A prime meridian is a meridian in a geographic coordinate system at which longitude is defined to be 0°. On a spheroid, a prime meridian and its anti-meridian form a great ellipse. This divides the body into two hemispheres: the Eastern Hemisphere and the Western Hemisphere. Unlike the equator, which also divides a spherical celestial body into two hemispheres, the prime meridian is astronomically arbitrary. For each of Earth's historic prime meridians, various conventions have been used or advocated in different regions throughout history, but none have a basis in physical geography. Earth's current international standard prime meridian is the IERS Reference Meridian. It is derived, but differs slightly, from the Greenwich Meridian, the previous standard.