Nautical Miles and Knots
A nautical mile is precise measurement based on the circumference of the earth. The equator divides the earth into two equal halves. This circular line is divided into 360 equal parts called degrees. Each degree is divided into 60 smaller parts, called minutes. A nautical mile is the length of one minute. The nautical mile is a standardized unit of measurement used by all nations for air and sea travel. It equals 1.1508 miles (1.852 kilometers).
If you are traveling at one nautical mile per hour, you are travelling at the speed of one knot. Why is it called a knot? To tell speed, a ship would carry a line wound on a reel. A chip of wood on the end of the line allowed to drag in the water behind the ship, causing the line to unreel. The line was knotted at intervals of 47 feet 3 inches and the line allowed to drag for exactly 28 seconds. (47 feet 3 inches are to 1.1508 miles what 28 seconds are to one hour) If the line unwound too the fifth knot in 28 seconds, the ship was moving at 5 knots per hour.