or magnetic inclination
is the angle made by a compass needle with the horizontal at any point on the Earth's surface. It was first described by Robert Norman
Magnetic dip results from the tendency of a magnet to align itself with lines of force. As the Earth's magnetic lines of force are not parallel to the surface, the north end of a compass needle will point downward on the northern hemisphere (positive dip) or upward on the southern hemisphere (negative dip). The lines joining parts of the Earth's surface with equal dip are called isoclinic lines
The phenomenon is especially important in aviation, as it causes the airplane's compass
to give erroneous readings during banked turns and airspeed changes.