In astronomy, the meridian is the great circle passing through the celestial poles, the zenith, and the nadir of an observer's 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 of a terrestrial meridian onto the celestial sphere. Hence, the number of astronomical meridians are infinite.
The celestial meridian is undefined when the observer is at the North Pole or South Pole, since at these two points, the zenith and nadir are on the celestial poles, and any great circle passing the celestial poles passes the zenith and nadir.
There are several ways in which the meridian can be divided into semicircles. In one way, it is divided into the local meridian and the antimeridian. The former semicircle contains the zenith and is terminated by the celestial poles; the latter semicircle contains the nadir. 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. At culmination, the object reaches its highest point in the sky when crossing, or transiting, the meridian. 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".
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