Ever wondered why ships and airplanes measure speed in knots instead of miles or kilometers per hour? It turns out the answer involves a rope, a chunk of wood, and some clever sailors from the 1600s.

Back then, ships didn’t have GPS, radar, or anything resembling a speedometer. To figure out how fast they were moving, sailors tossed a wooden “chip log” overboard attached to a rope with knots tied every 47 feet. As the ship sailed away, the rope unspooled while a sailor flipped a 28-second sand timer and counted how many knots passed through his hands. Five knots counted? Congrats. You’re sailing at five knots. Literally.

The spacing wasn’t random either. It was calculated so that the count matched nautical miles per hour, a unit based on the Earth itself: one nautical mile equals one minute of latitude. In other words, knots are basically speed measured using the planet’s grid instead of a road map.

And airplanes? They inherited the system because early pilots navigated using maritime charts and the same latitude-longitude math sailors used at sea. Turns out, when you’re flying across a curved planet, nautical measurements just make the math easier.

So the next time a pilot announces you’re cruising at 500 knots, remember: that high-tech jet is still using a speed system invented by sailors throwing wood into the ocean and counting knots on a rope.