Prime Meridian

The prime meridian is an imaginary half-circle reference line extending north and south from one pole to the other and passing through Greenwich, England. It marks 0° longitude and divides the western and eastern hemispheres of the earth. The line that meets it at both poles and is exactly 180 degrees away from the prime meridian is the international date line. The prime meridian is the standard basis for determining time throughout the world. It is the starting point for all the world's time zones. This meridian is the location of UTC (universal time coordinated) as well as Greenwich mean time (GMT). Noon GMT is defined as the time at which the sun crosses the Greenwich (prime) meridian.

An international agreement in 1884 established the prime meridian in its current location. This line runs through the transit circle telescope, built in 1850 by Sir George Biddell Airy, the seventh astronomer royal, at the Royal Observatory Meridian Building in Greenwich, England. The crosshairs in the eyepiece of the transit circle telescope precisely define 0° longitude for the world. It is located at 51° 28' 38' north latitude. As the earth's crust is moving very slightly all the time, this exact position moves very slightly.

Longitude is a measure of both time and location on the earth. As the earth spins on its axis, a specific location and time can be determined relative to the prime meridian. This north-south line marks noon of the day that begins at the international date line and is the origin from which east or west longitude is measured. One degree of longitude equals 4 minutes of time the world over, but the distance of 1° longitude varies depending on the latitude of the location. Every 15° east measured from the prime meridian marks another hour later; every 15° to the west marks another hour earlier.