The simplest magnetic coordinate system is based on the geomagnetic dipole. This is a spherical
polar coordinate system with coordinates of radius, latitude and longitude, just like the familiar
geographic coordinate system. However, the geomagnetic coordinate system differs from the geographic
one because its polar axis is aligned along the magnetic dipole and not the rotation axis.
Thus we can convert any geographic coordinate into a geomagnetic coordinate by means of a simple
rotational transformation. Zero geomagnetic longitude is defined by the great
circle path that passes through the northern geomagnetic and geographic poles.
Hence Halley is at 75.5 ^{o}S, 26.6 ^{o}W geographic, but at 65.8 ^{o}S, 24.3 ^{o}E
geomagnetic. This coordinate system is fine for rough work but there are other, more accurate, coordinate systems:

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