A 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.
The meridian is divided several ways into semicircles. In one way, there are the local meridian and the antimeridian. The former semicircle contains the zenith and is terminated by the celestial poles; the later semicircle has the nadir. This implies that an observer has no local meridian or antimeridian when the zenith is on a celestial pole. In the horizontal coordinate system the meridian is divided into halves terminated by the horizon's north and south points. The upper meridian passes through the zenith, and the lower meridian, through the nadir.
A celestial object will appear to drift past the local meridian as the Earth spins, for the meridian is fixed to the local horizon. The object reaches its highest point in the sky when crossing the meridian (culmination). Using an object's right ascension and the local sidereal time it is possible to determine the time of its culmination. (See hour angle).
The term "meridian" comes from the Latin meridies, which means both "midday" and "south".
- Millar, William (2006). The Amateur Astronomer's Introduction to the Celestial Sphere. Cambridge University Press.
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