Tuesday, March 04, 2008

North Poles?

Scientific American explains that the Earth has not one North Pole, but several. While you might have heard of the difference between "true" geographic north and "magnetic" north, there are some other alternatives as well. There is also the Alaskan town named "North Pole" - and yes, they do receive quite a few cards addressed to Santa from children around Christmas time.

"Somewhat related to the geographic north pole is the considerably less famous instantaneous north pole, where Earth's rotational axis meets its surface, as well as the celestial north pole, where the axis spears the night sky (in an imaginary extension kind of way). The instantaneous north pole is not fixed. Rather, it moves in an irregular circle caused by "the Chandler wobble"—named for astronomer Seth Carlo Chandler, who discovered in 1891 that our planet wobbles as it rotates. His discovery gives rise to the "north pole of balance," which lies at the center of this circle."

While I knew about the wobble, (more correctly referred to as precession, a complete cycle of which last 26,000 years) which is the reason the star Polaris will not always be the "pole star", one thing I did not know myself regarding the magentic pole is that a compass points down at the north magentic pole but upward at the south magnetic pole, which makes a certain amount of sense. The other thing I didn't know is kind of complicated, but also pretty interesting.

"But there is another magnetically based north pole: the north geomagnetic pole. "One thing that's very confusing is the fact that there's a magnetic pole and a geomagnetic pole and that they're different," says Stefan Maus, a geomagnetic field modeler at the National Oceanic & Atmospheric Administration's (NOAA) National Geophysical Data Center. "It's a historical and slightly outdated definition."

The geomagnetic poles are almost an artifact of reducing Earth's complex and varied magnetic field to that of a simple bar magnet, or dipole. "The only thing that we really want to know is where the field is really vertical," Maus says. "This other pole, which is just an approximation, is generally not very useful and often leads to confusion." So while the north dip pole lies in Northern Canada, the northern dipole is roughly off the northwest coast of Greenland.

But the geomagnetic pole is useful, if you're in space, argues Jeffrey J. Love, a U.S. Geological Survey geophysicist. The farther away from Earth you get, the more its magnetic field actually does act like a dipole, or a bar magnet—even if in reality it is no such thing."

The complicated part of this is these dip poles/dipoles also move around, as often as every day, and this north pole is moving toward Siberia at a rate of 6 to 30 miles every year. If I count correctly there are at least six north poles, seven if you count the town, so you had better be clear what you're referring to when you refer to the North Pole. :)

No comments: