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Sunday, January 9, 2011

A star to steer her by ('cuz my compass is effed)

I had planned to write a fairly in-depth post about the motion of the magnetic poles this evening, because it's one of my favorite topics to teach in the classroom. But as it turns out, there's already a really good article on the topic from NASA, so I'm going to link to that instead. The NASA article is linked at the very bottom. The short of it is this:

Illustration from NASA, so I didn't have to render it in MS Paint!

The earth is not just one solid sphere spinning around its axis, but two concentric solid spheres spinning around different independent axes. It is the innermost solid sphere, which is mostly iron, which generates our magnetic field. In the same way that the outer (mostly silicon) sphere functions as a spinning gyroscope as it revolves around the sun, the inner sphere does as well. And in the same way that the outer sphere, like all gyroscopes, has a tendency to wobble (process), so does the inner sphere, independently of the outer sphere. The net result of this is, the magnetic poles move relative to the surface of the silicon sphere, which is the part we live on.

It happens that for most of the 20th century, the poles moved at a fairly constant rate of about 10 km per year. However, for the past decade this rate has been steadily increasing; right now it is traveling northward at more than 55 km per year, and still accelerating. At the same time, the overall strength of the earth's magnetic field has decreased some 13% since 1900. Does all of this mean that the earth's magnetic poles are about to "flip"? Possibly, but there are other explanations as well. If it is, then the process will take hundreds of years, and the worst outcome is that during the time of magnetic minimum we may experience significantly more solar and cosmic radiation than normal, which may not go well for people of European descent but will otherwise hardly be catastrophic. The last magnetic field reversal happened 780,000 years ago, geologically speaking in the very recent past, our homo erectus ancestors survived it just fine.

Regardless of whether or not this proves to be the beginning of a field reversal, from a navigation standpoint it means that the magnetic variation printed on our charts is basically useless if the chart happens to be more than a year or so old.

For example, in the illustration above the variation (hikers call it "declination", but it's the same thing) is listed as 12° 15'W in 2004, with an annual increase of 2'. It's 2011 now, so 7 years times 2' is 14' added to the 2004 variation, or 12° 29'. Once upon a time, we actually figured variation this way. Now we can't, because that 2' per year increase is just a wild guess. This isn't trivial. As an experiment I took some ten-year-old charts with me the last time I went up the Inside Passage, and had I relied on the printed annual variation corrections my variation would have been wrong by 3° - 5°, more than plenty to run aground in the fog.

Moral of the story is, before leaving on a major trip, buy new charts, and get Print-on-Demand so the variation is as up to date as possible.

Here's the full article from NASA .

On the same topic, here's a more recent article from Scientific American .

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