In the study of the Earth’s magnetic field, the term isogon or isogonic line refers to a line that connect the points of the earth that have equal magnetic declination, the variation of magnetic north from geographic north.
In other words, it is a line connecting all points on the Earth’s surface in which there is the same value and sign of the magnetic declination angle at a certain time. It is also called isogonic line. The isogon corresponding to zero declination is called agonic line.
There are planispheres and special charts in which these lines are drawn, each marked by the value of the magnetic declination of the places touched by it: these cards are valid for a given period (usually expressed in years and tenths of a year), but can easily serve, taking into account the annual variation of declination, even for a certain number of years around the assigned period.
The first chart of isogons was built by E. Halley in 1701 probably using, in addition to his own observations, those collected by A. Kircher (Magnes, Rome 1641) from observations made in the ‘600 by several Italian Jesuits in the Indies and East Asia. The English Admiralty and the Hydrographic Office of the United States currently publish, at intervals of a few years, the planisphere with isogons, taking into account the latest measurements made: the individual nations build and keep up to date their own special maps.