Why Ships and Planes Measure Speed in Knots Instead of Miles or Kilometers per Hour

The informative channel Simple Things – Surprising Histories explained why ships and planes use knots as a measurement tool rather than miles and kilometers per hour.

You are measuring a distance across the land over a specific amount of time. But the moment you step onto a boat or strap yourself into an airplane, those rules disappear. Suddenly, nobody cares about miles or kilome. Instead, the captain speaks in knots. 

This term originated in the 17th century, when sailors developed an innovative process that used knots tied in a long rope to measure how fast the boat was going. The knots were spaced 47 feet apart, which accounted for the Earth’s curvature.

We use the word knot today because 400 years ago, sailors were counting physical knots in a piece of hemp rope. …The sailors did the math. They calculated that if they spaced the knots 47 ft apart and timed it for 28 seconds, the count would perfectly equal the number of nautical miles hour. So a knot isn’t just a random speed. It is a direct link to the geometry of the planet

The same formula also applies to air travel.

Since they were using naval maps and naval formulas, it made sense to use the naval unit of speed. Furthermore,planes travel long distances. If a plane flies from New York to London, it is traveling across the curvature of the Earth.