The celestial meridian in astronomy is the half-circle that connects the zenith with the astronomical north and south poles, explains Swinburne University. It is a part of the celestial sphere.
The celestial meridian changes by the position of the observer. The zenith is the point directly above the observer, expounds University of Tennessee’s Department of Physics and Astronomy. The north and south are on the horizon of the observer. The celestial meridian runs as a line through all three of these points. It helps guide observers to constellations and planets. The celestial sphere contains all of the night sky that an observer sees at once, and the observer only sees half of the sky.