The number on the runway

The big number painted at the end of a runway is its compass direction, in degrees, divided by ten and rounded. A runway pointing 268° wears 27; a runway pointing due north, 360°, wears 36, never 0.

The two ends of one strip of concrete point opposite ways, and opposite directions differ by 180°. So the far end is the near number plus or minus 18: runway 27 is runway 09 from the other end. Every runway number pair differs by 18.

The directions are magnetic, and the magnetic pole drifts. When the drift moves a runway’s heading past a rounding line, the airport repaints it: a runway can be renamed without anyone moving it. On GeometryForge older children turn a compass needle, find the painted number, and then find the other end.