Earth Magnetism — Page 62, Lesson 72

I want to walk you through the core definitions that underpin everything we do with direction on the Earth. These are the building blocks for understanding how a compass works and how we navigate using it. Let's start with Heading. Heading is simply the direction in which the fore and aft axis of the aircraft is pointing. That's the nose-to-tail line. Now, that direction can be measured from three different references: True North, Magnetic North, or Compass North. So you'll hear terms like True Heading, Magnetic Heading, or Compass Heading, depending on which north you're measuring from. Next, Variation. Variation is the angular difference at any point on the Earth between the direction of True North and the direction a freely suspended compass needle would point if it were influenced only by the Earth's magnetic field. In other words, it's the difference between where the geographic North Pole is and where the magnetic field pulls the needle. We call it East Variation if Magnetic North lies to the East of True North, and West Variation if it lies to the West. Then we have Deviation. Deviation is a different angle. It's measured at the aircraft's compass location between the direction the compass needle actually points and the direction of Magnetic North. This error is caused by local magnetic fields inside the aircraft — things like electrical systems, metal structure, or other equipment. Again, it's termed East or West depending on whether Compass North lies to the East or West of Magnetic North. Now, to help us map these values, we use Isogonals. These are pecked lines on a map or chart that join places of equal magnetic variation. So if you're flying along an isogonal, the variation is constant. A special case of an isogonal is the Agonic Line. That's the name given to isogonals that join places where the variation is zero — where True North and Magnetic North are aligned. Next, let's talk about The Angle of Dip. This is the angle in the vertical plane between the horizontal and the Earth's magnetic field at a point. Imagine the Earth's magnetic field lines entering the ground at an angle — that angle from the horizontal is the dip. It's zero at the magnetic equator and 90 degrees at the magnetic poles. To map dip, we use Isoclinals. These are lines on a map or chart joining places of equal magnetic dip. And the special case of an isoclinal is the Aclinic Line, which joins places of zero dip — that's the magnetic equator. One important note: Isoclinals and Aclinic lines do not appear on navigation charts. You won't find them on your standard aeronautical charts; they're more of a geophysical tool. Finally, the excerpt touches on the magnetic equator itself. It tells us that the magnetic equator is the shorter distance between the respective True and Magnetic North and South poles, and that it follows separate paths out of the North polar regions — one currently running through Western Europe and the other through the USA. That's a key point: the magnetic equator isn't a simple straight line; it has this dual path structure in the northern hemisphere. These figures on screen show you the 6 µT zone in the Arctic and more recent surveys of that zone, which help visualise the magnetic field behaviour near the poles.