THE secret of the magnetic compass lies in the fact that the earth behaves as if it had a giant magnet buried deep inside. The imaginary magnet is arranged with its "south" pole somewhere below the northern coast of Canada, and its "north" pole below a spot on the other side of the world, on the rim of the Antarctic ice pack. This apparent inversion of the conventional magnetic wisdom makes sense when you recall that "opposite" poles attract; the "north" end of the compass points north because it is drawn by the "south" end of the notional subterranean attractor.
But as we know, a compass needle rarely points exactly north. Both magnetic poles at present lie about 15 degrees of latitude from their respective geographic counterparts and as a result there is nearly always a slight difference between what is called tribe (or geographic) north and magnetic north (the direction in which a compass needle points). The angle between the two is called by scientists the "declination", and by mariners the "compass variation".
The magnetic declination is not constant; it differs from place to place and at any particular spot it changes gradually over the years. At Valentia Observatory in Co Kerry, for example, the variation has steadily decreased from 21 degrees in 1900 to about seven degrees today, and it decreases by about two degrees as one moves east-wards across the country from Galway to Dublin.
Lines drawn on a map connecting places with the same magnetic declination are called isogonics, and they run crookedly in a more or less north-south direction. Of particular interest is the isogonic line of zero declination, connecting those places in the world where a compass reads exactly right; it almost bisects both North arid South America, running from Hudson Bay through the Great Lakes, cutting the island of Cuba into two, before heading down through western Brazil to Buenos Aires. At points to the east of this line the declination is to the west of geographic north, while all points to the west share an easterly variation.
A compass needle remains horizontal only because it is constrained to do so by its mounting. A magnetic needle fitted with a horizontal axis, however, and therefore free to move in the vertical plane, adopts its own orientation; in general, in the northern hemisphere it comes to rest with its north pole pointing downwards, the angle it makes with horizontal at any place being called the dip or inclination. The magnetic dip is currently about 66 degrees in the vicinity of Ireland.