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A meridian is the great circle passing through the celestial poles and the zenith of a particular location. Consequently, it also contains the horizon's north and south points and it is perpendicular to the celestial equator and the celestial horizon. This celestial meridian matches the projection, onto the celestial sphere, of the terrestrial meridian. The term "meridian" comes from the Latin meridies, which means both "midday" and "south". The great circle defining the meridian is divided into the local meridian (containing the zenith and terminated by the celestial poles) and the antimeridian (opposite half containing the nadir). In the horizontal coordinate system a similar division is made between the halves terminated by the horizon's north and south points: the upper meridian (through the zenith) and the lower meridian (through the nadir). Because the meridian is fixed to the local horizon, a celestial object will appear to drift past the local meridian as the Earth spins. It 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). |
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