In 2009, Fairbanks International Airport in Alaska closed a runway, sent out crews, and repainted the enormous white numbers at both ends. Runway 1L–19R became runway 2L–20R.

Nothing had been built. Nothing had moved. The concrete was in exactly the same place it had always been, pointing in exactly the same direction.

What had moved was the North Magnetic Pole — and it is coming back for that runway again around 2033.

The Number Is a Compass Bearing

Every runway number is simply a direction, written in shorthand.

Point down the runway, read the magnetic compass, round to the nearest ten degrees and knock off the final zero. A runway aligned at 268 degrees rounds to 270 and is painted 27. Turn around and fly the other way and you are heading 088 degrees, so that end is painted 09. The two numbers on any runway always differ by 18.

It is a beautifully practical system. A pilot lining up can glance at the compass and at the paint and instantly confirm they are on the correct strip of concrete, pointing the correct way — a check that matters enormously at an unfamiliar airport at night.

How runway numbering works, including the parallel-runway letters.

The Problem With Magnetic North

Here is the catch. Runways are numbered against magnetic north, not true north, because that is what a compass actually shows a pilot.

True north is a fixed geographic point. Magnetic north is not a point at all so much as a wandering region, produced by the movement of molten iron in Earth’s outer core — flows that generate around 95 per cent of the planet’s magnetic field, and which nobody can predict far in advance.

That wandering is not trivial. Over recent decades the North Magnetic Pole has been drifting from the Canadian Arctic towards Siberia, and its speed has itself changed — accelerating for years to roughly 50 kilometres annually before slowing markedly to something closer to 35 kilometres a year, the sharpest deceleration ever recorded. It has now passed closer to Russia than to Canada.

The gap between true north and magnetic north at any location is called declination, and it is different everywhere on Earth and different every year.

Which Means the Paint Goes Stale

If the magnetic bearing of a runway drifts far enough that it no longer rounds to the number painted on it, the number is wrong — and a wrong number on a runway is a safety issue, not a cosmetic one.

So the paint has to change. The aviation authority notifies the airport, charts and approach plates are amended, navigation databases are updated, and crews go out with paint.

The reference everyone works from is the World Magnetic Model, built by NOAA’s National Centers for Environmental Information together with the British Geological Survey. It is assembled from satellite measurements and a global network of around 120 magnetic observatories, and reissued every five years.

The same model sits underneath the compass app on your phone, NATO navigation, and every ship at sea.

How Often, and Where

Not every airport is affected equally. Magnetic change happens fastest near the poles, so high-latitude airports repaint far more often than tropical ones.

Fairbanks, deep in Alaska, faces the problem roughly every 24 years — hence 2009, and hence the expectation of another change around 2033. Denver is at the other end of the scale. Its runway 17L–35R sits at about 172.5 degrees magnetic; it needs roughly another four degrees of drift before it becomes 18L–36R.

Other airports have already been through it. Tampa International repainted its runways in 2011, turning 18R/36L into 19R/1L. Manchester in England went from 06L/24R to 05L/23R in 2009. Austin-Bergstrom’s 17L/35R is due to reopen as 18L/36R.

Why the numbers change, and what happens at the airport when they do.

The full logic behind runway designations, letters and all.

Why Not Just Use True North?

It is the obvious question, and a few places have effectively answered yes. Airports in the far north, where declination is extreme and changes fast, sometimes reference true north instead — the alternative being renumbering every few years and a compass that is close to useless.

Everywhere else, magnetic wins for one stubborn reason: the magnetic compass is the last instrument standing. Lose electrical power, lose the flight management system, lose GPS, and there is still a compass in front of the pilot, working on physics alone. Numbering runways to match the one instrument that cannot fail is a deliberate choice, and the periodic repainting is the price of it.

So the next time you are on final approach and you see two enormous white digits rushing towards you, remember that they are not a name. They are a measurement — of a magnetic field generated by molten metal three thousand kilometres beneath the wheels you are about to put down. And that measurement has an expiry date.

Sources: NOAA National Centers for Environmental Information; British Geological Survey; FAA Order 8260.3; NBAA; Denver International Airport; Fairbanks International Airport.