The disagreement between the current value and that in Griffiths
is due to the fact that magnetic north pole (which is actually a
south magnetic pole, of course) has been drifting
approximately northwest at about 40 km per year for the last few years
(a blistering pace on a geological time scale);
it has always wandered around like this, and has reversed direction
more than once! Sailors (and students in Power Squadron courses)
must learn how to correct their compass readings for this gradual drift.
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