Answer: At
most places on the Earth's surface, the compass doesn’t point exactly
toward geographic north. The deviation of the compass from true north
is an angle called 'declination'. It is a quantity that has been a nuisance
to navigators for centuries, especially since it varies with geographic
location. It might surprise you to know that at very high latitudes the
compass can even point south! Declination is simply a manifestation of
the complexity of the geomagnetic field. The field is not perfectly symmetrical,
it has non-dipolar ‘ingredients’, and the dipole itself is
not perfectly aligned with the rotational axis of the Earth. Interestingly,
if you were to stand at the north geomagnetic pole, your compass, held
horizontally as usual, would not have a preference to point in any particular
direction, and the same would be true if you were standing at the south
geomagnetic pole. Moreover, if you were to hold your compass on its side
the north-pointing end of the compass would point down at the north geomagnetic
pole, and it would point up at the south geomagnetic pole. Maps of declination,
such as that shown above (contours of 10 degrees east), as well as other
field components, and a program for determining the magnetic field at
any geographic location, are given in the Models,
Charts, and Movies
pages of Geomagnetism Program website.
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