prime meridian

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

n 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

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