Runway numbers are a map of the Earth’s magnetic field

#What a runway number means

Runway 08R at Bucharest Otopeni points 83.94 degrees from true north, by the Romanian air navigation service’s survey. A compass needle does not point to true north: in Bucharest, in autumn 2026, it is 6.4 degrees east of it.

For the pilot, the runway therefore lies at 77.5 degrees by the compass. Divided by ten and rounded, that is 8: the runway is called 08, and its other end, 180 degrees round, 26. North is written 36, not 00. L and R tell two parallel runways apart: the left one and the right one.

#The wind chose the direction; the Earth’s core sets the number

Runways are usually laid along the prevailing wind, so that aircraft take off and land into it; terrain and airspace sometimes dictate otherwise. Hour-by-hour wind in 2022 and 2023 at 1,226 airports bears this out: at half of them, the main runway is within 17 degrees of the wind’s axis. For randomly oriented runways the median angle would be 45 degrees.

At the 378 airports with a strongly dominant wind axis (weighting by squared speed), the median angle falls to 12 degrees. The concrete stays put; the number on it depends on the molten iron that flows nearly 3,000 kilometres below the runway and makes the magnetic field. When that flow changes, the compass needle turns, and the runway ends up carrying the wrong number.

#The runways draw the magnetic field

Each runway’s number says how far the compass deviates where it lies, give or take five degrees: the true direction minus ten times the number. Coloured by that sum, the 13,389 runways gather into bands: blue where the needle pulls west, orange where it pulls east.

The magenta lines show the magnetic declination (how far the compass deviates) in the international IGRF-14 model, computed from observatories and satellites. They are drawn for the year chosen below and move with it.

#The same lines, drawn from the paint alone

The yellow lines are drawn only from the runways’ numbers and surveyed directions, without a single magnetometer: a weighted average of the runways within 350 kilometres of each point. Over the 182 five-by-five-degree regions with at least 15 runways, the field read from runways differs from the field of 2003 by a median 1.1 degrees, and from today’s by 2.6 degrees.

If every runway carried its ideal number for the field of 2003, the same calculation would give 0.57 degrees: that is what rounding to ten degrees costs. Old numbers and wrong coordinates in the register probably account for much of the remaining gap.

#The numbers are old: the world’s runways best fit the field of 2003

Today’s field fits the numbers of 73% of runways (true north, only 46%). Turn the clock back and the fit rises to 82% in 2003, then falls again: 64% in 1950.

In the United States the best-fitting year is 1997; in Europe, 2015. Where the needle has turned more than eight degrees since 1950, at 3,167 runways, only 36% would fit the compass of 1950, and 66% fit today’s.

#When the field moves, runways change their numbers

The register of September 2011, kept in the Internet Archive, can be compared with today’s runway by runway. Of the 12,885 runways that kept their number or changed it by one, 1,090, at 918 airports, changed their number by exactly one. 97% moved the way the field turned, and where the field turned by at least half a degree, 98%.

In Europe the numbers went down, because the needle pulls ever further east: Luton went from 08 to 07, Geneva from 05 to 04. In the United States they went up: Las Vegas from 07 to 08, Tampa from 18 to 19. The test shows the changes follow the field’s drift; on its own it does not prove the year above.

#In the United States, the numbers tend to follow the year in the file

The FAA records of American airports usually give the compass deviation and the year it was set for. The most common year is 1985, first noticed by the magnetic-drift-atlas project. At 2,162 runways, the record still holds the 1985 value.

Where the 1985 value and today’s field give different numbers, at 839 runways, 73% carry the 1985 number and 25% today’s. The numbers follow the value in the record more often than today’s field; the comparison does not say when the record or the paint last changed.

#Near the magnetic pole, the compass becomes hard to use

In northern Canada, Greenland and Alaska the needle is tens of degrees off and turns quickly from one year to the next. There, 31 runways carry numbers that fit true north and not the compass, a sign that they were probably numbered from true north.

The field lines crowd towards the magnetic pole, which over the past 30 years has moved from the Canadian Arctic towards Siberia.

#Britain and Ireland: the needle has crossed true north

By IGRF-14, the line where the compass points to true north crossed Greenwich in 2019, moving west. Everywhere in Britain and Ireland the needle is turning east, by a sixth to a fifth of a degree a year: east of that line it now pulls east, west of it (Scotland, Northern Ireland, Ireland) it still pulls west, but less each year. Either way, runway numbers here go down, about one step every fifty or sixty years.

Luton went from 08/26 to 07/25 in 2020. Shannon went from 06/24 to 05/23 on 11 June 2026 and Gloucestershire from 09/27 to 08/26 the same year; by the calculation here, Shannon’s old number had gone out of tolerance in 2024. Birmingham’s runway 15 and Liverpool’s 09 reach the threshold before 2030 at today’s rate, by the register’s coordinates.

For comparison, in Romania the deviation is six to seven degrees east. Timișoara’s runway 11/29 points 109.75 degrees from true north; by today’s compass that is 103.8 degrees, which makes it 10. By the model and the rounding rule, 11 last fitted in 2015, and even with the deviation printed in Romania’s own aeronautical publication, 5 degrees (2017), it comes out as 10.