prime meridian

/praɪm məˌrɪdiən/

Other forms: prime meridians

The prime meridian is the planet’s line of zero degrees longitude. Slicing the earth along the circle of the prime meridian would divide it into the Eastern and Western hemispheres — but it’s probably better to leave it in tact.

Sometimes called the Greenwich Meridian or the International Meridian, Earth’s prime meridian crosses the original site of the Royal Observatory in Greenwich, England. It is the circular line by which other longitudinal lines, or meridians, are referenced — otherwise it wouldn’t be prime. You can still have a prime meridian without Greenwich, England, though; you just need to be on the moon, Jupiter, or some other celestial body.

Definitions of prime meridian

1. noun

   meridian at zero degree longitude from which east and west are reckoned (usually the Greenwich longitude in England)
   examples:

   Greenwich Meridian

   the meridian passing through Greenwich; was internationally adopted as the earth's zero of longitude in 1884

   type of:

   line of longitude, meridian

   an imaginary great circle on the surface of the earth passing through the north and south poles at right angles to the equator