antimeridianen

The antimeridian, or antimeridianen in some languages, is the meridian at 180 degrees longitude, directly opposite the Prime Meridian at 0 degrees. It forms a great circle with the Prime Meridian and divides the Earth into eastern and western hemispheres along that line. In geographic coordinates, longitudes are measured east or west from the Prime Meridian, and the 180th meridian represents the boundary where the values wrap around from +180 to −180.

In practice, the antimeridian is closely associated with the International Date Line, the line where calendar days change when crossing from one side to the other. The Date Line generally follows the 180th meridian but is not a perfectly straight line: it deviates around certain landmasses and political boundaries to keep neighboring regions on the same calendar date. These deviations help avoid splitting countries or territories into multiple dates.

Geographically, the 180th meridian passes largely through the Pacific Ocean, with only a few land crossings. In mapping and timekeeping, the antimeridian serves as a reference for a longitude that, together with the Prime Meridian, completes the global grid. In many coordinate systems, longitudes are expressed from −180° to +180°, with the antimeridian marking the boundary where the sign of the longitude changes and where data may be wrapped in digital maps.

Historically and practically, the antimeridian provides a simple, opposing reference to the Prime Meridian and underpins the system used to define time zones and calendar dates across the globe.