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A (terrestrial) meridian is a semi-great circle on Earth's surface whose endpoints are Earth's poles.
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.
- 1933: D.M.Y. Sommerville: Analytical Conics (3rd ed.) ... (previous) ... (next): Chapter $\text I$. Coordinates: $2$. Coordinates
- 1976: W.M. Smart: Textbook on Spherical Astronomy (6th ed.) ... (previous) ... (next): Chapter $\text I$. Spherical Trigonometry: $4$. Terrestrial latitude and longitude.
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): meridian: 1.
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): meridian