1911 Encyclopædia Britannica/Meridian

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MERIDIAN (from the Lat. meridianus, pertaining to the south or mid-day), in general a direction toward the south or toward the position of the sun at mid-day. The terrestrial meridian of a place is the great circle drawn on the earth’s surface from either pole through the place. As determined astronomically the celestial meridian is the great circle passing through the celestial pole and the zenith. The terrestrial meridian as practically determined is the circle on the earth’s surface in which the plane of the celestial meridian cuts that surface. Owing to local deviations of the plumb-line the meridian thus determined does not strictly coincide with the terrestrial meridian as ordinarily defined, but the deviation, though perceptible in mountainous regions, is so minute that it is generally ignored.