Prime Meridian Line

A prime meridian is a meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. On a spheroid, a prime meridian and its anti-meridian (the 180th meridian in a 360-degree system) form a great ellipse. This divides the body (e.g. Earth) into two hemispheres: the Eastern Hemisphere and the Western Hemisphere (for an east-west notational system). 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 (unlike the prime meridian of Mars) 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. Longitudes for the Earth and Moon are measured from their prime meridian (at 0°) to 180° east and west. For all other Solar System bodies, longitude is measured from 0° (their prime meridian) to 360°. West longitudes are used if the rotation of the body is prograde (or 'direct', like Earth), meaning that its direction of rotation is the same as that of its orbit. East longitudes are used if the rotation is retrograde.