Museo Galileo
Virtual Museum
Magnetic declination

The vertical plane intersecting the longitudinal axis of a magnetic needle free to rotate horizontally indicates the plane of the magnetic meridian of the locality. Since the magnetic poles do not coincide with the geographic poles, the plane of the magnetic meridian and that of the geographic (or astronomical) meridian form an angle, usually different from zero, called the magnetic declination. The value of the declination can range from 0° to 180°, and can be east, or positive, if the north tip of the magnetic needle deviates, or declines, eastward from the geographic meridian; west, or negative, if the needle deviates westward. The points of equal declination are joined by lines called isogonous, all of which necessarily intersect both geographic poles or both magnetic poles. The east-declination regions of the globe are separated the west-declination ones by an isogonous line indicating zero declination. This line, called the agonic line, comprises the points in which the magnetic meridian and geographic meridian coincide. In any given point of the Earth's surface, the declination is subject to secular, annual, and diurnal variations. Our regions display a west magnetic declination of a few degrees. In the course of a single day, it reaches a low toward 8 A.M. and a high toward 1-2 P.M.; between 10 and 11 A.M. and around sunset, it reaches its middle value. Already known to the Chinese since c. 1100, magnetic declination remained unknown in the West until 1492, when it was discovered by Christopher Colombus (1451-1506) during his Atlantic voyages. The declination was first calculated by George Hartmann (1489-1564), who, in 1510, measured an east declination of 6° in Rome.