Declination

Definition

The angular difference, in the horizontal plane, between true north and magnetic north at a particular place.

Discussion

In his book, The American Practical Navigator, Nathanial Bowditch defined variation as “the angle between the magnetic and geographic meridians at any place, expressed in degrees and minutes east or west to indicate the direction of magnetic north from true north." In more practical terms, magnetic variation, more often referred to as declination in non-aviation / non-nautical contexts, is the angle on the horizontal plane between magnetic north and true north. This angle varies with position on the Earth's surface and it changes over time, at varying rates in different locations. If a compass at your location is pointing to the right of true north, declination is positive or east, and if it points to the left of true north, declination is negative or west.