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 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 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.
- The word antimeridian can also mean the meridian opposite to any given meridian. E.g. 20° west is the antimeridian of 160° east.
- "RFC 7946 - The GeoJSON Format".