Note:  Do not rely on this information. It is very old.

Meridian

Meridian, in astronomy, is the great circle in which any plane containing the earth's axis cuts the celestial sphere. The meridian of any place passes through the zenith of that place, and cuts the horizon in the meridian line. This line gives the direction of north or south. When the sun crosses the meridian of any place, it is noon there. Stars are higher in the heavens when they reach the meridian than at other parts of their path, and are more accurately observed then, since the distorting effect of refraction is in that position as little as possible. These observations are made with meridian circles.