Thursday, August 14, 2008

Map Confusion, Why is North Up?, Magnetic vs True North, Polar Motion
On maps, why is North always up? Apparently, it's been this way only since the middle ages when Ptolomey's maps were rediscovered. Before that, it was a free-for-all.

Even the word "orientation" refers to the fact that the "orient" i.e. the "East" was placed in the UP position- North thereby being moved to where West is today. According to baby-naming websites scoured prior to my son's birth- the name "Benjamin" means "son of the right hand" ... and is also translated as "Son of the South". This makes sense if the map has East at the top, or if you are facing the "East" towards Jerusalem- presumably you'd have to be West of the "Middle East" for that to be true!

Even more confusing is the fact that Magnetic North keeps moving , flipping around like a fish in a bucket. It's on it's way from Canada to Siberia at 41km/year as we speak. Actually, it may jiggle up to 85km in a single day and apparently is rarely found where it is supposed to be! average path on disturbed days

Compasses - despite popular belief- do not point North, or even to Magnetic North! Compasses point along LOCAL magnetic field lines- these are wobbly, and depend on moving flows of iron and nickel deep within the earth. The difference between where your compass is pointing and where true (geographic) north is, is known as Magnetic Declination. Off the coast of Madagascar, this can be up to 40 degrees!

Image:IGRF 2000 magnetic declination.gif
In any case, the north pole of any magnet (including that in a compass) is attracted to another south pole- hence the North Magnetic Pole should really be renamed "The Earth's South Magnetic Pole Which is Currently Located Vaguely Around Somewhere Near True North".

Even True North- the geographic North Pole is moving ("Polar Motion") - albeit very slowly, and very slightly. It takes a cycle of about 400 years, but the point the Earth is twisting around moves by up to 20m. (see chart below). I guess we are tumbling rather than twisting our way around the sun. Fascinatingly, some of this is due to melting of the Greenland ice sheets and the subsequent rebound of the unburdened earth! Global warming is literally throwing the Earth "off kilter".

Finally, thanks to "precession" - like the wobbling of a spinning top- True North points to varying regions of space. Currently (in the northern hemisphere) it points to Polaris (aptly named!).
Image:Earth precession.svg

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