What is a knot in aviation?
A knot is one nautical mile per hour, and the international nautical mile is exactly 1,852 metres. Both figures are fixed by agreement, not by measurement.
The United States National Institute of Standards and Technology sets it out in its guide to the SI: 1 nautical mile = 1852 m, with a knot defined as one nautical mile per hour. The same document records where the number came from. It was adopted at the First International Extraordinary Hydrographic Conference in Monaco in 1929, under the name international nautical mile.
So a knot is a shade over 1.15 statute miles per hour. A speed in knots always reads lower than the same speed in mph, which catches people out constantly.
Why aviation keeps it
Because the nautical mile is tied to the chart rather than to the ground. It was built around a minute of latitude, so a distance measured off a navigation chart and a speed measured in knots use the same unit, and the sums work without a conversion in the middle. Sailing kept it for the same reason, which is why the two trades still speak in the same numbers.
NIST, for what it's worth, files the knot among units it discourages its own authors from using. Aviation is one of the fields where established practice won, and it shows up everywhere once you notice it: wind speeds, approach speeds, and the jet stream figures quoted in how fast planes fly. It's also the unit painted on the instruments described in what a cockpit is. The whole set is on the sky.
