26th of June 1946
The day knots and nautical miles became the standard.
The USAAF (US Army Air Force, which became the US Air Force on the 26th of July 1947 with the enactment of the National Security Act of 1947 (61 Stat. 495) and the US Navy adopted the "knot" as the aeronautical standard for speed and the "nautical mile" as the aeronautical standard for distance on this day in aviation history. Where do these knots and nautical miles find their origin?
First will look at the nautical mile because without the nautical mile there is no knot!
The history of the nautical mile dates back to the 16th century. And actually, the word mile comes from the Latin word for a thousand paces, 'mille passus'. In history (assuming that the earth is a perfect globe) it was described as the distance between minutes (1/60th of a degree) of latitude, which equates to ~1852 meters.
How did they get to that value of ~1852 meters? The earth circumference is ~40.000 km, if you follow the historical assumption that the earth is a globe, a quarter of the circumference of the earth is ~10.000.000 meter. With 90 degrees of latitude between the Equator and the North or South Pole, that equates to 5.400 minutes of latitude. So the distance per latitudinal degrees is 10.000.000 / 5400 = 1.851, 85 meters
In 1929 the First International Extraordinary Hydrographic Conference in Monaco defined the nautical mile as exactly 1852 metres. Then the knot as a measure of speed.
The knot as a unit of speed is derived from covering 1 nautical mile (1852 meters) in one hour.
The term 'knot' comes from an ancient method of measuring the speed of a ship using a chip log. The chip log was attached to a rope and every 47 ft 3 inches (14.4018 m) a knot was tied in the rope. Using an hourglass the time was measured and the number of knots the rope passed through a sailors hand in a set time gave a speed. A 28 second time glass is currently the accepted timing device, although historically a 30-second hourglass was used.
47 ft 3 inches equals 567 inches. If it takes 28 seconds for one knot to go through the hands of a sailor (567 / 28 ) the speed is 20.52 in/s.
And guess what, 20.52 in/s equals 1,852 km/h, 1 nautical mile per hour, 1 knot!