First, let's lock in a hard limit — Page 49, Lesson 55

All right, let's pick this up right where the real-world picture starts to get messy. We've just established the idealized model, and now I want to walk you through the reality of variation on a global scale. First, let's lock in a hard limit. Because the maximum angular difference between True North and Magnetic North is 180°, the maximum possible value of variation is 180°. And that extreme value occurs at both the North and the South Poles. Keep that in your head as the absolute ceiling. Now, the real variation map. Figure 3.3 showed a somewhat idealized situation. The truth is, the North and South Magnetic Poles are not actually antipodal — that's the technical term for directly opposite each other on the globe. There's no reason they should be. And here's the key correction: Earth's magnetism is not really caused by a large bar magnet. That's simply a convenient analogy. The actual cause is the swirling of molten magnetic magma below the surface of the Earth. The effect is more like a bent bar magnet, not a straight one. Let me give you some real-world positions so you can feel how off-centre this is. The National Environmental Research Council — that's NERC — published a magnetic map for 1st January 2000. It positions the North Magnetic Pole at approximately 81°N 110°W, and the South Magnetic Pole at 63°S 135°E. By 2009, the North Magnetic Pole had moved to 84°N 120°W. Notice that — the pole itself drifts over time. Now, the actual situation is shown on the charts at Figure 3.4, Figure 3.5, and Figure 3.6. The disposition of variation is not quite as geometrically neat as the diagram in Figure 3.3, but there definitely are two lines of zero variation. One runs southwards from the True North Pole, and the other runs southwards from the Magnetic Pole. One of these agonic lines — that's the name for a line of zero variation — runs down through Europe. In fact, the variation near Stuttgart, in Germany, is zero. The other runs down through the USA. Let me trace these two lines for you, because they behave very differently. The agonic line running down through the USA continues through South America, as you see in Figure 3.4, and then continues on to the True South Pole, as in Figure 3.5. That one behaves much as you'd expect from the idealized model. But the other agonic line — the one running down out of the North True Pole — is the wild one. It passes through Stuttgart, into Central Africa, then curves upward again, back onto the North Polar chart in Figure 3.4, into North Central Asia. Then it turns southwards again, through Australia, and finally ends at the South Magnetic Pole. So one agonic line ends at the True South Pole, and the other ends at the South Magnetic Pole. And I want to close with a distinction that pilots sometimes blur, and you must not. Isogonals are not the actual magnetic lines of flux. The lines of flux are a natural phenomenon — they're the real magnetic field. Isogonals are something we construct. They represent the difference between the alignment of those lines of flux and the local direction of True North at any given point. So when you read an isogonal on a chart, you're reading an angular difference, not a physical field line. That's the whole point of them. So to summarise what we've got: maximum variation is 180°, occurring at both poles; the magnetic poles are not antipodal and they drift; and the two agonic lines — one through the USA to the True South Pole, one through Stuttgart, Central Africa, North Central Asia, and Australia to the South Magnetic Pole — are the real-world zero-variation paths. And isogonals are differences in alignment, not flux lines.