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(circle around the Earth passing through both poles)

A meridian is a circle passing around the Earth, through both poles. It is an example of a great circle, a circle passing fully around the Earth as the equator does. At any given point on Earth excluding the poles, only one meridian passes through, termed its local meridian. The prime meridian (aka Greenwich meridian), passing through Greenwich, England, is used as a reference for maps, Earth coordinates, and standardized time.

The term meridian is also be used for defining positions on other spherical bodies, such as stars, planets or moons, or their surroundings. It is also be used for corresponding lines around the celestial sphere and the term celestial meridian (often shortened to the meridian) refers to such a meridian that joins the zenith with the celestial poles, i.e., the celestial sphere's meridian that corresponds at that point in time to the local meridian. The portion of the celestial meridian above observer's the horizon is termed the upper meridian and the rest (below the horizon) is termed the lower meridian.

Further reading:

Referenced by pages:
celestial meridian
ecliptic coordinate system
equatorial coordinate system
gravitational potential model
meridian circle
spherical harmonics
time standard
transit telescope