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Ahoy! For land-lubbers who don't know a sail from a sale, knots measure the speed of ships. A knot is one nautical mile per hour, so a ship traveling at five knots is traversing five nautical miles per hour. That's simple enough, but why, inquiring pirates want to know, is it called a knot? The answer dates way back to the 17th century. To measure the speed and distance of a ship, knots were tied into a "log line." This line was thrown overboard, an hourglass was tipped, and the knots were counted. When the sand ran out, the counting stopped, and a general speed was determined. Which brings us to the next seafaring query -- how long is a nautical mile? We're glad
you asked, matey. A nautical mile is equal to about 1.15 survey miles, or 6080 feet. According to this great page of nautical units and angles, a nautical mile is the angular distance of one minute of arc on the Earth's surface. As HowStuffWorks explains, if you break up the equator into 360 degrees, then break up those degrees into sixty minutes each, a nautical mile is the equivalent of one minute. Alas, the Earth isn't a perfect sphere, so the arc minute distances tend to vary from the pole to the equator.
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