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Related to isogonic lines: aclinic line
the isolines of the orientation of any physical variable.
In meteorology, isogonic lines are lines of identical wind direction. In terrestrial magnetism, isogonic lines are lines where the values of magnetic declination are equal. On magnetic maps, isogonic lines in the middle latitudes move in an approximately meridional direction, with the exception of eastern Asia, where they are closed. In higher latitudes isogonic lines converge toward the geographic and magnetic poles. Astronomy deals with the isogonic lines of a solar eclipse—the lines passing through the points on the earth’s surface from which the lines from the center of the sun to the pointsat which the darkened lunar disc touches the bright solar disc at the beginning and end of a partial eclipse have the same position angle. Isogonic lines are calculated in advance and used in setting up observations of solar eclipses.