Earth Magnetism — Page 49, Lesson 53

I want to walk you through the concept of variation in Earth magnetism, which is fundamental to understanding how a compass works in navigation. Let's start with the definition. Variation is the angular difference between the directions of True North and Magnetic North at any point on the Earth's surface. We measure it in degrees East or West from True North. So if your compass needle points to the right of True North, that's East variation; if it points to the left, that's West variation. Now, a line on the Earth's surface that joins points of equal magnetic variation is called an Isogonal. That's a term you'll see on navigation charts. How variation changes depending on where you are The amount and direction of variation depend on the relative geometry between three things: your position as the observer, the True Poles, and the Magnetic Poles. The True North Pole is the geographic North Pole — the top of the Earth's axis. The Magnetic North Pole is where the Earth's magnetic field lines converge, and it's not at the same location. Currently, the Magnetic North Pole sits at about 120° West longitude. Let me walk you through the idealized model shown in Figure 3.2. Imagine an observer at point A. For them, True North is the direction straight up their meridian toward the True North Pole. But their compass needle points toward the Magnetic North Pole. From point A, the Magnetic North Pole lies to the right — that is, East — of True North. So for observer A, Magnetic North is East of True North, meaning variation is East. Now take observer B. The geometry of their position is different. From B, the direction to the Magnetic North Pole is to the left of True North. So Magnetic North is West of True North, and variation is West. Consider observer C. For them, just like observer A, Magnetic North is East of True North. But here's the key difference: observer C is much farther away from both poles than A is. Because they're farther, the angular difference between the two directions — the variation — is a smaller angle. So distance from the poles affects the magnitude of variation. Similarly, observer D is like observer B — Magnetic North is West of True North, so variation is West — but again, because D is farther away, the angle of variation is smaller than for B. Now look at observer E. This is a special case. We start at the True Pole, take the Great Circle to the Magnetic Pole, and continue that Great Circle around the Earth. For observer E, the line joining them to the Magnetic North Pole and the line joining them to the True North Pole lie along the same Great Circle. That means the direction to True North — straight up the meridian — is exactly the same direction their compass needle points. For observer E, variation is zero. That line connecting points of zero variation is called the Agonic Line. What happens at the poles themselves? This is a critical point for professional pilots. If an aircraft is 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 degrees. Look at Figure 3.3. An aircraft is at position A, somewhere on the line between the North True Pole and the North Magnetic Pole. The meridian connecting that aircraft to the True Pole gives the direction of True North. But the compass needle points toward the Magnetic North Pole, which from that position is in exactly the opposite direction. So the variation at that point is 180 degrees. The zero variation line is shown in yellow, and the 180-degree variation line is shown in green. This means the maximum possible value of variation is 180 degrees. So to summarize: variation is the angle between True North and Magnetic North, measured East or West. It changes depending on where you are relative to the poles. Lines of equal variation are isogonals, and the line of zero variation is the agonic line. And in the region between the True and Magnetic North Poles, variation can reach 180 degrees.