# Definition:Meridian (Terrestrial)

## Definition

A (terrestrial) meridian is a semi-great circle on Earth's surface whose endpoints are Earth's poles.

### Principal Meridian

The principal meridian is the meridian of Earth which passes through the Royal Observatory in Greenwich, Greater London, England, which is used as the internationally-recognised $0 \degrees$ reference longitude meridian.

## Also defined as

Some sources recognise a (terrestrial) meridian to be a complete great circle passing through both of Earth's poles.

However, this definition is less convenient.

## Also known as

In common parlance, a terrestrial meridian is usually referred to as just a meridian.

The terrestrial descriptor is added so as to distinguish it from a celestial meridian.