prime meridian

prime meridian meridian that is designated zero degree (0°) longitude, from which all other longitudes are measured. By international convention, it passes through the original site of the Royal Observatory in Greenwich, England; for this reason, it is sometimes called the Greenwich meridian. Universal time , the standard basis for determining time throughout the world, is civil time measured at the prime meridian.

prime meridian A fixed circle of longitude on the surface of a body, used as a reference from which other longitudes can be measured west or east. The prime meridian on Earth passes through the transit circle established by G. B.Airy at Greenwich, and defines the zero of terrestrial longitude. On other bodies the prime meridian is defined with respect to a known surface feature, such as a small crater.

meridian, from Latin medius meaning middle, and dies meaning day, a semi-great circle joining the earth's poles. Meridians, better known as lines of longitude, cross the equator and all parallels of latitude at right angles. Owing to the rotation of the earth, all celestial bodies appear to revolve around the earth towards the west, making one revolution in a day. When the sun crosses an observer's meridian, the local time is midday.

See also prime meridian.