Meridian (astronomy)

An (astronomical) meridian is the great circle passing through the celestial poles, the zenith, and the nadir of a particular location. Consequently, it contains also the horizon's north and south points, and it is perpendicular to the celestial equator and to the celestial horizon. A celestial meridian matches the projection, onto the celestial sphere, of a terrestrial meridian. Hence there are an infinite number of meridians.