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the straight line along which the celestial meridian plane intersects the plane of the celestial horizon. The meridian line intersects the celestial sphere at the north and south points. The line in the horizontal plane perpendicular to the meridian line determines the east and west points on the celestial sphere. The sun reaches its greatest height above the horizon almost exactly at apparent noon, at. which point the center of its disk intersects the meridian, although the greatest height may be reached somewhat earlier or later than noon owing to variations in the declination of the sun. At this time, shadows cast by objects on the earth point along the meridian line and are at their shortest. This makes it possible to determine approximately the moment at which noon begins and the cardinal points—north, south, east, and west.