<P> 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 horizon . A celestial meridian is coplanar with the analogous terrestrial meridian projected onto the celestial sphere . Hence, the number of astronomical meridians is infinite . </P> <P> 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 through the celestial poles also passes through the zenith and nadir . </P>

How to find the meridian in the sky