meridian

A prime meridian is the meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system) form a great circle. This great circle divides a spheroid into two hemispheres. If one uses directions of East and West from a defined prime meridian, then they can be called the Eastern Hemisphere and the Western Hemisphere.

A prime meridian for a body not tidally locked (or at least not synchronous) is ultimately arbitrary, unlike an equator, which is determined by the axis of rotation. For celestial objects that are tidally locked (more specifically, synchronous), however, their prime meridians are determined by the face always inward of the orbit (a planet facing its star, or a moon facing its planet), just as equators are determined by rotation. For Earth's prime meridian, various conventions have been used or advocated in different regions throughout history. The most widely used modern meridian is the IERS Reference Meridian. It is derived but deviates slightly from the Greenwich Meridian, which was selected as an international standard in 1884.
Longitudes for the Earth and Moon are measured from their prime meridian at 0° to 180° east and to 180° 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 direct, that is, it follows the right hand rule. East longitudes are used if the rotation is retrograde. However, °E that are greater than 180 can be converted to °W by subtracting the value from 360. The same is true of °W greater than 180, converting to °E.

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