In the days of the old sailing vessels, it was firmly believed that both garlic and onions affected the accuracy of the magnetic compass. For this reason, neither was allowed aboard a ship, and the mariners of old were deprived of the benefit of either form of seasoning. But the reason the compass does not always point directly north is more elusive.
The magnetic north pole, unlike its geographical counterpart, does not remain fixed in one spot, but drifts gradually with the passing years. These days it lies close to the northern coast of Canada, but we have reason to believe that centuries ago it may have been in the Pacific Ocean near Guam.
With the passage of time it drifted to the mainland of Asia, passing near the present city of Shanghai, and by AD1300 it seems to have been somewhere near North Korea. It then followed a long, slow, circular path across Siberia, passing under Iceland on its way to Canada.
A consequence of this meandering of the pole is that the magnetic declination, the angle between "true" north and "magnetic" north, is continually changing. Since 1800 or thereabouts, the westerly declination in these parts has gradually decreased from about 24 degrees to its present value of about seven, and it decreases by about two degrees as one moves eastwards across Ireland from Galway to Dublin.
Columbus was somewhat taken aback by this phenomenon of declination on his first voyage towards the New World, but he wisely kept his worries to himself; he was afraid that if his crew became aware of this strange occurrence they might become terrified and insist on turning back.
But once the concept became familiar, later mariners began to notice that the declination seemed to change more or less uniformly as one moved from east to west or vice versa. Moreover, it could be easily measured by taking a bearing on the pole star and comparing this to that given by the compass.
So it struck them that here was a convenient way of establishing one's longitude: if you knew the rate at which the declination changed as you moved westwards, all you had to do was compare the declination at your place of observation with that at places whose longitude had already been determined.
But a wider experience taught them that it was not as simple as it seemed. The lines of equal declination do not always run north-south, and even where they do, declination could not be measured accurately enough at sea to make the method a viable one for estimating longitude.