Antimeridian

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.

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.

Other prime meridians were presented as candidates for the world's baseline: Jersualem; Ferro; Kamchatka; the mid-Atlantic; the Bering Strait; and Greenwich's antimeridian.