Geek!

In times of confusion and trial, it's nice to know some things can be relied on, some things don't change. "I am as constant as a northern star," Joni Mitchell has one partner in a stormy love affair say in one of her songs. Yes, indeed, no matter what else, north is north and south is south.

Except, of course, it's not. First, consider the North Star, Polaris. Which hasn't always been the North Star and after a time won't be - that is, until it is again. See, the Earth is tilted on it's axis relative to the plane of its orbit around the Sun. A pretty good thing as far as I'm concerned, since the varying length of days and amount of sunlight that produces over the course of a year is what gives us seasons. No tilt, no seasons. (Yes, the Earth does vary in its distance to the Sun over the course of a year but the variance isn't actually all that much, just a couple of percent. Not enough to produce significant changes.) That axis points at Polaris (within about a degree), so those of us in the Northern Hemisphere have a ready nighttime guide: Find Polaris, face it, and you're facing north.

However, as the Earth spins on that axis (producing day and night), it also wobbles, very slowly in a wide circle, like a spinning top. It's called precession. Which means that over time that axis points to different places in the sky. Not really a concern for us, as it takes about 26,000 years to complete one circle, but it does mean that at the beginning of recorded history Polaris was not nearly as useful for navigation as it became more recently and that there have been times in the past and there will be times in the future when the Earth's axis points at another star (which would mean that one, and not Polaris, would be the North Star) and other times when it points to nothing visible to the naked eye at all.

Yeah, right, big deal. But north is still north, even when there isn't a North Star. If you doubt it, pick up any decent compass.

Well, yeah, except that, as most of us know, the North Pole (latitude 90 degrees north) and the magnetic north pole are not in the same place. In fact, magnetic north is drifting across the Canadian Arctic; right now it's about 82.3 degrees north latitude and 113.4 degrees west longitude, putting it in the Beaufort Sea north of the Queen Elizabeth Islands.

And at some point in the future, magnetic north won't be magnetic north at all. It'll be south.

Earth's magnetic field is thought to be generated deep inside the planet. An inner core of solid iron is surrounded by an outer core of molten iron. They rotate at different rates, and the interaction between the regions creates what scientists call a "hydromagnetic dynamo." It's something like an electric motor, and it generates a magnetic field akin to a giant bar magnet.

The process is not completely understood. In fact, one study suggests the planet's mantle, which surrounds the core, also plays a role.

However it works, the setup has been in place for at least 3 billion of Earth's 4.6 billion years, scientists figure. But the field is shifty, periodically growing stronger and weaker, moving around, and even flipping its polarity entirely.

In the past 15 million years, there have been four reversals every 1 million years, or about one shift each 250,000 years,

says Brad Clement, of Florida International University, leader of a study group on the phenomenon. Since the last flip took place 790,000 years ago, we might seem overdue for another, but there is no regular schedule of flips, so no one can be sure. However, the strength of the magnetic field has been dropping for about 2,000 years; one study even says it has declined 10% in just the last 150 years. That has lead some to speculate another shift is beginning, but, again, there's no way to tell for sure.

It's not really something to worry about too much, though: On average, the flips have taken about 7,000 years to complete. Plenty of time to get used to the idea.

On the other hand,

the dramatic turnarounds occur more quickly nearer the equator than at higher latitudes closer to the poles.

That means folks living during the next reversal - which some scientists speculate might be underway - will see compasses change and behave differently in different locations,

making them essentially useless until things settle down a couple of thousand years later.

Maybe instead of "constant as the North Star" we should use as our cliche the I Ching philosophy that "the only thing that doesn't change is the fact that everything else changes."