A meridian is a great circle on the celestial sphere that passes through the celestial poles and the observer's zenith. The meridian angle of a celestial object is the angle measured eastward along the celestial equator from an observer's meridian to that of the object. The meridian altitude of an object is its altitude when it crosses the observer's meridian. Meridian transit, also known as meridian passage or culmination, is the moment when a celestial object crosses, or transits, an observer's meridian. A meridian observation is an observation of the object's declination (and, occasionally, brightness) at this time and is made by a transit circle.
A meridian is also a line of longitude on Earth, or on some other astronomical body. On Earth, the meridian that passes through Greenwich, London, is known as the prime meridian.