The 180th meridian or antimeridian is the meridian which is 180° east or west of the Prime Meridian with which it forms a great circle which divides the earth into the Western Hemisphere and Eastern Hemisphere. It is common to both east longitude and west longitude. It is used as the basis for the International Date Line because it for the most part passes through the open waters of the Pacific Ocean. However, the meridian passes through Russia and Fiji as well as Antarctica.
The word antimeridian can also mean the opposite of another meridian (or Local meridian) that is 180 degrees east or west of that meridian. Eg the 90th meridian east is opposite the 90th meridian west. Opposite meridians form straight lines at the poles so if you a the Prime Meridian and you keep going north or south you would end up on the antimeridian going in the opposite direction you were going before and vice versa. Meridians that are 90 degrees away from each other form right angles at the poles.
Country, territory or sea Notes Arctic Ocean Russia Chukotka Autonomous Okrug — Wrangel Island Chukchi Sea Russia Chukotka Autonomous Okrug Bering Sea Amchitka Pass Passing just east of Semisopochnoi Island, Alaska, United States (at ) Pacific Ocean Passing just east of Nukulaelae atoll, Tuvalu (at )
Passing just west of the island of Cikobia, Fiji (at )
Fiji Islands of Vanua Levu, Rabi, and Taveuni Pacific Ocean Passing just east of the island of Moala, Fiji (at )
Passing just west of the island of Totoya, Fiji (at )
Passing just east of the island of Matuku, Fiji (at )
Southern Ocean Antarctica Ross Dependency, claimed by New Zealand
The meridian also passes between (but not particularly close to):
- the Gilbert Islands and the Phoenix Islands of Kiribati;
- between North Island and the Kermadec Islands of New Zealand;
- between the Bounty Islands and the Chatham Islands, also of New Zealand.
The only place where roads cross this meridian, and where there are buildings very close to it, is in Fiji.
Software representation problems
Many geographic software libraries or data formats project the world to a rectangle; very often this rectangle is split exactly at the 180th meridian. This often makes it impossible to do simple tasks (like representing an area, or a line) over the 180th meridian. Some examples:
- The GeoJSON specification doesn't mention handling of the 180th meridian in its specification, as such, representations of lines crossing the 180th meridian can just as well be interpreted as going around the world.
- In OpenStreetMap, areas (like the boundary of Russia) are split at the 180th meridian.
- The word antimeridian can also mean the meridian opposite to any given meridian. E.g. 20° west is the antimeridian of 160° east.