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The celestial meridian is the meridian (north-south great circle around the celestial sphere, i.e., that passes through the celestial poles) that passes through the zenith. Like the zenith, its position on the celestial sphere varies by place and time. The term is often shortened to meridian, which must be interpreted in context. That portion of the celestial meridian above the observer's horizon is termed the upper meridian and the rest (below the horizon) is termed the lower meridian.