Deep inside the center of the earth, some 1800 miles down beneath the surface, is a 1500 mi. thick layer of liquid molten iron and nickel, which eddies and flows turbulently between the mantle and the solid central core. The flow of this material creates electrical currents which generate a magnetic field, which shields the planet from the solar wind, and without which the atmosphere would likely be rapidly stripped away, leaving the earth as barren and lifeless as Mars, the Moon, or parts of North Dakota. Coincidentally, that electrical field, having due to inescapable laws of physics a magnetic polarity, creates a field detectable upon the surface, that, while wandering slowly, is fairly constant over time (at least until periodic - in a geological timeframe - polarity reversals), and helpfully, that polarity is centered near enough on the actual rotational poles of the planet as to be directionally useful. It has been constant enough for several pivotal centuries to allow the brighter among humankind to note first its existence, its direction, and finally to deduce a fairly simple yet robust means for finding our way, by using a small piece of metal to align with the waves produced, point generally north, and thus develop the magnetic compass, useful for wayfinding since about the 11th century A.D. For reference, this was about the time a French-speaking lout named William from Normandy, no cheese-eating surrender monkey he, sailed to the south of Britanica, bringing with him horse cavalry unpossessed by the luckless army of King Harold, and thereby subjugated the Saxons of the kingdom, giving rise to modern England.
Neither William nor Harold possessed the magnetic compass, but the travels of Marco Polo to China retrieved the concept, the Renaissance spread it from Arabia to the Mediterranean and thence to all of Europe, soon remedying that lack even in far-off England, and the English soon sailed everywhere with one, eventually conquering and subjugating a great portion of the known world, until with their usual obtuseness and pedantry, they mismanaged most of their colonies in North America, leading inevitably to the colossally overweaning leviathan behemoth Deep State which we all loathe and despise in the present day.
So having covered the past up until now, let's talk about what the phenomenon of magnetism, and the invention of the magnetic compass means for you.
The slowly-wandering current-producing field of molten outer core I told you about creates a magnetic north pole. Its wanderings are intensively tracked and measured, getting more precise over time as both scientific curiosity, laboratory instrumentation, and the dependence of commerce and the vast wealth involved have cooperatively required such a keen interest in the otherwise unremarkable results of bubbling molten metal hundreds of miles beneath the frozen tundra and arctic seas of Canada.
Those measurements, coupled with the space-age ability to more precisely locate and compare the location of the magnetic north pole to the actual planetary North Pole have led to depictions of the variation to prevent someone from, for instance, crashing their ship into rocks, losing the cargo, and dumping millions of gallons of crude oil onto someone else's beach.
This rather important difference between magnetic north and actual north is called Magnetic Declination.
|2015 map of magnetic declination worldwide.|
This is all important to you, because it means your magnetic compass doesn't point at north on the map, and it doesn't point to the North Pole, ever, except occasionally along a very thin strip of the earth's surface, which also moves year by year.
|Movement from 1590-1990 AD|
So somewhere on your map, if it's not a touristy p.o.s. map, and definitely on both USGS and US/UK et al military maps, you'll find something in the margin like this:
Which shows you the difference between magnetic north (MN -where your compass is pointing), grid north (GN - which way the map is presenting the surface of a 3D sphere in a 2D representation), and true north (★-the actual direction in which the actual North Pole is rotating about dead-on beneath Polaris, the astronomically named "North Star").
I told you about that so we can get to the Compass next.