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magnetic meridian[mag′ned·ik mə′rid·ē·ən]
the projection of a line of force of the geomagnetic field on the earth’s surface. All the magnetic meridians are complicated curves that converge at the north and south magnetic poles of the earth. The vertical plane that passes through the point at which the observer (instrument) is situated and that contains the vector of the geomagnetic field intensity at this point is called the plane of the magnetic meridian. The angle between the plane of the magnetic meridian (the needle of a magnetic compass will lie in this plane) and the plane of the geographical meridian at this point of the earth’s surface is called the magnetic declination.
In addition to the earth’s magnetic meridian we often consider the geomagnetic meridian, which is the line of intersection between the surface of the earth and the plane that passes through both a given point of the earth’s surface and the straight line connecting the north and south geomagnetic poles. The geomagnetic meridians coincide with arcs of great circles passing through the magnetic poles. Unlike the magnetic meridians, which describe the actual magnetic field of the earth, the geo-magnetic meridians describe the first approximation of the earth’s magnetic field, that is, the field of a homogeneously magnetized terrestrial sphere.