 a.m. The next day.

But you"re no done yet. Due to the fact that all the moment calculations room taken indigenous Greenwich, you must subtract four much more hours from the time since Yakutsk sits four time zones west the the IDL. So, when the time in mountain Francisco is 1:00 p.m. On Wednesday, that is 5:00 a.m. ~ above Thursday in Yakutsk.

Depending on which direction you"re going as soon as you overcome the international date line, you"ll either run forward or behind a day. (Image credit: Getty Images/ Yaorusheng)

How the IDL began

The IDL was developed in 1884 throughout the international Meridian Conference organized in Washington, D.C., follow to short article Card History. President Chester A. Arthur convened the conference, which included representatives that 26 nations. The president, favor other people leaders, saw the requirement of establishing standards because that an international covenant on time and longitude. By the finish of the 19th century, as global commerce progressed, interaction technologies grew an ext sophisticated and nations prospered increasingly interdependent, it to be imperative the all clocks be set to a civilization standard.

With this in mind, the allude of the conference, as summed up in the Protocols of the Proceedings, to be "for the purpose of addressing upon a meridian suitable to it is in employed as a usual zero the longitude and standard of time-reckoning throughout the globe."

The conference led to the recognition of the element meridian in Greenwich, which had already been in use for number of decades, as the world"s solitary "initial meridian." Although various other prime meridians to be in presence at the time, such as ones in Rome, Paris, Jerusalem, St. Petersburg and Oslo, for example, the Greenwich meridian was adopted primarily since over two-thirds of ships currently used it because that zero longitude.

In all, 7 resolutions were passed at the finish of the conference. This ranged native adopting a global solar work (beginning at midnight in Greenwich and counted on a 24-hour clock) come the proposal that all nautical and also astronomical days anywhere would begin at 12:00 a.m.

The IDL was developed as a an outcome of the conference"s 3rd Resolution, i beg your pardon stated: "That indigenous this meridian longitude shall be counted in 2 directions approximately 180 degrees, eastern longitude being plus and west longitude minus."

The logic behind this is that on a discrete sphere, the day and also date have to be separated attwolocations — friend can"t separation a sphere right into two parts with a solitary "cut" top top one side. You need a beginning and an finishing point.

None of the resolutions, however, were binding. It was approximately each nation to enforce these proposals. Even today, because that example, the IDL is not a matter of worldwide law, no one is it imposed by the dictates that an global governing board. That is embraced by all nations and also has been adopted because that is critical for worldwide interconnectivity, instantaneous communication, time measurement and also consistent international databases.

Why is the international day line not straight?

An excellent means of visualizing the IDL is come think of it in terms comparable to a line of longitude. In fact, it around follows the 180-degree meridian, which is located halfway ring the world from the prime meridian. But the IDL, unlike a meridian line, does no run in a directly path. Because its inception, the IDL has undergone several major deviations, and also it now swerves, zig-zags and jogs in a look at arbitrary pattern approximately prominent landmasses and details Pacific islands. Most of these deviations room the result of practical considerations, such as to avoid separating a nation into 2 time zones, or because that political and also economic reasons.

The IDL runs southern from the north Pole, adhering to the 180-degree meridian until it swerves eastern to pass through the Bering Strait, keeping big Diomede Island, a part of Russia, west that the IDL, and tiny Diomede Island, i m sorry is component of the joined States, come the east. Return the islands are only separated by the narrowest of margins — simply 2 miles (3.2 kilometers) — lock experience different days, with huge Diomede Island constantly one day front of small Diomede Island. The IDL climate jogs earlier west to skirt roughly the united States’ Aleutian Islands, keeping Russia and Alaska on opposite sides.

Simple map showing the location of the international date line in 1921 and 1995. (Image credit: Shutterstock/Nasky)

It then tracks back, complying with the 180-degree meridian southern again for numerous thousand miles, happen west the the Hawaiian Islands and east of the independent nation of the Marshall Islands, until reaching the Pacific island country of Kiribati. Here, the IDL renders a major deviation east to incorporate the Kiribati and Line Islands, the last of i m sorry is also known as the Teraina or Equatorial Islands. This deviation reaches practically as far east as the 150-degree meridian and also forms a very huge and noticeable hammerhead-looking configuration.

Kiribati, which got its self-reliance from the united kingdom in 1979, is a team of 33 greatly uninhabited islands and atolls that span a substantial area encompassing no much less than 1.9 million square miles (5 million square kilometers). Prior to the 1990s, the IDL bisected the island nation, throwing its western fifty percent a day ahead of its eastern half, follow to the CIA"s people Fact Book. This caused considerable political and economic hardships together the nation tried come conduct regular everyday business with one fifty percent of its country a day ahead and the various other a day behind. In 1995, Kiribati determined it had actually had enough of this arrangement, and seeking greater economic ties with Australia and brand-new Zealand, prolonged the IDL eastward to encompass the nation"s far-flung eastern islands, thus developing the strange configuration.

Shifting the day line is a fairly easy matter, the BBC reported. It doesn’t require the commitment of other nations or the ratify of an worldwide governing body. The country can decide because that itself. It’s simply a matter of publicizing the event and informing the worldwide community and map-makers.

Map of the international date line in the southern Pacific. (Image credit: Wikimedia/Jailbird, license:

Leaving Kiribati, the IDL passes in between Samoa and also American Samoa, keeping the previous west and the latter eastern of the IDL. It climate swerves back sharply toward the 180-degree meridian, however does no follow the meridian"s specific path. Instead, it stays parallel to the meridian, maintaining to the east of the heat so the Tonga, the Kermadec islands and brand-new Zealand room all eastern of the IDL.

South of new Zealand, in the vicinity that the Chatham Islands, i beg your pardon are part of brand-new Zealand, the IDL jogs ago and connects increase again v the 180-degree meridian. It complies with this route until reaching Antarctica, which has multiple time zones. The IDLisnot drawninto Antarcticaon many maps.

Universal coordinated Time

Today, a an approach known as global Coordinated Time, abbreviated as UTC, has end up being the timekeeping standard. In essence, that is a more sophisticated variation of GMT due to the fact that it is still based upon the zero levels of latitude that passes through Greenwich yet relies on atom clocks. This devices, designed in the 1950s, are extremely accurate and also are based exclusively on Earth"s rotation. The clocks take into account slim variations in the Earth"s rotation rate that can reason timekeeping to vary.

Additional resources

This short article has to be updated from a previous version written by Dan Helm in 2018.

See more: I Want To Fucking Tear You Apart, Tear You Apart

Tom Garlinghouse is a reporter specializing in general science stories. He has a Ph.D. In archaeology native the college of California, Davis, and also was a practicing excavator prior to receiving his MA in scientific research journalism from the college of California, Santa Cruz. His job-related has showed up in one eclectic selection of print and also online publications, consisting of the Monterey Herald, the san Jose Mercury News, history Today,,, Current people Archaeology and many others. The is additionally a novelist whose first novelMind Fields, was newly published