antimeridian

The reference line at 180° longitude, on the opposite side of earth to the prime meridian.

On the opposite side of the Earth from Greenwich is the antimeridian, a line of longitude that is 180° both east and west of the prime meridian. It cuts through parts of eastern Russia, the Fiji Islands and the middle of the Pacific Ocean.

A meridian of longitude opposite to any given meridian used as a point of reference.

123° west is the antimeridian of the 57° east meridian.

The antimeridian contains your nadir and the opposite celestial pole for your hemisphere. The local meridian and the antimeridian together form the great circle simply called the meridian.