he variations between magnetic north and "true" or geographic north have been understood for several hundred years. As early as 1585, a Portuguese map showed "isogonic lines" or lines indicating the declination or difference between north as it appeared on a compass and north as a direction of the geographic grid system.
The English scientist, Edmund Halley (for whom the comet was named), drew a map of the world in 1702, showing isogonic lines for the entire planet and enabling explorers everywhere to correct their compasses for the difference between magnetic and true north.
The great English explorer, Captain James Cook, was the first to use compass declination widely in his mapping of the islands and coasts of the Pacific Ocean. Magnetic poles shift over time and magnetic north in 1804-1806 was different from today.
--John Logan Allen