The Earth Has More Than One North Pole

NORTH POLE?: There are at least seven different possible definitions of the North Pole, including the conceptual point in the Arctic Ocean that corresponds to where all the lines of longitude converge on a map.
©ISTOCKPHOTO/MICHAEL DE NIJS

You may think of the North Pole only as the top of the world—its northernmost point and, if you're younger, Santa's home. But it turns out there are a host of "north (and south) poles" on our planet.

First, and most simply, there is a town in Alaska called "North Pole" which isn't near any of the other north poles (but it does get snow and receives a lot of mail addressed to Santa Claus). Then there is the geographic north pole, also known as "true north." This is the spot in the Arctic Ocean where all the man-made lines of longitude converge on a map as well as the conceptual point on the ice-encrusted waters that countless explorers sought to stab with their national banner–bearing flagpoles, beginning in 1827 with British rear admiral, Sir William Edward Parry.

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.

All of this jargon separates into unique, if not pedantic, definitions. So although they all share the term "north pole," each has clearly staked out its own semantic territory. The same cannot be said, however, of the last two "north poles" in this rundown, and both relate to Earth's very real magnetic field, which is generated by fluid motion inside the planet's core. That motion—affected by Earth's rotation—sets up a naturally occurring electric generator that sustains the magnetic field.

The magnetic pole describes the two locations (north and south) where the planet's magnetic field is vertical. So if you're standing over the north magnetic pole with a compass, the needle would dip and try to point straight down—hence its other name: the magnetic dip pole. Over the south magnetic pole, your compass needle would point upward.

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.

"A space physicist usually thinks in terms of this tilted dipole that the earth has," Love says, "whereas a navigator would probably be more interested in the magnetic dip poles."

To further confuse things the dip poles move around—sometimes with daily frequency. The north magnetic pole in recent years has started shifting quickly toward Siberia. Its annual movement has accelerated from 10 to 50 kilometers (6.2 to 31 miles), says Larry Newitt, an emeritus scientist with the Geological Survey of Canada, who has pegged the pole's location on many expeditions since 1973.

And here's something to add even more confusion to the north magnetic pole (aka dip pole) versus north geomagnetic pole (aka dipole): the magnetic pole in Earth's northern hemisphere acts like the south pole of a bar magnet.

"If you look at the north pole of the bar magnet you have the field lines going from the north pole to the south pole, but for the earth it's exactly opposite," Maus explains. So the north magnetic pole is where the earth's magnetic field lines pull toward the planet, acting like the south pole of a bar magnet.

From a physics standpoint, then, the north needle of a compass (or any magnet) points to what is physically—but not in name—the south magnetic pole of the earth, in other words, in the direction of the Arctic.

"The north pole of your bar magnet is attracted to the north [magnetic] pole of the earth," Maus adds, the reverse of the usual situation in which like poles on magnets repel one another. "That is why some people have suggested that to avoid this confusion we should call the north magnetic pole the 'north seeking pole.'"

Whether that would add or subtract from the confusion remains unclear. What is clear is that—even in Santa Claus–related matters—one must be very precise in specifying exactly what one is talking about when referring to the "north pole."

Original here