Australian_Geographic_-_October_2015_

September–October 2015 17

Where today becomes tomorrow

The International Date Line marks the point on the planet where globetrotters get to travel in time.

TOPOGR APHICAL

Y

OU’D THINK the International

Date Line (IDL) would be

geographically precise – but it’s

more an ill-defi ned north-to-south

path, roughly following 180° longitude.

The IDL is the place where we

declare that time shifts by exactly 24

hours to accommodate the fact that

Earth’s movement causes confusion

on an international scale, when the

definition of a day is defined by sun-

rise and sunset. When you cross it

travelling west, the day and the date

skip forward, but the time of day

usually remains the same – cross it

travelling east and the reverse happens.

Discussion about the need for a

defined marker arose during the 1884

International Meridian Conference,

which convened to set the prime

meridian – 0° longitude. This was

needed for geographical and nautical

charts. The conference overwhelmingly

agreed to define that line as passing

through both poles via the district of

Greenwich in London.

But there was, and never has been,

any international treaty, agreement or

protocol to precisely define the IDL’s

path. And so this imaginary linear

marker has shifted according to

political and cultural alliances and

sometimes trade pressures. The IDL

shown on most modern maps is

largely the one drawn up by the

British Admiralty in 1921.

The biggest kink in the IDL occurs

around the Republic of Kiribati, which

comprises 32 island atolls that once

straddled the line. On 1 January 1995, part

of the IDL was moved eastwards so that all of

Kiribati’s islands were on the same side. Before

this, government offices working across the line

could only communicate for four days each week.

In the early 1800s, Alaska – then part of Russia –

was west of the IDL, because most travellers

arrived via Siberia. When the USA acquired

it in 1867, the IDL was shifted, so

Alaska now sits to its east.

BACK TO THE FUTURE

In 1917 the Anglo-French

Conference on Time-keeping at Sea

set the Nautical Date Line. Unlike the

IDL, this is a legal construct deter-

mined by international agreement.

It precisely follows the 180°

line of longitude, except where

interrupted by territorial waters.

Four landmasses sit on 180° of longitude –

Vanua Levu, Rabi and Taveuni islands, all part

of Fiji; and Russia’s Wrangel Island, in

the Arctic Ocean.

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In 1892 trading partners convinced the king of Samoa

to move the line to better match the USA. This was done by

repeating Monday 4 July 1892 – the USA’s Independence Day.

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Because they are either side of the IDL,

the neighbouring territories of Samoa and

American Samoa (just 70km apart) experience

a 24–25 hour time difference, depending on

daylight savings time.

Friday 30 December 2011 did not exist for Samoa’s

186,000 citizens, as the country jumped to the west of the

IDL again to align with trading partners Australia and New

Zealand. Samoa’s now among the first countries to ring in

the New Year, rather than the last.