Episode Details

The speed readout on a jetliner's seatback screen uses a unit born from sailors throwing a weighted wooden chip over the stern. The chip log floated upright to anchor itself in the water while the ship pulled away, and a sailor counted knots tied every 47 feet 3 inches in the line as it ran out over a 28 second sandglass. That count works out to within about 0.02 percent of today's international knot, defined as one nautical mile per hour, or exactly 1.852 kilometers per hour. Accurate speed fed dead reckoning, the only way to fix position when storms hid the stars.

The knot survived metrication because one nautical mile equals one minute of latitude. On Mercator charts, where the horizontal scale can double between Florida and Greenland, navigators walk dividers to the latitude margin instead of trusting a scale bar. Currents, tides, and winds are all measured in knots, which makes ground speed and velocity made good simple arithmetic. Aviation adopted the unit more slowly: U.S. regulations moved to nautical miles and knots only in 1969, and ICAO's 1979 permission for knots was labeled temporary and never given an end date.

• The ISO and IEEE abbreviate the knot as kn, while aviation's ICAO prefers kt.
• Saying knots per hour technically describes acceleration, and seafaring readers complained to the Kipling Journal about Rudyard Kipling's knots an hour.
• British Admiralty charts print a latitude scale down the middle of the page.
• At 36,000 feet a plane with a 500 knot true airspeed may show only 300 knots indicated.
• Pilots rely on indicated airspeed because the wings, like the pitot tube, respond to the air actually flowing past them.