Earth Magnetism — Page 49, Lesson 53

Let’s start with the core definition, because everything else in this chapter hangs off it. Variation is the angular difference between the directions of True North and Magnetic North at any point. In other words, it’s the angle between those two directions, and we measure it in degrees East or West from True North. So if your compass needle points a little to the right of the true meridian, that’s variation East; if it points to the left, variation West. Now, a line on the surface of the Earth joining points of equal magnetic variation is called an Isogonal. So an isogonal is a contour line of constant variation — every point along it has the same variation value. The amount and the direction of variation depends on the relative geometry of the observer, the True Poles, and the Magnetic Poles. That’s the key idea: variation isn’t a fixed number; it changes depending on where you are on the Earth, because the True North Pole and the Magnetic North Pole are in different places. Let’s look at the idealized model. The red line on the globe is the current longitude of the Magnetic North Pole, which is about 120°W. So the Magnetic North Pole sits roughly along that meridian. Now consider an observer at point A. For him, True North is the direction up the meridian at A toward the True North Pole. Magnetic North is the direction his compass needle points — toward the Magnetic North Pole. From A, that direction is to the right, or East, of True North. So we say Magnetic North is East of True North, and variation is East. Now take an observer at B. The geometry of the relative positions is different, so the direction of Magnetic North will be left of True North. That means Magnetic North is West of True North, and variation is West. Now consider an observer at C. For him, just like at A, Magnetic North is East of True North. But here’s the subtle point: because he is so much further away from both poles, the difference in their directions — that is, the variation — is a smaller angle. So distance from the poles matters; the further you are, the smaller the angular separation between the two directions appears. Similarly, for an observer at D, like at B, Magnetic North is West of True North, so variation is West. But again, it will be a smaller angle of variation than for the observer at B, because D is further away. Now here’s the special case. Consider an observer at E. We start at the True Pole, take the Great Circle to the Magnetic Pole, and then continue it in a Great Circle round the Earth. To an observer at E, the line joining him to the Magnetic North Pole and to the True North Pole will be the same Great Circle. That means the direction of True North — straight up the meridian — is also the direction his compass needle will point. So for him, the value of variation is zero. That line is called the Agonic Line. It is the line connecting points of zero variation. Now let’s look at the situation at the poles. For an aircraft flying between the North True Pole and the North Magnetic Pole, the variation on that shorter arc of the Great Circle is not zero — it is 180°. Consider an aircraft at position A, somewhere on the line between the North True Pole and the North Magnetic Pole. The meridian connecting him to the True Pole is the direction of True North. However, his compass needle will point at the Magnetic North Pole, which is in exactly the opposite direction. So the variation at this point is 180°. The zero variation line is shown in yellow, and the 180° variation line is shown in green. Therefore, the maximum possible value of variation is 180° — that’s the extreme case, right between the two poles. So to tie it together: variation is the angle between True and Magnetic North, measured East or West from True North. Isogonals join points of equal variation; the Agonic Line joins points of zero variation. The value changes with your position relative to the True and Magnetic Poles, and it reaches its maximum of 180° on the arc between the two poles.