Earlier on EarthDate, we talked about the three North Poles.

Of these, geographic north, the tip of Earth’s axis, defines latitude and longitude on our maps, while magnetic north is what compass needles point toward.

Unfortunately, geographic north and magnetic north are not in the same place. So the north of our maps and the north of our compasses don’t match.

The difference between them is called declination.

To complicate matters, declination varies depending on your location. Compass navigations have to be offset by the declination of that particular spot to make them accurate to the map.

Now for the real curveball: magnetic north is always moving!

It’s based on Earth’s magnetic field, which is created by turbulent currents in Earth’s molten metal outer core as they swirl around its solid iron inner core.

Shifting currents mean the magnetic field is always moving. Which means magnetic north is always changing. And declinations around the world are always adjusting!

We can see this effect all around us. Streets and walls built to compass north a few centuries ago may now be off by several degrees.

Runways, which are named by their compass orientation to help pilots navigate them, periodically have to be renamed when the declination of that spot changes.

To keep planes and ships safe, scientists track the movements of magnetic north and the constantly changing declinations and keep them updated every 5 years.