Earth Magnetism — Page 49, Lesson 51

I want to walk you through the start of Earth Magnetism, which is a core topic in general navigation. We're going to begin by understanding the difference between True Direction and Magnetic Direction, because that distinction is the foundation of how we steer aircraft. Let's start with True Direction. In the earlier 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 is the basis of True Direction. True Direction is what is shown on maps. If you need to fly from, say, Oxford to Leicester, you draw a straight line on the map between the two airports and measure its direction. That measured direction is a True direction. Notice that, at this stage, there has been no reference to magnetic compasses at all. If there were such a thing as a compass that works in True Direction, we would not even need to consider Earth magnetism. However, until about 50 years ago, no such thing existed. They do exist now — the Inertial Navigation System, or INS, finds its direction by establishing the direction of Earth rotation, so it is a genuine "true compass". But these are very expensive and are fitted only to sophisticated aircraft. Most aircraft are still steered by a magnetic compass, and even aircraft with INS will have a standby compass, which will be magnetic. Now let's move to Magnetic Direction. It has been known for hundreds, and probably thousands, of years, that you can hold a reasonably constant direction by making use of Earth magnetism. Because the centre of the Earth is still cooling down, at its centre is a mass of molten liquid. This has the effect of making 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. However, the alignment of this magnetic effect 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. Furthermore, 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 is like a bent bar magnet. Now, let's define Magnetic North precisely. Magnetic North is the horizontal direction indicated by a freely suspended magnet influenced only by the Earth's magnetic field. This direction is sometimes referred to as the 'magnetic meridian' at that point. So when we talk about Magnetic Direction, it is measured from Magnetic North clockwise through 360 degrees, and it is suffixed by the letter 'M' — for example, 043(M) and 270(M). That suffix tells you the direction is magnetic, not true. So to summarise: True Direction comes from the Earth's spin axis and is what you see on maps. Magnetic Direction comes from the Earth's magnetic field, which behaves like a bent bar magnet inside the planet, and it is what most aircraft compasses actually show. The difference between them is something we will explore further, but for now, understand that they are not the same, and that is why we need to study Earth magnetism in the first place.