This article is about the astronomical concept. For other uses of the word, see Meridian.
In the sky, a meridian is an imaginary great circle on the celestial sphere. It passes through the north point on the horizon, through the celestial pole, up to the zenith, through the south point on the horizon, and through the nadir, and is perpendicular to the local horizon. The term "meridian" comes from the Latin meridies, which means both "midday" and "south".
Because it is fixed to the local horizon, stars will appear to drift past the local meridian as the earth spins. You can use an object's right ascension and the local sidereal time to determine when it will cross your local meridian, or culminate (see hour angle).
The upper meridian is the half above the horizon, the lower meridian is the half below it.