Why sailors measure speed in knots
Ships measure speed in knots, not miles per hour. One knot is one nautical mile per hour, and a nautical mile is one minute of latitude — about 1,852 meters.
The name comes from the old chip log: sailors threw a rope with knots tied at regular intervals overboard and counted how many knots ran out in a fixed time, measured with a sandglass. The count was the speed.
The unit survived because it fits navigation. Charts are drawn in latitude and longitude, so a speed in nautical miles per hour converts directly into distance on the chart: one hour at 20 knots moves you 20 minutes of latitude.
Aircraft adopted it too, for the same reason. So when a pilot says 450 knots, they mean 450 nautical miles per hour — about 833 kilometers per hour, or 518 miles per hour.