We are used to having north at the top of our maps. This has been the most common orientation for hundreds of years, largely because of the use of the magnetic compass. Compasses do not, however, point exactly north. The northern magnetic pole wanders around the Canadian Arctic, and anyone requiring precise direction for navigational purposes needs to keep this in mind. It is common for maps to have a diagram showing the difference between magnetic and true north, as in this sea chart from 1870 (which also includes a date for the declination and, elsewhere on the chart, the current rate of change).
The discovery that the earth’s magnetic field fluctuates, and does not line up with its geographical axis, is nothing new. European navigators were aware of this issue from the fifteenth century. Edmond Halley had begun charted the magnetic declination across much of the world at the end of the seventeenth century, and this map by John Senex from 1725, based on his work, shows the “Line of no variation in the year 1700” curving sinuously across the Atlantic. Lines of equal declination – isogonic lines – are marked around it.
This line where magnetic and true north coincide – properly called the agonic – is also in constant motion and we recently heard the exciting news that it is about to reach the Royal Observatory at Greenwich, so compasses there will point to true north for the first time in 360 years. More information can be found here on the website of the British Geological Survey.