- 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
- 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.