Vertical circle

In astronomy, a vertical circle is a great circle on the celestial sphere that is perpendicular to the horizon. Therefore it passes through the zenith and the nadir. There is a vertical circle for any given azimuth, where azimuth is the angle measured east from the north on the celestial horizon. The vertical circle which is on the east-west direction is called the prime vertical. The vertical circle which is on the north-south direction is called the Local Celestial Meridian (LCM) or Principal Vertical. Vertical circles are part of the horizontal coordinate system.