What imaginary line splits the earth into east and west hemispheres?

The Prime Meridian is the meridian (line of longitude) at which longitude is defined to be 0°. The Prime Meridian and the opposite 180th meridian (at 180° longitude), which the generally follows, form a International date line great circle that divides the Earth into the Eastern and Western Hemispheres.

Unlike the parallels of latitude, which are defined by the rotational axis of the Earth (the poles being 90° and the Equator 0°), the Prime Meridian is arbitrary. By international convention, the modern Prime Meridian is one passing through Greenwich, London, United Kingdom, known as the International Meridian or Greenwich Meridian. Historically, various meridians have been used, including four different ones through Greenwich.