Magnetic dip, dip angle, or magnetic inclination is the angle made with the horizontal by the compass needle of a vertically held compass. This angle varies at different points on the Earth's surface. Positive values of inclination indicate that the magnetic field of the Earth is pointing downward, into the Earth, at the point of measurement. The value can be measured with an instrument typically known as a dip circle.
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). Contour lines along which the dip measured at the Earth's surface is equal are referred to as isoclinic lines. The locus of the points having zero dip is called the magnetic equator or aclinic line. The range of dip is from −90 degrees to 90 degrees.
The phenomenon is especially important in aviation, as it causes the airplane's compass to give erroneous readings during banked turns and airspeed changes.