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.

 

Central meridian

The central meridian is an imaginary line bisecting the apparent disk of a planet or the Sun and joining the poles of rotation. The passage of a surface feature across the central meridian, due to the axial rotation of a planet, is known as a central meridian transit. The timing of central meridian transits is an important means of determining the planetographic longitudes of planetary surface features.

 

Prime meridian

The prime meridian is the meridian adopted as the zero of longitude measurement on the surface of a planet. In the case of Earth, the prime meridian adopted by international agreement in 1894 is that of the Airy Transit Circle at the Old Royal Greenwich Observatory. The prime meridian on Mars is defined to be the line of longitude that passes through a 0.5 kilometer-wide crater known as Airy-0 (named after George Airy who built the transit at Greenwhich).