Storm in a Teacup: The Physics of Everyday Life(79)

IT’S EASY TO take a compass for granted, especially if you do a lot of walking, when it’s very handy to be accompanied by a needle that always points north. But imagine getting ten compasses, or twenty, or two hundred. You spread them out across the floor, they all point north, and suddenly you see that this isn’t just something that happens when you get a compass out. It’s there all the time, and it’s consistent. You can take your compass collection anywhere on the globe, unpack it, set it out, and all the compasses will swing around and agree on where north is. The Earth’s magnetic field is always there, flowing through cities, deserts, forests, and mountain ranges. We live inside it, and although we never feel it, a compass will always remind us that it’s there.

A compass is a brilliantly simple measuring device. The needle is a magnet, and so one end of it behaves very differently from the other end. Unhelpfully, these two ends of the magnet are called the north and south poles, but it’s just a way of saying that one behaves like the magnetic north pole of the Earth, and the other behaves like the magnetic south pole. If you take two magnets and move them about near to each other, you’ll see very quickly that it’s very hard to push the two north poles together, but that a north pole and a south pole will attract each other very strongly. This is why it’s easy to detect the direction of a magnetic field; if you put a small mobile magnet inside a magnetic field, it will spin around until its north and south ends are aligned with the field. And that’s all a compass is: a mobile magnet that gives away the direction of any magnetic field you bring it into. We can’t see the vast magnetic field of the Earth, but we can see the compass needle respond to it. It’s not just the Earth’s field that compasses sense, either. Take a compass around your home, and you’ll detect for yourself the magnetic fields that surround plug sockets, steel pans, electronics, fridge magnets, and even any iron that’s been close to a magnet recently.

Compasses, obviously, are mostly used for navigation. Finding your way about on the surface of a sphere is always going to be tricky, but the Earth’s magnetic field has provided a fabulously reliable tool for explorers for centuries. The Earth has a magnetic north pole and a magnetic south pole, and anyone with a compass can orient themselves toward one or the other. As a navigational tool magnetism is straightforward, it’s cheap, and it never runs out. However, it does have a few caveats attached to it. Caveat number one sounds unexpectedly serious: The magnetic poles aren’t fixed in one place. They wander, and they can travel a very long way.

On the day I’m typing this, the magnetic north pole is in the far north of Canada, about 270 miles from “true north,” which is the real North Pole defined by the Earth’s spin axis. Since this time last year, the magnetic north pole has moved 26 miles—it’s on its way across the Arctic Ocean toward Russia. This sounds spectacularly unhelpful for navigators, although since the world is a big place it’s not as bad as it seems. But the magnetic field moves because of where it comes from, and it’s a reminder that the innards of our planet are more than just a static ball of rock.

Deep down beneath our feet, the iron-rich outer core of the Earth is churning slowly. It’s shifting heat from the center out toward the surface, and the rotation of the planet forces the molten rock to rotate, too. Because of the iron, the sluggish outer core is an electrical conductor, and that means it can behave like the electromagnet in the toaster. It’s thought that the currents running through the Earth’s outer core as it turns are responsible for generating our planet’s magnetic field. The process is based on the slow shifting of molten rock; and because the details of the rock movements change with time, the magnetic poles meander. They stay approximately aligned with the spin axis of the Earth because the rotation of the iron-rich rock is caused by the rotation of the whole planet, but the alignment is only approximate.

So if you really care about accurate navigation, you need to correct for the current position of the magnetic pole, because it’s not the same as the true North Pole. Today’s maps show the direction of both poles. I just had a look at an Ordnance Survey map of part of the south coast of the UK, and both magnetic and spin north are marked at the top. I can see that if you followed a compass directly north for 40 miles, you’d end up about 1 mile west of the line toward true north. A map seems like such a permanent record, and yet the magnetic field that you may use to help you navigate with it is fickle. Modern technology means that you and I won’t often get lost because of this. But the aviation industry, with one of the most sophisticated modern navigation systems humans have developed, certainly pays attention. For a start, it has to keep relabeling its runways.

Next time you’re at or near an airport, take a look at the large signs at the start of each runway. Every runway around the world is labeled by a number, which is its direction in degrees from north, divided by ten. So the runway at Tampa International Airport in Florida was given the number 18, because a plane landing on it will fly in on a heading of 180 degrees. Each runway will have a specific designation that’s a number between 01 and 36.§§ But this heading is relative to magnetic north, because that’s what a compass is telling you. So in 2011, runway 18 at Tampa became runway 19, to keep up with the movement of the magnetic pole. The runway hadn’t moved, but the Earth’s magnetic field had. Aviation authorities keep an eye on it all, and correct the runway designations when it becomes necessary. Since the poles move relatively slowly, the changes are manageable.