Meridian (geography), name given to an imaginary line on the earth's surface, stretching from pole to pole. A meridian is one-half of a great circle that passes through the poles and forms a right angle with the equator. It is midday at any place on the surface of the earth when the sun's direct rays pass over the meridian of that particular place. Stars and planets appear to be directly overhead when they are observed passing over the meridian. A meridian is also known as a line of longitude (see Latitude and Longitude).