The Earth is like a giant magnet, surrounded by a magnetic field. This magnetic field, which is a vector with both direction and intensity, is generated by a dynamo process in the fluid outer core of the Earth. Due to the chaotic movement of the core fluid, the Earth's magnetic field gradually changes over the years. The first animation (sample image on left) shows the horizontal direction of the magnetic field lines at the surface of the Earth. The magnetic North and South poles are shown as blue and red stars, respectively (note the change in location of the magnetic poles and the change in the speed of movement). Where the lines are blue, the magnetic field dips into the Earth, where they are red it emerges from the Earth. The transition from red to blue, where the field lines are horizontal, is called the magnetic equator.

When using a magnetic compass for navigation, the compass needle points in the direction of the lines displayed on the left. Obviously, this direction is not equivalent with True North. The compass pointing direction can also differ substantially from the direction to the Magnetic North Pole, since magnetic field lines are not just great circles connecting the magnetic poles. The second animation (middle in sample image) illustrates the orientation of compass needles distributed over the surface of the Earth. True North is indicated by the direction of the black lines.

The angle between the pointing direction of a compass needle and True North is called magnetic declination, or magnetic variation, and is measured in degrees east (positive) or west (negative) of True North. Shown in the third animation (right in sample image) are the lines of equal declination (isogonic lines). On the black, agonic line (declination = 0) True North and Magnetic North are identical. In areas of red lines (positive declination) the compass points East of True North, and in areas of blue lines it points West of True North. Line spacing is 5 degrees. The magnetic North and South poles are indicated by black stars. Erratic looking features at the geographic poles are due to a mathematical complication arising from the definition of declination.

In these animations, the magnetic field from 1590 to 1980 is given by the GUFM-1 model of Jackson et al. (2000), while the field from 1980 to 2010 is given by the 10th generation of the International Geomagnetic Reference Field.

Further infos and related animations:
Historical main field change and declination at geomag.org
NGDC/WDC answers to Geomagnetic Frequently Asked Questions
NGDC World Magnetic Anomaly Map
Earth's magnetism on NOAA's Science on a Sphere