Earth Magnetism — Page 49, Lesson 51

Let's start with the idea of True Direction, because that's the baseline everything else is measured against. In the chapter on the Form of the Earth, we defined direction using the poles the Earth spins around — North and South — and the direction it spins, which is East. That whole system is what we call True Direction, and it's exactly what's drawn on maps. If you need to fly from, say, Oxford to Leicester, you draw a straight line on the map between the two airports — assuming no air traffic or obstacles get in the way — and measure its direction. That measured direction is a True direction. Now, notice something important: at this stage, we haven't mentioned magnetic compasses at all. If there were such a thing as a compass that worked in True Direction, we wouldn't even need to think about Earth magnetism. But here's the reality — until about 50 years ago, no such instrument existed. They do exist now, though. The Inertial Navigation System, the INS, finds its direction by establishing the direction of the Earth's rotation, so it genuinely is a "true compass." But those systems are very expensive and are fitted only to sophisticated aircraft. Most aircraft are still steered by a magnetic compass, and even aircraft that carry an INS will have a standby compass, and that standby compass will be magnetic. So that brings us to Magnetic Direction. It's been known for hundreds, probably thousands, of years, that you can hold a reasonably constant direction by using Earth's magnetism. Why does the Earth behave magnetically at all? Because the centre of the Earth is still cooling down, and at its centre there's a mass of molten liquid. That molten mass makes the Earth act magnetically as though there were a huge bar magnet running through it, aligned fairly closely to the North and South True poles. But — and this is the key point — that alignment is only approximately in line with the True poles. This can lead to marked differences between True and Magnetic direction in some parts of the world. And there's more: the magnetic pattern is not symmetrical in the north–south sense. The North and South magnetic poles are not exactly opposite each other on the Earth. In effect, it's like a bent bar magnet, not a straight one. Let me define Magnetic North precisely, because it's a formal definition you'll need. Magnetic North is the horizontal direction indicated by a freely suspended magnet influenced only by the Earth's magnetic field. That direction is sometimes referred to as the "magnetic meridian" at that point. So when you let a magnet hang freely and it settles, the horizontal line it points along is the magnetic meridian, and that's Magnetic North for your location. Finally, Magnetic Direction itself: it's measured from Magnetic North clockwise through 360 degrees, and it's suffixed with the letter M. So you'd write 043(M) or 270(M). That suffix is part of the notation — it tells anyone reading the number that this direction is referenced to Magnetic North, not True North. So the whole picture is this: True Direction is the map reference, based on the spin axis. Magnetic Direction is what a compass actually shows, based on the Earth's magnetic field, which is only roughly aligned with the true poles and is bent like a bar magnet. The difference between the two is what we'll build on next.