At some point over the next two weeks, compasses at Greenwich will point true north for the first time in about 360 years.
And for some parts of the UK, this may not happen for another 20 years. Either way, it is a once-in-a-lifetime event.
The angle a compass needle makes between true north and magnetic north is called declination. As the magnetic field changes all the time, so does declination at any given location.
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