Knot is a unit of speed used for ships and aircraft. It equals one nautical mile an hour. A ship with a 20-knot speed can go 20 nautical miles in an hour.
The international nautical mile equals one-sixtieth of one degree, or a minute of arc, of the earth’s circumference. Navigators use the nautical mile because of its simple relationship to the degrees and minutes by which latitude and longitude are measured. The international nautical mile equals exactly 1.852 kilometers, or 6,076.115 international feet, or 1.151 statute miles.
The term knot came into use in the earlier days of sailing, when ships carried a speed-measuring device called a log chip and line. The line was wound up on a reel. The chip, a piece of wood, was allowed to drag in the water behind the ship. The chip caused the line to unreel as the ship moved. The line was knotted at intervals of 47 feet 3 inches (14.4 meters). At the end of the first interval was one knot. Two knots marked the end of the second, and so on. The line was allowed to run for 28 seconds. An interval of twenty-eight seconds is to one hour approximately what a distance of 47 feet 3 inches is to 6,076 feet. Therefore, if the log had pulled out 5 intervals of line in 28 seconds, the sailors knew the ship was moving at 5 knots, or 5 nautical miles an hour.
