The 180th meridian or antimeridian  is the meridian 180° both east and west of the Prime Meridian, with which it forms a great circle dividing the earth into the Western and Eastern Hemispheres. It is common to both east longitude and west longitude. It mostly passes through the open waters of the Pacific Ocean, but passes across land in Russia, Fiji and Antarctica. This meridian is used as the basis for the International Date Line, but the latter deviates from it to maintain date consistency within the territories of Russia, USA, Kiribati, Fiji and New Zealand.
|Country, territory or sea||Notes|
|Russia||Chukotka Autonomous Okrug — Wrangel Island|
|Russia||Chukotka Autonomous Okrug|
|Amchitka Pass||Passing just east of Semisopochnoi Island, Alaska, United States (at )|
|Pacific Ocean||Passing just east of
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
Fiji (at )|
Passing just west of the island of Totoya, Fiji (at )
Passing just east of the island of Matuku, Fiji (at )
|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.
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 non-trivial to do simple tasks (like representing an area, or a line) over the 180th meridian. Some examples:
- The GeoJSON specification strongly suggests splitting geometries so that neither of their parts cross the antimeridian. 
- In OpenStreetMap, areas (like the boundary of Russia) are split at the 180th meridian.