Meridian (astronomy)

In astronomy, the meridian is the great circle passing through the celestial poles, as well as the zenith and nadir of an observer's location.

Meridians, celestial and geographical, are determined by the pencil of planes passing through the Earth's rotation axis.

In another approach known as the horizontal coordinate system, the meridian is divided into the local meridian, the semicircle that contains the observer's zenith and the north and south points of their horizon,[1][2] and the opposite semicircle, which contains the nadir and the north and south points of their horizon.

At culmination, the object contacts the upper meridian and reaches its highest point in the sky.

The term meridian comes from the Latin meridies, which means both "midday" and "south", as the celestial equator appears to tilt southward from the Northern Hemisphere.

The meridian on the celestial sphere. An observer's upper meridian , a semicircle contains their zenith and both celestial poles; the observer's local meridian is the semicircle that passes through their zenith and the north and south points of their horizon.