The message in the numbers on the runway

The plane is ready for take-off. It taxies to the end of the runway and turns. There you see the big number… 27. Ever wondered what it means? A nickname? Wrong.

The number is more or less the runway’s name, Popular Mechanics explains from Atlas Obscura. By the number, pilots tell one runway from another. But there is more: the numbers have not been chosen randomly. A runway always has a number between 1 and 36; each runway number is its degree from the magnetic north.

This way, Runway 27 is the runway on a particular airport that runs 270 degrees from the magnetic north. Unfortunately, the magnetic north shifts over time. A pilot preparing for take-off can make sure he's about to head down the right runway by just taking a look at his compass.

Interestingly, this magnetic north moves in loops, up to 50 miles every day. In the last 150 years, the pole has wandered a total of 685 miles. When the shift in magnetic north has become too big, the number on the runway no longer corresponds with its real degree from the magnetic north. Basically that means that the runway number has to change. A new number needs to be painted on the tarmac and the number has to be altered in all procedures that airports and pilots use.

Further reading on popularmechanics.com