A great circle on the celestial sphere passing through the celestial poles and an observer's zenith. An observer's celestial meridian is not a fixed reference on the celestial sphere but rather changes with the observer's location on Earth. Stars transit an observer's celestial meridian (that is, cross directly overhead in the sky at that location) once every 24 hours. Also called local meridian, meridian.
In the equatorial coordinate system, a great circle on the celestial sphere passing through the celestial poles and the vernal equinox. It represents the zero point for the horizontal coordinate in this system, having a right ascension of 0 hours.