Learn & Understand

The Magnetic Pole Is Moving, and It Keeps Rewriting the Charts

The companion calculator converts true heading to magnetic by applying variation with the east-least-west-best rule. That is the right rule, but it treats variation as a fixed local constant - and it quietly omits a second compass error entirely. The full story involves a magnetic pole that physically wanders across the Arctic, dragging the world's aeronautical charts along behind it.

True North and Magnetic North Are Different Places

A chart is drawn to true north, the geographic pole about which the Earth spins. A magnetic compass points to magnetic north, a separate location set by the planet's molten-iron core - currently in the Arctic, but not at the geographic pole. The angle between them, at your specific location, is magnetic variation (or declination). It is not a single global number: it depends entirely on where you are standing, because the geometry between you and two different "norths" changes across the globe.

The Third Error the Conversion Skips

Variation converts true to magnetic - but the compass in the aircraft has its own, additional error called deviation.

The full chain is true heading, then variation to get magnetic heading, then deviation to get the compass heading you actually steer. The calculator handles the first correction; the little placard by the compass - unique to each airframe - handles the second. Ignoring deviation is why a precisely computed magnetic heading can still leave the needle a few degrees off.

Isogonic Lines: Mapping an Invisible Field

Because variation changes with location, charts print isogonic lines connecting points of equal variation, with a special agonic line marking where variation is zero and compass north happens to align with true north. Fly across the country and the variation to apply slides continuously from many degrees east to many degrees west - which is exactly why the correction is a local lookup, not a universal constant.

The Pole That Will Not Sit Still

Here is the part that surprises people: magnetic north physically migrates, driven by churning in the Earth's core, and in recent decades it has been moving unusually quickly across the Arctic. Because variation depends on where magnetic north is, this secular change means the variation at every location slowly shifts over the years - which is why aeronautical charts carry a date and are periodically reissued. It even reaches the runways: runways are numbered for their magnetic heading, so as variation drifts, airports occasionally have to physically renumber and repaint a runway to keep its number matching the compass. The wandering pole eventually rewrites the paint on the ground.

Applying the Magnetic Heading Correctly

Use the calculator's magnetic heading as the correct output of the variation step - just remember it is the middle link, not the last. Apply your aircraft's deviation card to reach the compass heading you will actually fly, use variation from a current chart rather than memory, and appreciate that the reason charts expire is a molten pole thousands of miles beneath the Arctic that never quite holds still.