Earth Magnetism — Page 56, Lesson 58

I want to walk you through what happens with variation at the poles, and then how variation changes over time. This is a key part of understanding Earth's magnetism for navigation. Let's start with the North Pole. If you look at the actual variation patterns around the True North Pole, you'll see something important. There's an agonic line — that's the line where variation is zero degrees — running up from Europe, shown in yellow on the chart. That agonic line changes to 180 degrees of variation, shown in black, right at the True North Pole itself. Now, the red isogonals — the lines connecting points of equal magnetic variation — that go around the True Pole range from 015 degrees East variation all the way up to 180 degrees East. The blue isogonals go from 015 degrees West variation to 180 degrees West. So the key point here is that the isogonals converge on the True North Pole. But here's the really interesting part: exactly the same thing happens around the Magnetic North Pole. The agonic line running up from the USA — again shown in yellow — changes to 180 degrees variation, shown in black, in the region between the Magnetic Pole and the True Pole. Around the Magnetic North Pole, the red isogonals go from zero to 180 degrees East variation, and the blue ones go from zero to 180 degrees West. So the isogonals also converge on the Magnetic North Pole. And if you study the same pattern at the South Poles, you'll see the same phenomenon. So the takeaway is this: isogonals converge on both the True and the Magnetic North and South Poles. Now, let's talk about how variation changes over time. The value of variation at any point on Earth is not fixed — it changes gradually. For example, as of 2012, the variation at Oxford was about 2 degrees West. But that value appears to be reducing at a rate of approximately one degree every nine years. To give you some perspective: back in 1960, Oxford's variation was about 8 degrees West. If the current trend continues, the variation should fall to zero around the year 2040, and after that it would become easterly. Similar changes are happening in other parts of the world too. So why does this happen? The reason is that the Magnetic North Pole — which lies to the west of Oxford, and by more than 90 degrees West of longitude — is moving westwards around the True North Pole. That movement is what's reducing the variation at Oxford. As for why the Magnetic Pole moves, the reasons aren't fully understood, but it's evidently associated with movement of the molten magma in the Earth's core.