knot

Before GPS, before radar, before chronometers, sailors measured ship speed with a chip log: a flat piece of wood tied to a rope with knots spaced at regular intervals. The wood was thrown overboard and remained roughly stationary in the water while the ship sailed away from it. A sailor held a sand glass (usually 28 or 30 seconds) and counted how many knots slipped through his hands before the time ran out. The number of knots was the ship's speed.

The spacing of the knots was calculated to correspond to nautical miles per hour. A nautical mile is one minute of arc of latitude — about 1,852 metres, or 6,076 feet. If the knots on the log line were spaced 47 feet 3 inches apart, and the sand glass ran for 28 seconds, then each knot that passed corresponded to one nautical mile per hour. The math was elegant: the ratio of 47.25 feet to 6,076 feet equals the ratio of 28 seconds to 3,600 seconds (one hour).

The chip log was standard equipment on European ships from the sixteenth century through the nineteenth. Columbus did not have one — he estimated speed by watching flotsam pass the hull. But by the time of Drake and the Armada, the log and line were standard. The Dutchman Leendert Willemsz is sometimes credited with the invention, around 1600, though the evidence is thin.

The knot is still the standard unit of speed at sea and in aviation. An Airbus A320 cruises at about 450 knots. A container ship makes 14 knots. Weather reports give wind speed in knots. The word has outlived the tool: no ship drags a knotted rope anymore, but every captain and every pilot speaks in knots. The rope is gone. The unit is permanent.